Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?
On Sun, Nov 14, 2010 at 07:21:50PM -0800, Linus Torvalds wrote: So when Richard Gunther says a memory clobber doesn't cover automatic storage, to me that very clearly spells gcc is buggy as hell. Because automatic storage with its address taken _very_ much gets clobbered by things like memset etc. If the compiler doesn't understand that, the compiler is just broken. I'll leave the discussion about meaning of memory clobber aside to Richard, And now, if even the (superfluous) +m isn't working, it sounds like we have no sane options left. Except to say that gcc-4.5.1 is totally just to say that of course there are sane options left. :=a(rc), +m (*regs) :a(regs) :%ebx, %ecx, %edx, %esi, %edi, memory); is simply too high register pressure for i386 if you force also -fno-omit-frame-pointer, there is not a single register left. Yes, reload should figure out it has address of regs already tied to %eax, unfortunately starting with IRA it doesn't (I'll file a GCC bug about that; so that leaves 4.4/4.5/4.6 currently not being able to compile it). That said, changing the inline asm to just clobber one less register would be completely sufficient to make it work well with all gccs out there, just push/pop one of the register around the whole body. I doubt calling out SMM BIOS is actually so performance critical that one push and one pop would ruin it. Of course x86_64 version can stay as is, there are enough registers left... Jakub
Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?
That said, changing the inline asm to just clobber one less register would be completely sufficient to make it work well with all gccs out there, just push/pop one of the register around the whole body. I doubt calling out SMM BIOS is actually so performance critical that one push and one pop would ruin it. Of course x86_64 version can stay as is, there are enough registers left... Yes traditionally clobbering all registers has been dangerous and it clearly can be done inside the asm too. Here's a untested patch to do some manual push/pops too. Could someone with the hardware please test it? (running a 32bit kernel) -Andi --- i8k: Clobber less registers gcc doesn't like inline assembler statements that clobber nearly all registers. Save a few registers manually on i386 to avoid this problem. Fix suggested by Jakub Jelinek Signed-off-by: Andi Kleen a...@linux.intel.com diff --git a/drivers/char/i8k.c b/drivers/char/i8k.c index f0863be..a2da38b 100644 --- a/drivers/char/i8k.c +++ b/drivers/char/i8k.c @@ -146,7 +146,10 @@ static int i8k_smm(struct smm_regs *regs) :a(regs) :%ebx, %ecx, %edx, %esi, %edi, memory); #else - asm(pushl %%eax\n\t + asm(pushl %%ebx\n\t + pushl %%ecx\n\t + pushl %%edx\n\t + pushl %%eax\n\t movl 0(%%eax),%%edx\n\t push %%edx\n\t movl 4(%%eax),%%ebx\n\t @@ -167,10 +170,13 @@ static int i8k_smm(struct smm_regs *regs) movl %%edx,0(%%eax)\n\t lahf\n\t shrl $8,%%eax\n\t - andl $1,%%eax\n + andl $1,%%eax\n\t + popl %%edx\n\t + popl %%ecx\n\t + popl %%ebx\n :=a(rc), +m (*regs) :a(regs) - :%ebx, %ecx, %edx, %esi, %edi, memory); + :%esi, %edi, memory); #endif if (rc != 0 || (regs-eax 0x) == 0x || regs-eax == eax) return -EINVAL; -- a...@linux.intel.com -- Speaking for myself only.
Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?
On Mon, Nov 15, 2010 at 09:56:05AM +0100, Jakub Jelinek wrote: Yes, reload should figure out it has address of regs already tied to %eax, unfortunately starting with IRA it doesn't (I'll file a GCC bug about that; http://gcc.gnu.org/PR46479 Jakub
Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?
On Mon, Nov 15, 2010 at 09:56:05AM +0100, Jakub Jelinek wrote: On Sun, Nov 14, 2010 at 07:21:50PM -0800, Linus Torvalds wrote: So when Richard Gunther says a memory clobber doesn't cover automatic storage, to me that very clearly spells gcc is buggy as hell. Because automatic storage with its address taken _very_ much gets clobbered by things like memset etc. If the compiler doesn't understand that, the compiler is just broken. I'll leave the discussion about meaning of memory clobber aside to Richard, And for this the starting point should be what has been requested, i.e. preprocessed source + gcc options + gcc version and some hints what actually misbehaves (with the , +m (*regs) change reverted) in gcc bugzilla. Only with that we can actually look at what has been happening, see whether it is the tree optimizations or RTL and which one makes a difference. If I've missed a PR about this I apologize. Jakub
Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?
On Mon, Nov 15, 2010 at 9:56 AM, Jakub Jelinek ja...@redhat.com wrote: On Sun, Nov 14, 2010 at 07:21:50PM -0800, Linus Torvalds wrote: So when Richard Gunther says a memory clobber doesn't cover automatic storage, to me that very clearly spells gcc is buggy as hell. Because automatic storage with its address taken _very_ much gets clobbered by things like memset etc. If the compiler doesn't understand that, the compiler is just broken. I'll leave the discussion about meaning of memory clobber aside to Richard, Of course GCC handles memset just fine. Note that I was refering to non-address taken automatic storage for memory (even though when double-checking the current implementation GCC even thinks that all address-taken memory is clobbered by asms as soon as they have at least one memory operand or a memory clobber). It's just that in future we might want to improve this and I think not covering non-address taken automatic storage for memory is sensible. And I see that you don't see address-taken automatic storage as a sensible choice to exclude from memory, and I have noted that. Btw, I still haven't seen an testcase for the actual problem we are talking about. Richard.
Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?
And for this the starting point should be what has been requested, i.e. preprocessed source + gcc options + gcc version and some hints what actually misbehaves (with the , +m (*regs) change reverted) in gcc bugzilla. Only with that we can actually look at what has been happening, see whether it is the tree optimizations or RTL and which one makes a difference. If I've missed a PR about this I apologize. I tried to file one, but I can't reproduce it currently (I don't have hardware, so have to rely on code reading and the 32bit code looks correct to me even without the additional +m) The preprocessed source is at http://halobates.de/tmp/i8k.i Options I used: -D__KERNEL__ -Wall -Wundef -Wstrict-prototypes -Wno-trigraphs -fno-strict-aliasing -fno-common -Werror-implicit-function-declaration -Wno-format-security -fno-delete-null-pointer-checks -O2 -m32 -msoft-float -mregparm=3 -freg-struct-return -mpreferred-stack-boundary=2 -march=i686 -mtune=pentium3 -mtune=generic -maccumulate-outgoing-args -Wa,-mtune=generic32 -ffreestanding -DCONFIG_AS_CFI=1 -DCONFIG_AS_CFI_SIGNAL_FRAME=1 -DCONFIG_AS_CFI_SECTIONS=1 -pipe -Wno-sign-compare -fno-asynchronous-unwind-tables -mno-sse -mno-mmx -mno-sse2 -mno-3dnow -Wframe-larger-than=2048 -fno-stack-protector -fno-omit-frame-pointer -fno-optimize-sibling-calls -pg -Wdeclaration-after-statement -Wno-pointer-sign -fno-strict-overflow -fconserve-stack -Andi -- a...@linux.intel.com -- Speaking for myself only.
Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?
On Mon, Nov 15, 2010 at 11:54:46AM +0100, Andi Kleen wrote: And for this the starting point should be what has been requested, i.e. preprocessed source + gcc options + gcc version and some hints what actually misbehaves (with the , +m (*regs) change reverted) in gcc bugzilla. Only with that we can actually look at what has been happening, see whether it is the tree optimizations or RTL and which one makes a difference. If I've missed a PR about this I apologize. I tried to file one, but I can't reproduce it currently (I don't have hardware, so have to rely on code reading and the 32bit code looks correct to me even without the additional +m) The preprocessed source is at http://halobates.de/tmp/i8k.i Options I used: -D__KERNEL__ -Wall -Wundef -Wstrict-prototypes -Wno-trigraphs -fno-strict-aliasing -fno-common -Werror-implicit-function-declaration -Wno-format-security -fno-delete-null-pointer-checks -O2 -m32 -msoft-float -mregparm=3 -freg-struct-return -mpreferred-stack-boundary=2 -march=i686 -mtune=pentium3 -mtune=generic -maccumulate-outgoing-args -Wa,-mtune=generic32 -ffreestanding -DCONFIG_AS_CFI=1 -DCONFIG_AS_CFI_SIGNAL_FRAME=1 -DCONFIG_AS_CFI_SECTIONS=1 -pipe -Wno-sign-compare -fno-asynchronous-unwind-tables -mno-sse -mno-mmx -mno-sse2 -mno-3dnow -Wframe-larger-than=2048 -fno-stack-protector -fno-omit-frame-pointer -fno-optimize-sibling-calls -pg -Wdeclaration-after-statement -Wno-pointer-sign -fno-strict-overflow -fconserve-stack Indeed, with this and 4.5.2 2010 (prerelease) from SVN as well as gcc-4.5.1-5.fc14: ... movl%eax, -16(%ebp) # regs, %sfp movl(%eax), %eax# regs_2(D)-eax, movl%eax, -20(%ebp) #, %sfp movl-16(%ebp), %eax # %sfp, #APP # 149 /home/lsrc/git/linux-work2/drivers/char/i8k.c 1 ... #NO_APP testl %eax, %eax # movl$-22, %edx #, D.18378 movl%eax, -24(%ebp) #, %sfp je .L7 #, .L2: movl-12(%ebp), %ebx #, movl%edx, %eax # D.18378, movl-8(%ebp), %esi #, movl-4(%ebp), %edi #, movl%ebp, %esp #, popl%ebp# ret .p2align 4,,7 .p2align 3 .L7: movl-16(%ebp), %eax # %sfp, movl(%eax), %ecx# regs_2(D)-eax, D.18371 cmpw$-1, %cx#, D.18371 je .L2 #, cmpl%ecx, -20(%ebp) # D.18371, %sfp cmovne -24(%ebp), %edx # %sfp,, D.18378 jmp .L2 # .size i8k_smm, .-i8k_smm I don't see any problems on the assembly level. i8k_smm is not inlined in this case and checks all 3 conditions. Guess we need somebody who actually reported the problem, state what gcc was actually used and post preprocessed source, gcc options from his case. Jakub
Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?
Guess we need somebody who actually reported the problem, state what gcc was actually used and post preprocessed source, gcc options from his case. Jim Bos, Can you please supply that? Please use rm drivers/char/i8k.o make V=1 drivers/char/i8k.o make drivers/char/i8k.i and supply the .i file and the output of the first make line Thanks, -Andi -- a...@linux.intel.com -- Speaking for myself only.
Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?
On Mon, Nov 15, 2010 at 3:16 AM, Jakub Jelinek ja...@redhat.com wrote:
I don't see any problems on the assembly level. i8k_smm is
not inlined in this case and checks all 3 conditions.
If it really is related to gcc not understanding that *regs has
changed due to the memory being an automatic variable, and passing in
regs itself as a pointer to that automatic variable together with
the memory clobber not being sufficient, than I think it's the lack
of inlining that will automatically hide the bug.
(Side note: and I think this does show how much of a gcc bug it is not
to consider memory together with passing in a pointer to an asm to
always be a clobber).
Because if it isn't inlined, then regs will be seen a a real pointer
to some external memory (the caller) rather than being optimized to
just be the auto structure on the stack. Because *mem is auto only
within the context of the caller.
Which actually points to a possible simpler:
- remove the +m since it adds too much register pressure
- mark the i8k_smm() as noinline instead.
Quite frankly, I'd hate to add even more crud to that inline asm (to
save/restore the registers manually). It's already not the prettiest
thing around.
So does the attached patch work for everybody?
Linus
drivers/char/i8k.c |6 +++---
1 files changed, 3 insertions(+), 3 deletions(-)
diff --git a/drivers/char/i8k.c b/drivers/char/i8k.c
index f0863be..101011e 100644
--- a/drivers/char/i8k.c
+++ b/drivers/char/i8k.c
@@ -114,7 +114,7 @@ static inline const char *i8k_get_dmi_data(int field)
/*
* Call the System Management Mode BIOS. Code provided by Jonathan Buzzard.
*/
-static int i8k_smm(struct smm_regs *regs)
+static noinline int i8k_smm(struct smm_regs *regs)
{
int rc;
int eax = regs-eax;
@@ -142,7 +142,7 @@ static int i8k_smm(struct smm_regs *regs)
lahf\n\t
shrl $8,%%eax\n\t
andl $1,%%eax\n
- :=a(rc), +m (*regs)
+ :=a(rc)
:a(regs)
:%ebx, %ecx, %edx, %esi, %edi, memory);
#else
@@ -168,7 +168,7 @@ static int i8k_smm(struct smm_regs *regs)
lahf\n\t
shrl $8,%%eax\n\t
andl $1,%%eax\n
- :=a(rc), +m (*regs)
+ :=a(rc)
:a(regs)
:%ebx, %ecx, %edx, %esi, %edi, memory);
#endif
Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?
On 11/15/2010 05:04 PM, Linus Torvalds wrote: On Mon, Nov 15, 2010 at 3:16 AM, Jakub Jelinek ja...@redhat.com wrote: I don't see any problems on the assembly level. i8k_smm is not inlined in this case and checks all 3 conditions. If it really is related to gcc not understanding that *regs has changed due to the memory being an automatic variable, and passing in regs itself as a pointer to that automatic variable together with the memory clobber not being sufficient, than I think it's the lack of inlining that will automatically hide the bug. (Side note: and I think this does show how much of a gcc bug it is not to consider memory together with passing in a pointer to an asm to always be a clobber). Because if it isn't inlined, then regs will be seen a a real pointer to some external memory (the caller) rather than being optimized to just be the auto structure on the stack. Because *mem is auto only within the context of the caller. Which actually points to a possible simpler: - remove the +m since it adds too much register pressure - mark the i8k_smm() as noinline instead. Quite frankly, I'd hate to add even more crud to that inline asm (to save/restore the registers manually). It's already not the prettiest thing around. So does the attached patch work for everybody? Linus Hmm, that doesn't work. [ Not sure if you read to whole thread but initial workaround was to change the asm(..) to asm volatile(..) which did work. ] Jim.
Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?
On Mon, Nov 15, 2010 at 06:36:06PM +0100, Jim Bos wrote: On 11/15/2010 12:37 PM, Andi Kleen wrote: See attached, note this is the vanilla 2.6.36 i8k.c (without any patch). And to be 100% sure, if I build this (make drivers/char/i8k.ko) it won't work. [ The i8k.i is rather big, even gzipped 80k, not sure if it'll bounce ] Please also say which exact gcc you are using. Note, I've compiled it with current 4.5 branch and made the function always_inline and still didn't see any issues in the *.optimized dump, regs.eax after the inline asm has always been compared to the constant that has been stored into regs.eax before the inline asm. Jakub
Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?
On Mon, Nov 15, 2010 at 9:40 AM, Jim Bos jim...@xs4all.nl wrote: Hmm, that doesn't work. [ Not sure if you read to whole thread but initial workaround was to change the asm(..) to asm volatile(..) which did work. ] Since I have a different gcc than yours (and I'm not going to compile my own), have you posted your broken .s file anywhere? In fact, with the noinline (and the removal of the +m thing - iow just the patch you tried), what does just the i8k_smm function assembly look like for you after you've done a make drivers/char/i8k.s? If the asm just doesn't exist AT ALL, that's just odd. Because every single call-site of i8k_smm() clearly looks at the return value. So the volatile really shouldn't make any difference from that standpoint. Odd. Linus
Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?
On 11/15/2010 06:44 PM, Jakub Jelinek wrote: On Mon, Nov 15, 2010 at 06:36:06PM +0100, Jim Bos wrote: On 11/15/2010 12:37 PM, Andi Kleen wrote: See attached, note this is the vanilla 2.6.36 i8k.c (without any patch). And to be 100% sure, if I build this (make drivers/char/i8k.ko) it won't work. [ The i8k.i is rather big, even gzipped 80k, not sure if it'll bounce ] Please also say which exact gcc you are using. Note, I've compiled it with current 4.5 branch and made the function always_inline and still didn't see any issues in the *.optimized dump, regs.eax after the inline asm has always been compared to the constant that has been stored into regs.eax before the inline asm. Jakub -- To unsubscribe from this list: send the line unsubscribe linux-kernel in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ # gcc -v Reading specs from /usr/lib/gcc/i486-slackware-linux/4.5.1/specs COLLECT_GCC=gcc COLLECT_LTO_WRAPPER=/usr/libexec/gcc/i486-slackware-linux/4.5.1/lto-wrapper Target: i486-slackware-linux Configured with: ../gcc-4.5.1/configure --prefix=/usr --libdir=/usr/lib --mandir=/usr/man --infodir=/usr/info --enable-shared --enable-bootstrap --enable-languages=ada,c,c++,fortran,java,objc --enable-threads=posix --enable-checking=release --with-system-zlib --with-python-dir=/lib/python2.6/site-packages --disable-libunwind-exceptions --enable-__cxa_atexit --enable-libssp --with-gnu-ld --verbose --with-arch=i486 --target=i486-slackware-linux --build=i486-slackware-linux --host=i486-slackware-linux Thread model: posix gcc version 4.5.1 (GCC) I'm re-reading this thread where I found the asm- asm volatine suggestion: https://bbs.archlinux.org/viewtopic.php?pid=752099#p752099 but nobody there reported their gcc version (but apparently first people started complaining May 1st). _ Jim
Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?
On Mon, Nov 15, 2010 at 07:17:31PM +0100, Jim Bos wrote: # gcc -v Reading specs from /usr/lib/gcc/i486-slackware-linux/4.5.1/specs COLLECT_GCC=gcc COLLECT_LTO_WRAPPER=/usr/libexec/gcc/i486-slackware-linux/4.5.1/lto-wrapper Target: i486-slackware-linux Configured with: ../gcc-4.5.1/configure --prefix=/usr --libdir=/usr/lib --mandir=/usr/man --infodir=/usr/info --enable-shared --enable-bootstrap --enable-languages=ada,c,c++,fortran,java,objc --enable-threads=posix --enable-checking=release --with-system-zlib --with-python-dir=/lib/python2.6/site-packages --disable-libunwind-exceptions --enable-__cxa_atexit --enable-libssp --with-gnu-ld --verbose --with-arch=i486 --target=i486-slackware-linux --build=i486-slackware-linux --host=i486-slackware-linux Thread model: posix gcc version 4.5.1 (GCC) Does it have any patches applied? The gcc options look the same as what I've been already trying earlier. Thus, can you run gcc with those options on i8k.i and add -fverbose-asm to make it easier to read and post i8k.s you get? Jakub
Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?
On 11/15/2010 07:08 PM, Linus Torvalds wrote: On Mon, Nov 15, 2010 at 9:40 AM, Jim Bos jim...@xs4all.nl wrote: Hmm, that doesn't work. [ Not sure if you read to whole thread but initial workaround was to change the asm(..) to asm volatile(..) which did work. ] Since I have a different gcc than yours (and I'm not going to compile my own), have you posted your broken .s file anywhere? In fact, with the noinline (and the removal of the +m thing - iow just the patch you tried), what does just the i8k_smm function assembly look like for you after you've done a make drivers/char/i8k.s? If the asm just doesn't exist AT ALL, that's just odd. Because every single call-site of i8k_smm() clearly looks at the return value. So the volatile really shouldn't make any difference from that standpoint. Odd. Linus Attached version with plain 2.6.36 source and version with the committed patch, i.e with the '+m (*regs)' _ Jim .file i8k.c # GNU C (GCC) version 4.5.1 (i486-slackware-linux) # compiled by GNU C version 4.5.1, GMP version 5.0.1, MPFR version 2.4.2-p3, MPC version 0.8.2 # GGC heuristics: --param ggc-min-expand=81 --param ggc-min-heapsize=96817 # options passed: -nostdinc -I/usr/src/linux-2.6.36/arch/x86/include # -Iinclude -D__KERNEL__ -DCONFIG_AS_CFI=1 -DCONFIG_AS_CFI_SIGNAL_FRAME=1 # -DCONFIG_AS_CFI_SECTIONS=1 -DMODULE -DKBUILD_STR(s)=#s # -DKBUILD_BASENAME=KBUILD_STR(i8k) -DKBUILD_MODNAME=KBUILD_STR(i8k) # -isystem /usr/lib/gcc/i486-slackware-linux/4.5.1/include -include # include/generated/autoconf.h -MD drivers/char/.i8k.s.d drivers/char/i8k.c # -m32 -msoft-float -mregparm=3 -mpreferred-stack-boundary=2 -march=i686 # -mtune=pentium3 -mtune=generic -mno-sse -mno-mmx -mno-sse2 -mno-3dnow # -auxbase-strip drivers/char/i8k.s -Os -Wall -Wundef -Wstrict-prototypes # -Wno-trigraphs -Werror-implicit-function-declaration -Wno-format-security # -Wno-sign-compare -Wframe-larger-than=1024 -Wdeclaration-after-statement # -Wno-pointer-sign -fno-strict-aliasing -fno-common # -fno-delete-null-pointer-checks -freg-struct-return -ffreestanding # -fno-asynchronous-unwind-tables -fno-stack-protector -fomit-frame-pointer # -fno-strict-overflow -fconserve-stack -fverbose-asm # options enabled: -falign-loops -fargument-alias -fauto-inc-dec # -fbranch-count-reg -fcaller-saves -fcprop-registers -fcrossjumping # -fcse-follow-jumps -fdefer-pop -fdwarf2-cfi-asm -fearly-inlining # -feliminate-unused-debug-types -fexpensive-optimizations # -fforward-propagate -ffunction-cse -fgcse -fgcse-lm # -fguess-branch-probability -fident -fif-conversion -fif-conversion2 # -findirect-inlining -finline -finline-functions # -finline-functions-called-once -finline-small-functions -fipa-cp # -fipa-pure-const -fipa-reference -fipa-sra -fira-share-save-slots # -fira-share-spill-slots -fivopts -fkeep-static-consts # -fleading-underscore -fmath-errno -fmerge-constants -fmerge-debug-strings # -fmove-loop-invariants -fomit-frame-pointer -foptimize-register-move # -foptimize-sibling-calls -fpeephole -fpeephole2 -freg-struct-return # -fregmove -freorder-blocks -freorder-functions -frerun-cse-after-loop # -fsched-critical-path-heuristic -fsched-dep-count-heuristic # -fsched-group-heuristic -fsched-interblock -fsched-last-insn-heuristic # -fsched-rank-heuristic -fsched-spec -fsched-spec-insn-heuristic # -fsched-stalled-insns-dep -fschedule-insns2 -fshow-column -fsigned-zeros # -fsplit-ivs-in-unroller -fsplit-wide-types -fthread-jumps # -ftoplevel-reorder -ftrapping-math -ftree-builtin-call-dce -ftree-ccp # -ftree-ch -ftree-copy-prop -ftree-copyrename -ftree-cselim -ftree-dce # -ftree-dominator-opts -ftree-dse -ftree-forwprop -ftree-fre # -ftree-loop-im -ftree-loop-ivcanon -ftree-loop-optimize # -ftree-parallelize-loops= -ftree-phiprop -ftree-pre -ftree-pta # -ftree-reassoc -ftree-scev-cprop -ftree-sink -ftree-slp-vectorize # -ftree-sra -ftree-switch-conversion -ftree-ter -ftree-vect-loop-version # -ftree-vrp -funit-at-a-time -fvect-cost-model -fverbose-asm # -fzero-initialized-in-bss -m32 -m96bit-long-double -malign-stringops # -mfused-madd -mglibc -mieee-fp -mno-fancy-math-387 -mno-red-zone # -mno-sse4 -mpush-args -msahf -mtls-direct-seg-refs # Compiler executable checksum: 7ba2dc3c015559b9d16b297ee7f8d354 .text .type i8k_smm, @function i8k_smm: pushl %ebp# movl%eax, %ebp # regs, regs pushl %edi# pushl %esi# pushl %ebx# subl$8, %esp#, movl(%eax), %eax# regs_2(D)-eax, movl%eax, 4(%esp) #, %sfp movl%ebp, %eax # regs, #APP # 148 drivers/char/i8k.c 1 pushl %eax movl 0(%eax),%edx push %edx movl 4(%eax),%ebx movl 8(%eax),%ecx movl 12(%eax),%edx movl 16(%eax),%esi movl 20(%eax),%edi popl %eax out %al,$0xb2 out %al,$0x84 xchgl %eax,(%esp) movl
Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?
On 11/15/2010 07:30 PM, Jim Bos wrote: On 11/15/2010 07:08 PM, Linus Torvalds wrote: On Mon, Nov 15, 2010 at 9:40 AM, Jim Bos jim...@xs4all.nl wrote: Hmm, that doesn't work. [ Not sure if you read to whole thread but initial workaround was to change the asm(..) to asm volatile(..) which did work. ] Since I have a different gcc than yours (and I'm not going to compile my own), have you posted your broken .s file anywhere? In fact, with the noinline (and the removal of the +m thing - iow just the patch you tried), what does just the i8k_smm function assembly look like for you after you've done a make drivers/char/i8k.s? If the asm just doesn't exist AT ALL, that's just odd. Because every single call-site of i8k_smm() clearly looks at the return value. So the volatile really shouldn't make any difference from that standpoint. Odd. Linus Attached version with plain 2.6.36 source and version with the committed patch, i.e with the '+m (*regs)' _ Jim And I just tried with your noninline patch which results in exactly the same .s file as with plain 2.6.36 source, i.e. the noninline patch is not doing anything here. _ Jim
Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?
On 11/07/10 15:41, Andreas Schwab wrote: Andi Kleena...@firstfloor.org writes: Jimjim...@xs4all.nl writes: After upgrading my Dell laptop, both OS+kernel the i8k interface was giving nonsensical output. As it turned out it's not the kernel but compiler upgrade which broke this. Guys at Archlinux have found the underlying cause (but don't seem to have submitted a patch yet): https://bbs.archlinux.org/viewtopic.php?pid=780692#p780692 gcc seems to optimize the assembly statements away. And indeed, applying this patch makes the i8k interface work again, i.e. replacing the asm(..) construct by asm volatile(..) The compiler really should not optimize the asm away, because it has both input and output arguments which are later used. asm volatile normally just means don't move significantly The asm fails to mention that it modifies *regs. But there's a memory clobber, that should be sufficient to indicate *regs is modified. jeff
Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?
On 11/08/10 03:49, Richard Guenther wrote: On Mon, Nov 8, 2010 at 12:03 AM, Andi Kleena...@firstfloor.org wrote: Andreas Schwabsch...@linux-m68k.org writes: The asm fails to mention that it modifies *regs. It has a memory clobber, that should be enough, no? No. A memory clobber does not cover automatic storage. A memory clobber should clobber anything in memory, including autos in memory; if it doesn't, then that seems like a major problem. I'd like to see the rationale behind not clobbering autos in memory. Jeff
Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?
On Mon, Nov 15, 2010 at 10:30 AM, Jim Bos jim...@xs4all.nl wrote: Attached version with plain 2.6.36 source and version with the committed patch, i.e with the '+m (*regs)' Looks 100% identical in i8k_smm() itself, and I'm not seeing anything bad. The asm has certainly not been optimized away as implied in the archlinux thread. There are differences, but they are with code generation *elsewhere*. To me it is starting to look like the real problem is that gcc has decided that the i8k_smm() function is __attribute__((const)). Which is clearly totally bogus. If a function has an inline asm that has a memory clobber, it is clearly *not* 'const'. But that does explain the bug, and does explain why +m makes a difference and why noinline does not. So what I _think_ happens is that - gcc logic for the automatic 'const' attribute for functions is broken, so it marks that function 'const'. - since the rule for a const function is that it only _looks_ at its attributes and has no side effects, now the callers will decide that 'i8k_smm()' cannot change the passed-in structure, so they'll happily optimize away all the accesses to it. Linus
Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?
On Mon, Nov 15, 2010 at 07:30:35PM +0100, Jim Bos wrote:
On 11/15/2010 07:08 PM, Linus Torvalds wrote:
On Mon, Nov 15, 2010 at 9:40 AM, Jim Bos jim...@xs4all.nl wrote:
Hmm, that doesn't work.
[ Not sure if you read to whole thread but initial workaround was to
change the asm(..) to asm volatile(..) which did work. ]
Since I have a different gcc than yours (and I'm not going to compile
my own), have you posted your broken .s file anywhere? In fact, with
the noinline (and the removal of the +m thing - iow just the patch
you tried), what does just the i8k_smm function assembly look like
for you after you've done a make drivers/char/i8k.s?
If the asm just doesn't exist AT ALL, that's just odd. Because every
single call-site of i8k_smm() clearly looks at the return value. So
the volatile really shouldn't make any difference from that
standpoint. Odd.
Linus
Attached version with plain 2.6.36 source and version with the committed
patch, i.e with the '+m (*regs)'
Thanks, this actually helped to see the problem.
The problem is not inside of i8k_smm, which is not inlined, but in the
callers.
ipa-pure-const.c pass thinks i8k_smm is a pure function, thus
regs = {};
regs.eax = 166;
x = i8k_smm (regs);
if (!x) x = regs.eax;
in the callers is optimized into
regs = {}
regs.eax = 166;
x = i8k_smm (regs);
if (!x) x = 166;
Now, not sure why this happens, as there is
case GIMPLE_ASM:
for (i = 0; i gimple_asm_nclobbers (stmt); i++)
{
tree op = gimple_asm_clobber_op (stmt, i);
if (simple_cst_equal(TREE_VALUE (op), memory_identifier_string) == 1)
{
if (dump_file)
fprintf (dump_file, memory asm clobber is not const/pure);
/* Abandon all hope, ye who enter here. */
local-pure_const_state = IPA_NEITHER;
}
}
Debugging...
Jakub
Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?
On Mon, Nov 15, 2010 at 10:45 AM, Jeff Law l...@redhat.com wrote:
A memory clobber should clobber anything in memory, including autos in
memory; if it doesn't, then that seems like a major problem. I'd like to
see the rationale behind not clobbering autos in memory.
Yes. It turns out that the asm optimized away was entirely wrong (we
never saw that, it was just a report on another mailing list).
Looking at the asm posted, it seems to me that gcc actually compiles
the asm itself 100% correctly, and the memory clobber is working
fine inside that function. So the code generated for i8k_smm() itself
is all good.
But _while_ generating the good code, gcc doesn't seem to realize that
it writes to anything, so it decides to mark the function
__attribute__((const)), which is obviously wrong (a memory clobber
definitely implies that it's not const). And as a result, the callers
will be mis-optimized, because they do things like
static int i8k_get_bios_version(void)
{
struct smm_regs regs = { .eax = I8K_SMM_BIOS_VERSION, };
return i8k_smm(regs) ? : regs.eax;
}
and since gcc has (incorrectly) decided that i8k_smm() is a const
function, it thinks that regs.eax hasn't changed, so it doesn't
bother to reload it: it knows that it is still I8K_SMM_BIOS_VERSION
that it initialized it with. So it will basically have rewritten that
final return statement as
return i8k_smm(regs) ? : I8K_SMM_BIOS_VERSION;
which obviously doesn't really work.
This also explains why adding volatile worked. The asm volatile
triggered this is not a const function.
Similarly, the +m works, because it also makes clear that the asm is
writing to memory, and isn't a const function.
Now, the memory clobber should clearly also have done that, but I'd
be willing to bet that some version of gcc (possibly extra slackware
patches) had forgotten the trivial logic to say a memory clobber also
makes the user function non-const.
Linus
Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?
On 11/15/2010 07:26 PM, Jakub Jelinek wrote: On Mon, Nov 15, 2010 at 07:17:31PM +0100, Jim Bos wrote: # gcc -v Reading specs from /usr/lib/gcc/i486-slackware-linux/4.5.1/specs COLLECT_GCC=gcc COLLECT_LTO_WRAPPER=/usr/libexec/gcc/i486-slackware-linux/4.5.1/lto-wrapper Target: i486-slackware-linux Configured with: ../gcc-4.5.1/configure --prefix=/usr --libdir=/usr/lib --mandir=/usr/man --infodir=/usr/info --enable-shared --enable-bootstrap --enable-languages=ada,c,c++,fortran,java,objc --enable-threads=posix --enable-checking=release --with-system-zlib --with-python-dir=/lib/python2.6/site-packages --disable-libunwind-exceptions --enable-__cxa_atexit --enable-libssp --with-gnu-ld --verbose --with-arch=i486 --target=i486-slackware-linux --build=i486-slackware-linux --host=i486-slackware-linux Thread model: posix gcc version 4.5.1 (GCC) Does it have any patches applied? The gcc options look the same as what I've been already trying earlier. Thus, can you run gcc with those options on i8k.i and add -fverbose-asm to make it easier to read and post i8k.s you get? Jakub Slackware is typically not patching much (and I'm just using the pre-compiled binary). Here is the link to how it's built: http://slackware.osuosl.org/slackware-current/source/d/gcc/ there doesn't appear to be anything relevant changed. I already posted the .s files, plain 2.6.36 and the one with working patch, I =think= that's already using -fverbose-asm, at least that shows in the output. _ Jim
Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?
On Mon, Nov 15, 2010 at 07:58:48PM +0100, Jakub Jelinek wrote:
Now, not sure why this happens, as there is
case GIMPLE_ASM:
for (i = 0; i gimple_asm_nclobbers (stmt); i++)
{
tree op = gimple_asm_clobber_op (stmt, i);
if (simple_cst_equal(TREE_VALUE (op), memory_identifier_string) ==
1)
{
if (dump_file)
fprintf (dump_file, memory asm clobber is not
const/pure);
/* Abandon all hope, ye who enter here. */
local-pure_const_state = IPA_NEITHER;
}
}
Debugging...
Ah, the problem is that memory_identifier_string is only initialized in
ipa-reference.c's initialization, so it can be (and is in this case) NULL in
ipa-pure-const.c.
Two possible fixes (the latter is apparently what is used in
tree-ssa-operands.c, so is probably sufficient). Guess ipa-reference.c
should be changed to do the same and just drop memory_identifier_string.
Jakub
--- gcc/ipa-pure-const.c.jj 2010-08-11 16:06:19.0 +0200
+++ gcc/ipa-pure-const.c2010-11-15 20:06:36.121310614 +0100
@@ -460,7 +460,10 @@ check_stmt (gimple_stmt_iterator *gsip,
for (i = 0; i gimple_asm_nclobbers (stmt); i++)
{
tree op = gimple_asm_clobber_op (stmt, i);
- if (simple_cst_equal(TREE_VALUE (op), memory_identifier_string) == 1)
+ if (TREE_CODE (TREE_VALUE (op)) == STRING_CST
+ TREE_STRING_LENGTH (TREE_VALUE (op)) == sizeof (memory)
+ memcmp (TREE_STRING_POINTER (TREE_VALUE (op)), memory,
+sizeof (memory)) == 0)
{
if (dump_file)
fprintf (dump_file, memory asm clobber is not
const/pure);
--- gcc/ipa-pure-const.c.jj 2010-08-11 16:06:19.0 +0200
+++ gcc/ipa-pure-const.c2010-11-15 20:07:51.463716989 +0100
@@ -460,7 +460,7 @@ check_stmt (gimple_stmt_iterator *gsip,
for (i = 0; i gimple_asm_nclobbers (stmt); i++)
{
tree op = gimple_asm_clobber_op (stmt, i);
- if (simple_cst_equal(TREE_VALUE (op), memory_identifier_string) == 1)
+ if (strcmp (TREE_STRING_POINTER (TREE_VALUE (link)), memory) == 0)
{
if (dump_file)
fprintf (dump_file, memory asm clobber is not
const/pure);
Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?
On Mon, Nov 15, 2010 at 11:12 AM, Jakub Jelinek ja...@redhat.com wrote: Ah, the problem is that memory_identifier_string is only initialized in ipa-reference.c's initialization, so it can be (and is in this case) NULL in ipa-pure-const.c. Ok. And I guess you can verify that all versions of gcc do this correctly for asm volatile? Because since we'll have to work around this problem in the kernel, I suspect the simplest solution is to remove the +m that causes register pressure problems, and then use asm volatile to work around the const-function bug. And add a large comment about why asm volatile is probably always a good idea when you have a memory clobber and don't have any other visible memory modifications. I do wonder if this explains some of the problems we had with the bitop asms too. Hmm? Linus
Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?
On Mon, Nov 15, 2010 at 11:21:30AM -0800, Linus Torvalds wrote:
On Mon, Nov 15, 2010 at 11:12 AM, Jakub Jelinek ja...@redhat.com wrote:
Ah, the problem is that memory_identifier_string is only initialized in
ipa-reference.c's initialization, so it can be (and is in this case) NULL in
ipa-pure-const.c.
Ok. And I guess you can verify that all versions of gcc do this
correctly for asm volatile?
Yes, reading 4.1/4.2/4.3/4.4/4.5/4.6 code ipa-pure-const.c handles
asm volatile correctly, in each case the function is no longer assumed to be
pure or const in the discovery (of course, user can still say the
function is const or pure). 4.0 and earlier didn't have ipa-pure-const.c.
Using the simplified
extern void abort (void);
__attribute__((noinline)) int
foo (int *p)
{
int r;
asm (movl $6, (%1)\n\txorl %0, %0 : =r (r) : r (p) : memory);
return r;
}
int
main (void)
{
int p = 8;
if ((foo (p) ? : p) != 6)
abort ();
return 0;
}
testcase shows that in 4.1/4.2/4.3/4.4 this is miscompiled only when using
-fno-ipa-reference, in 4.5 it is miscompiled always when optimizing
unless -fno-ipa-pure-const (as 4.5 added local-pure-const pass which is run
before ipa-reference) and in 4.6 this has been fixed by Honza when
doing ipa cleanups.
Because since we'll have to work around this problem in the kernel, I
suspect the simplest solution is to remove the +m that causes
register pressure problems, and then use asm volatile to work around
the const-function bug.
Yes.
Jakub
Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?
On 11/15/2010 11:12 AM, Jakub Jelinek wrote: - if (simple_cst_equal(TREE_VALUE (op), memory_identifier_string) == 1) + if (strcmp (TREE_STRING_POINTER (TREE_VALUE (link)), memory) == 0) I prefer this solution. I think memory_identifier_string is over-engineering. Patch to remove it entirely is pre-approved. r~
Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?
On Mon, Nov 15, 2010 at 11:53:05AM -0800, Richard Henderson wrote: On 11/15/2010 11:12 AM, Jakub Jelinek wrote: - if (simple_cst_equal(TREE_VALUE (op), memory_identifier_string) == 1) + if (strcmp (TREE_STRING_POINTER (TREE_VALUE (link)), memory) == 0) I prefer this solution. I think memory_identifier_string is over-engineering. Patch to remove it entirely is pre-approved. Honza even committed this to the trunk in May, it is just release branches that are broken (and only in 4.5 it matters a lot because it happens with the default flags). Jakub
Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?
On 11/15/2010 08:51 PM, Jakub Jelinek wrote:
On Mon, Nov 15, 2010 at 11:21:30AM -0800, Linus Torvalds wrote:
On Mon, Nov 15, 2010 at 11:12 AM, Jakub Jelinek ja...@redhat.com wrote:
Ah, the problem is that memory_identifier_string is only initialized in
ipa-reference.c's initialization, so it can be (and is in this case) NULL in
ipa-pure-const.c.
Ok. And I guess you can verify that all versions of gcc do this
correctly for asm volatile?
Yes, reading 4.1/4.2/4.3/4.4/4.5/4.6 code ipa-pure-const.c handles
asm volatile correctly, in each case the function is no longer assumed to be
pure or const in the discovery (of course, user can still say the
function is const or pure). 4.0 and earlier didn't have ipa-pure-const.c.
Using the simplified
extern void abort (void);
__attribute__((noinline)) int
foo (int *p)
{
int r;
asm (movl $6, (%1)\n\txorl %0, %0 : =r (r) : r (p) : memory);
return r;
}
int
main (void)
{
int p = 8;
if ((foo (p) ? : p) != 6)
abort ();
return 0;
}
testcase shows that in 4.1/4.2/4.3/4.4 this is miscompiled only when using
-fno-ipa-reference, in 4.5 it is miscompiled always when optimizing
unless -fno-ipa-pure-const (as 4.5 added local-pure-const pass which is run
before ipa-reference) and in 4.6 this has been fixed by Honza when
doing ipa cleanups.
Because since we'll have to work around this problem in the kernel, I
suspect the simplest solution is to remove the +m that causes
register pressure problems, and then use asm volatile to work around
the const-function bug.
Yes.
Jakub
Linus,
In case you didn't already fixed this, here's the follow-up patch.
---
The fix to work around the gcc miscompiling i8k.c to add +m (*regs)
caused register pressure problems. Changing the 'asm' statement to
'asm volatile' instead should prevent that and works around the gcc
bug as well.
Signed-off-by: Jim Bos jim...@xs4all.nl
--- linux/drivers/char/i8k.c.ORIG 2010-11-15 21:04:19.0 +0100
+++ linux/drivers/char/i8k.c2010-11-15 21:02:32.0 +0100
@@ -119,7 +119,7 @@
int eax = regs-eax;
#if defined(CONFIG_X86_64)
- asm(pushq %%rax\n\t
+ asm volatile(pushq %%rax\n\t
movl 0(%%rax),%%edx\n\t
pushq %%rdx\n\t
movl 4(%%rax),%%ebx\n\t
@@ -141,11 +141,11 @@
lahf\n\t
shrl $8,%%eax\n\t
andl $1,%%eax\n
- :=a(rc), +m (*regs)
+ :=a(rc)
:a(regs)
:%ebx, %ecx, %edx, %esi, %edi, memory);
#else
- asm(pushl %%eax\n\t
+ asm volatile(pushl %%eax\n\t
movl 0(%%eax),%%edx\n\t
push %%edx\n\t
movl 4(%%eax),%%ebx\n\t
@@ -167,7 +167,7 @@
lahf\n\t
shrl $8,%%eax\n\t
andl $1,%%eax\n
- :=a(rc), +m (*regs)
+ :=a(rc)
:a(regs)
:%ebx, %ecx, %edx, %esi, %edi, memory);
#endif
Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?
On Mon, Nov 15, 2010 at 7:45 PM, Jeff Law l...@redhat.com wrote: On 11/08/10 03:49, Richard Guenther wrote: On Mon, Nov 8, 2010 at 12:03 AM, Andi Kleena...@firstfloor.org wrote: Andreas Schwabsch...@linux-m68k.org writes: The asm fails to mention that it modifies *regs. It has a memory clobber, that should be enough, no? No. A memory clobber does not cover automatic storage. A memory clobber should clobber anything in memory, including autos in memory; if it doesn't, then that seems like a major problem. I'd like to see the rationale behind not clobbering autos in memory. Non-address taken automatic storage. (note that we don't excercise this in optimization yet) It's difficult to model thins kind of non-aliased memory with this kind of aliasing mechanism (apart from taking all asms as clobbering everything as we currently do). Richard. Jeff
Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?
testcase shows that in 4.1/4.2/4.3/4.4 this is miscompiled only when using -fno-ipa-reference, in 4.5 it is miscompiled always when optimizing unless -fno-ipa-pure-const (as 4.5 added local-pure-const pass which is run before ipa-reference) and in 4.6 this has been fixed by Honza when doing ipa cleanups. Maybe it would be better to simply change the kernel Makefiles to pass -fno-ipa-pure-const instead of adding volatiles everywhere. -Andi
Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?
On Mon, Nov 15, 2010 at 11:43:22PM +0100, Andi Kleen wrote: testcase shows that in 4.1/4.2/4.3/4.4 this is miscompiled only when using -fno-ipa-reference, in 4.5 it is miscompiled always when optimizing unless -fno-ipa-pure-const (as 4.5 added local-pure-const pass which is run before ipa-reference) and in 4.6 this has been fixed by Honza when doing ipa cleanups. Maybe it would be better to simply change the kernel Makefiles to pass -fno-ipa-pure-const instead of adding volatiles everywhere. If you do this, please do it for 4.5.[012] only. If you disable all gcc passes that ever had any bugs in it, you'd need to disable most of them if not all. Jakub
Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?
On 11/15/10 15:07, Richard Guenther wrote: On Mon, Nov 15, 2010 at 7:45 PM, Jeff Lawl...@redhat.com wrote: On 11/08/10 03:49, Richard Guenther wrote: On Mon, Nov 8, 2010 at 12:03 AM, Andi Kleena...@firstfloor.orgwrote: Andreas Schwabsch...@linux-m68k.orgwrites: The asm fails to mention that it modifies *regs. It has a memory clobber, that should be enough, no? No. A memory clobber does not cover automatic storage. A memory clobber should clobber anything in memory, including autos in memory; if it doesn't, then that seems like a major problem. I'd like to see the rationale behind not clobbering autos in memory. Non-address taken automatic storage. (note that we don't excercise this in optimization yet) If the address of the auto isn't taken, then why is the object in memory to begin with (with the obvious exception for aggregates). Jeff
Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?
On Mon, Nov 15, 2010 at 11:58 PM, Jeff Law l...@redhat.com wrote: On 11/15/10 15:07, Richard Guenther wrote: On Mon, Nov 15, 2010 at 7:45 PM, Jeff Lawl...@redhat.com wrote: On 11/08/10 03:49, Richard Guenther wrote: On Mon, Nov 8, 2010 at 12:03 AM, Andi Kleena...@firstfloor.org wrote: Andreas Schwabsch...@linux-m68k.org writes: The asm fails to mention that it modifies *regs. It has a memory clobber, that should be enough, no? No. A memory clobber does not cover automatic storage. A memory clobber should clobber anything in memory, including autos in memory; if it doesn't, then that seems like a major problem. I'd like to see the rationale behind not clobbering autos in memory. Non-address taken automatic storage. (note that we don't excercise this in optimization yet) If the address of the auto isn't taken, then why is the object in memory to begin with (with the obvious exception for aggregates). Exactly sort of my point. If people pass the address of x to an asm and modify x + 8 expecting the adjacent stack location to be changed I want to tell them that's not a supported way to get to another stack variable (even if they clobber memory). Or consider the C-decl guy who wants to access adjacent parameters by address arithmetic on the address of the first param ... Richard. Jeff
Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?
On 11/15/10 16:07, Richard Guenther wrote: If the address of the auto isn't taken, then why is the object in memory to begin with (with the obvious exception for aggregates). Exactly sort of my point. If people pass the address ofx to an asm and modifyx + 8 expecting the adjacent stack location to be changed I want to tell them that's not a supported way to get to another stack variable (even if they clobber memory). Or consider the C-decl guy who wants to access adjacent parameters by address arithmetic on the address of the first param ... Well, in that case, I think we can easily say that the programmer has gone off the deep end and has entered the realm of undefined behavior. Presumably we rooted out all relevant instances of the latter over the last 20 years... It was fairly common in the past, but I doubt anyone worth caring about is still writing code assuming they can take the address of parameter A, offset it and get parameters B, C, D, etc. jeff
Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?
Gcc 4.5.1 running on an amd64 box cross-compiling for a P3 i8k fails to compile the module since commit 6b4e81db2552bad04100e7d5ddeed7e848f53b48 with: CC drivers/char/i8k.o drivers/char/i8k.c: In function ‘i8k_smm’: drivers/char/i8k.c:149:2: error: can't find a register in class ‘GENERAL_REGS’ while reloading ‘asm’ drivers/char/i8k.c:149:2: error: ‘asm’ operand has impossible constraints -JimC -- James Cloos cl...@jhcloos.com OpenPGP: 1024D/ED7DAEA6
Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?
On Sun, Nov 14, 2010 at 4:52 PM, James Cloos cl...@jhcloos.com wrote: Gcc 4.5.1 running on an amd64 box cross-compiling for a P3 i8k fails to compile the module since commit 6b4e81db2552bad04100e7d5ddeed7e848f53b48 with: CC drivers/char/i8k.o drivers/char/i8k.c: In function ‘i8k_smm’: drivers/char/i8k.c:149:2: error: can't find a register in class ‘GENERAL_REGS’ while reloading ‘asm’ drivers/char/i8k.c:149:2: error: ‘asm’ operand has impossible constraints At this point, I think this falls clearly under unresolvable gcc bug. Quite frankly, I think gcc was buggy to begin with: since we had a memory clobber, the +m (*regs) should not have mattered. The fact that *regs may be some local variable doesn't make any difference what-so-ever, since we took the address of the variable. So the memory clobber _clearly_ can change that variable. So when Richard Gunther says a memory clobber doesn't cover automatic storage, to me that very clearly spells gcc is buggy as hell. Because automatic storage with its address taken _very_ much gets clobbered by things like memset etc. If the compiler doesn't understand that, the compiler is just broken. And now, if even the (superfluous) +m isn't working, it sounds like we have no sane options left. Except to say that gcc-4.5.1 is totally broken wrt asms. Can we just get gcc to realize that when you pass the address of automatic storage to an asm, that means that memory really does clobber it? Because clearly that is the case. Linus
Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?
My speculation is, that the asm is not removed but rather that regs.eax isn't reloaded after the asm because the memory clobber doesn't clobber automatic variables. Yes that makes sense. I wasn't able to verify it so far though. Maybe the original poster could try the obvious patch instead of the volatile change. i8k: tell gcc that regs gets clobbered Signed-off-by: Andi Kleen a...@linux.intel.com diff --git a/drivers/char/i8k.c b/drivers/char/i8k.c index 3bc0eef..f3bbf73 100644 --- a/drivers/char/i8k.c +++ b/drivers/char/i8k.c @@ -142,7 +142,7 @@ static int i8k_smm(struct smm_regs *regs) lahf\n\t shrl $8,%%eax\n\t andl $1,%%eax\n - :=a(rc) + :=a(rc), =m (*regs) :a(regs) :%ebx, %ecx, %edx, %esi, %edi, memory); #else @@ -167,7 +167,7 @@ static int i8k_smm(struct smm_regs *regs) movl %%edx,0(%%eax)\n\t lahf\n\t shrl $8,%%eax\n\t - andl $1,%%eax\n:=a(rc) + andl $1,%%eax\n:=a(rc), =m (*regs) :a(regs) :%ebx, %ecx, %edx, %esi, %edi, memory); #endif -Andi
Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?
Hi,
On Mon, 8 Nov 2010, Dave Korn wrote:
void foo (void)
{
int x, y, z;
x = 23;
asm (do something : =r (y) : r (x) );
z = y + 1;
}
The case in i8k.c really is different. It does use the value by
influencing the return value and the callers use the returned value in
conditionals and the like. It really, really _is_ used :-) and if GCC
removes the asm (which up to now is only speculation) then it's a GCC bug.
The code outlines like so:
int i8k_smm (regs) {
int rc;
asm (... : =r(rc) ...);
if (rc != 0 || ...)
return -EINVAL;
return 0;
}
...
struct regs regs = {.eax = ...}
return i8k_smm(regs) ?: regs.eax;
...
My speculation is, that the asm is not removed but rather that regs.eax
isn't reloaded after the asm because the memory clobber doesn't clobber
automatic variables.
Ciao,
Michael.
Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?
Andi Kleen a...@firstfloor.org writes: @@ -142,7 +142,7 @@ static int i8k_smm(struct smm_regs *regs) lahf\n\t shrl $8,%%eax\n\t andl $1,%%eax\n - :=a(rc) + :=a(rc), =m (*regs) I think this should be +m. Andreas. -- Andreas Schwab, sch...@redhat.com GPG Key fingerprint = D4E8 DBE3 3813 BB5D FA84 5EC7 45C6 250E 6F00 984E And now for something completely different.
Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?
On 11/09/2010 02:57 PM, Andreas Schwab wrote: Andi Kleen a...@firstfloor.org writes: @@ -142,7 +142,7 @@ static int i8k_smm(struct smm_regs *regs) lahf\n\t shrl $8,%%eax\n\t andl $1,%%eax\n -:=a(rc) +:=a(rc), =m (*regs) I think this should be +m. Andreas. Just tested Andi's patch with Andreas' suggestion to make it +m, i.e. like attached and can confirm it solves the issue. Thanks guys, Jim Bos --- i8k.c.ORIG 2010-08-02 17:20:46.0 +0200 +++ i8k.c 2010-11-09 17:31:29.0 +0100 @@ -141,7 +141,7 @@ lahf\n\t shrl $8,%%eax\n\t andl $1,%%eax\n - :=a(rc) + :=a(rc), +m (*regs) :a(regs) :%ebx, %ecx, %edx, %esi, %edi, memory); #else @@ -166,7 +166,7 @@ movl %%edx,0(%%eax)\n\t lahf\n\t shrl $8,%%eax\n\t - andl $1,%%eax\n:=a(rc) + andl $1,%%eax\n:=a(rc), +m (*regs) :a(regs) :%ebx, %ecx, %edx, %esi, %edi, memory); #endif
Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?
On Mon, Nov 8, 2010 at 12:03 AM, Andi Kleen a...@firstfloor.org wrote: Andreas Schwab sch...@linux-m68k.org writes: The asm fails to mention that it modifies *regs. It has a memory clobber, that should be enough, no? No. A memory clobber does not cover automatic storage. Btw, I can't see a testcase anywhere so I just assume Andreas got it right as usual. Richard. Besides in any case it cannot be eliminated because it has valid non dead inputs and outputs. -Andi -- a...@linux.intel.com -- Speaking for myself only.
Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?
Richard Guenther richard.guent...@gmail.com writes: On Mon, Nov 8, 2010 at 12:03 AM, Andi Kleen a...@firstfloor.org wrote: Andreas Schwab sch...@linux-m68k.org writes: The asm fails to mention that it modifies *regs. It has a memory clobber, that should be enough, no? No. A memory clobber does not cover automatic storage. That's a separate problem. Btw, I can't see a testcase anywhere so I just assume Andreas got it right as usual. An asm with live inputs and outputs should never be optimized way. If 4.5.1 started doing that it's seriously broken. -Andi -- a...@linux.intel.com -- Speaking for myself only.
Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?
On Mon, Nov 8, 2010 at 12:20 PM, Andi Kleen a...@firstfloor.org wrote: Richard Guenther richard.guent...@gmail.com writes: On Mon, Nov 8, 2010 at 12:03 AM, Andi Kleen a...@firstfloor.org wrote: Andreas Schwab sch...@linux-m68k.org writes: The asm fails to mention that it modifies *regs. It has a memory clobber, that should be enough, no? No. A memory clobber does not cover automatic storage. That's a separate problem. Btw, I can't see a testcase anywhere so I just assume Andreas got it right as usual. An asm with live inputs and outputs should never be optimized way. If 4.5.1 started doing that it's seriously broken. Please provide a testcase, such asms can be optimized if the outputs are dead. Richard. -Andi -- a...@linux.intel.com -- Speaking for myself only.
Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?
On Nov 8, 2010, at 6:20 AM, Richard Guenther wrote: On Mon, Nov 8, 2010 at 12:20 PM, Andi Kleen a...@firstfloor.org wrote: Richard Guenther richard.guent...@gmail.com writes: On Mon, Nov 8, 2010 at 12:03 AM, Andi Kleen a...@firstfloor.org wrote: Andreas Schwab sch...@linux-m68k.org writes: The asm fails to mention that it modifies *regs. It has a memory clobber, that should be enough, no? No. A memory clobber does not cover automatic storage. That's a separate problem. Btw, I can't see a testcase anywhere so I just assume Andreas got it right as usual. An asm with live inputs and outputs should never be optimized way. If 4.5.1 started doing that it's seriously broken. Please provide a testcase, such asms can be optimized if the outputs are dead. I don't know about 4.5, but I noticed that with 4.6 (trunk), testcasese like gcc.c-torture/compile/2804-1.c optimize away the asm and all the operand generation except for -O0. paul
Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?
On Mon, Nov 08, 2010 at 06:47:59AM -0500, Paul Koning wrote: I don't know about 4.5, but I noticed that with 4.6 (trunk), testcasese like gcc.c-torture/compile/2804-1.c optimize away the asm and all the operand generation except for -O0. That's fine, the asm isn't volatile and the output is not used. Jakub
Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?
Hi, On Mon, 8 Nov 2010, Andi Kleen wrote: Richard Guenther richard.guent...@gmail.com writes: On Mon, Nov 8, 2010 at 12:03 AM, Andi Kleen a...@firstfloor.org wrote: Andreas Schwab sch...@linux-m68k.org writes: The asm fails to mention that it modifies *regs. It has a memory clobber, that should be enough, no? No. A memory clobber does not cover automatic storage. That's a separate problem. Btw, I can't see a testcase anywhere so I just assume Andreas got it right as usual. An asm with live inputs and outputs should never be optimized way. If 4.5.1 started doing that it's seriously broken. You know the drill: testcase - gcc.gnu.org/bugzilla/ (In particular up to now it's only speculation in some forum that the asm really is optimized away, which I agree would be a bug, or if it isn't merely that regs-eax isn't reloaded after the asm(), which would be caused by the problem Andreas mentioned) Ciao, Michael.
Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?
On 08/11/2010 11:20, Andi Kleen wrote:
An asm with live inputs and outputs should never be optimized
way. If 4.5.1 started doing that it's seriously broken.
I don't see that. Consider:
void foo (void)
{
int x, y, z;
x = 23;
y = x + 1;
z = y + 1;
}
So far, you'd agree the compiler may optimise the entire function away? So
why not this:
void foo (void)
{
int x, y, z;
x = 23;
asm (do something : =r (y) : r (x) );
z = y + 1;
}
?
cheers,
DaveK
Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?
Jim jim...@xs4all.nl writes: After upgrading my Dell laptop, both OS+kernel the i8k interface was giving nonsensical output. As it turned out it's not the kernel but compiler upgrade which broke this. Guys at Archlinux have found the underlying cause (but don't seem to have submitted a patch yet): https://bbs.archlinux.org/viewtopic.php?pid=780692#p780692 gcc seems to optimize the assembly statements away. And indeed, applying this patch makes the i8k interface work again, i.e. replacing the asm(..) construct by asm volatile(..) The compiler really should not optimize the asm away, because it has both input and output arguments which are later used. asm volatile normally just means don't move significantly I tested it with gcc version 4.5.0 20100604 [gcc-4_5-branch revision 160292] (SUSE Linux) and the asm statement is there for both 32bit and 64bit (with an allmodconfig, with both -O2 and -Os) If gcc 4.5.1 broke that over 4.5.0 you should really file a bug report for the compiler, it seems like a serious regression in 4.5.1 -Andi -- a...@linux.intel.com -- Speaking for myself only.
Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?
Andi Kleen a...@firstfloor.org writes: Jim jim...@xs4all.nl writes: After upgrading my Dell laptop, both OS+kernel the i8k interface was giving nonsensical output. As it turned out it's not the kernel but compiler upgrade which broke this. Guys at Archlinux have found the underlying cause (but don't seem to have submitted a patch yet): https://bbs.archlinux.org/viewtopic.php?pid=780692#p780692 gcc seems to optimize the assembly statements away. And indeed, applying this patch makes the i8k interface work again, i.e. replacing the asm(..) construct by asm volatile(..) The compiler really should not optimize the asm away, because it has both input and output arguments which are later used. asm volatile normally just means don't move significantly The asm fails to mention that it modifies *regs. Andreas. -- Andreas Schwab, sch...@linux-m68k.org GPG Key fingerprint = 58CA 54C7 6D53 942B 1756 01D3 44D5 214B 8276 4ED5 And now for something completely different.
Re: gcc 4.5.1 / as 2.20.51.0.11 miscompiling drivers/char/i8k.c ?
Andreas Schwab sch...@linux-m68k.org writes: The asm fails to mention that it modifies *regs. It has a memory clobber, that should be enough, no? Besides in any case it cannot be eliminated because it has valid non dead inputs and outputs. -Andi -- a...@linux.intel.com -- Speaking for myself only.
