[Bug tree-optimization/100363] gcc generating wider load/store than warranted at -O3
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100363 Nick Desaulniers changed: What|Removed |Added CC||ndesaulniers at google dot com --- Comment #20 from Nick Desaulniers --- (In reply to Alexander Monakov from comment #7) > Most likely the issue is that sout/sfrom are misaligned at runtime, while > the > vectorized code somewhere relies on them being sufficiently aligned for > a 'short'. > It is unsafe to dereference a misaligned pointer. The pointed-to-type must > have reduced alignment: C 6.3.2.3p7 (N1548) says: A pointer to an object type may be converted to a pointer to a different object type. If the resulting pointer is not correctly aligned) for the referenced type, the behavior is undefined. === We're working on adding diagnostics and UBSAN checks for these. Perhaps with those in place, we'd be able to spot such a case in the kernel's initramfs decompression code.
[Bug tree-optimization/100363] gcc generating wider load/store than warranted at -O3
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100363 --- Comment #19 from rguenther at suse dot de --- On Tue, 4 May 2021, vgupta at synopsys dot com wrote: > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100363 > > --- Comment #18 from Vineet Gupta --- > (In reply to Richard Biener from comment #9) > > (In reply to Linus Torvalds from comment #8) > > > (In reply to Alexander Monakov from comment #7) > > > > > > > > Most likely the issue is that sout/sfrom are misaligned at runtime, > > > > while > > > > the vectorized code somewhere relies on them being sufficiently aligned > > > > for > > > > a 'short'. > > > > > > They absolutely are. > > > > > > And we build the kernel with -Wno-strict-aliasing exactly to make sure the > > > compiler doesn't think that "oh, I can make aliasing decisions based on > > > type > > > information". > > > > > > Because we have those kinds of issues all over, and we know which > > > architectures support unaligned loads etc, and all the tricks with > > > "memcpy()" and unions make for entirely unreadable code. > > > > > > So please fix the aliasing logic to not be type-based when people > > > explicitly > > > tell you not to do that. > > > > > > Linus > > > > Note alignment has nothing to do with strict-aliasing (-fno-strict-aliasing > > you mean btw). > > > > One thing we do is (I'm not 50% sure this explains the observed issue) > > assume > > that if you have two accesses with type 'short' and they are aligned > > according to this type then they will not partly overlap. Note this has > > nothing to do with C strict aliasing rules but is basic pointer math when > > you know lower zero bits. > > OK, given that source code has type short, they will assume these things are > short aligned and thus won't overlap for short accesses. But then the code > actually generated by loop vectorizer assumes they are 8 bytes apart - since > that is what it is generating. That's guarded by a runtime check but this check again assumes the accesses are aligned as short and thus will fail if not
[Bug tree-optimization/100363] gcc generating wider load/store than warranted at -O3
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100363 --- Comment #18 from Vineet Gupta --- (In reply to Richard Biener from comment #9) > (In reply to Linus Torvalds from comment #8) > > (In reply to Alexander Monakov from comment #7) > > > > > > Most likely the issue is that sout/sfrom are misaligned at runtime, while > > > the vectorized code somewhere relies on them being sufficiently aligned > > > for > > > a 'short'. > > > > They absolutely are. > > > > And we build the kernel with -Wno-strict-aliasing exactly to make sure the > > compiler doesn't think that "oh, I can make aliasing decisions based on type > > information". > > > > Because we have those kinds of issues all over, and we know which > > architectures support unaligned loads etc, and all the tricks with > > "memcpy()" and unions make for entirely unreadable code. > > > > So please fix the aliasing logic to not be type-based when people explicitly > > tell you not to do that. > > > > Linus > > Note alignment has nothing to do with strict-aliasing (-fno-strict-aliasing > you mean btw). > > One thing we do is (I'm not 50% sure this explains the observed issue) assume > that if you have two accesses with type 'short' and they are aligned > according to this type then they will not partly overlap. Note this has > nothing to do with C strict aliasing rules but is basic pointer math when > you know lower zero bits. OK, given that source code has type short, they will assume these things are short aligned and thus won't overlap for short accesses. But then the code actually generated by loop vectorizer assumes they are 8 bytes apart - since that is what it is generating. > > I suggest to try the fix suggested in comment#7 and report back if that > fixes the observed issue.
[Bug tree-optimization/100363] gcc generating wider load/store than warranted at -O3
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100363 Richard Biener changed: What|Removed |Added Status|UNCONFIRMED |RESOLVED Resolution|--- |INVALID --- Comment #17 from Richard Biener --- Not a bug.
[Bug tree-optimization/100363] gcc generating wider load/store than warranted at -O3
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100363 --- Comment #16 from Andrew Pinski --- (In reply to Vineet Gupta from comment #15) > The problem is is indeed gone. I need to analyze the assembly fully how it > prevents the bad case. e.g. I'm still not comfortable seeing the loop > entered with following and it doing 8 byte ldd/std when we know it should > only do 2 at a time. Why? It is called a "vectorization" optimization. Where we are vectorizing the 2 byte load/stores into a 4x2 vector load/stores.
[Bug tree-optimization/100363] gcc generating wider load/store than warranted at -O3
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100363 --- Comment #15 from Vineet Gupta --- (In reply to Linus Torvalds from comment #14) > (In reply to Vineet Gupta from comment #13) > > Sorry the workaround proposed by Alexander doesn't seem to cure it (patch > > attached), outcome is the same > > Vineet - it's not the ldd/std that is necessarily buggy, it's the earlier > tests of the address that guard that vectorized path. > > So your quoted parts of the code generation aren't necessarily the > problematic ones. /me slaps myself. How can I be so stupid. > Did you actually test the code and check whether it has the same issue? > Maybe it changed the address limit guards before that ldd/std? The problem is is indeed gone. I need to analyze the assembly fully how it prevents the bad case. e.g. I'm still not comfortable seeing the loop entered with following and it doing 8 byte ldd/std when we know it should only do 2 at a time. r21 = 0xbf178036 (pre-increment so 0x3e will be first src) r22 = 0xbf1780b2 LPC = 4 80d9a360: lp 12 ;80d9a36c 80d9a364: ldd.a r18r19,[r21,8] 80d9a368: std.ab r18r19,[r22,8] > I also sent you a separate patch to test if just upgrading to a newer > version of the zlib code helps. Although that may be buggy for other > reasons, it's not like I actually tested the end result.. But it would be > interesting to hear if that one works for you (again, ldd/std might be a > valid end result of trying to vectorize that code assuming the aliasing > tests are done correctly in the vectorized loop headers). Thx for that. And this seems to boot as well.
[Bug tree-optimization/100363] gcc generating wider load/store than warranted at -O3
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100363 --- Comment #14 from Linus Torvalds --- (In reply to Vineet Gupta from comment #13) > Sorry the workaround proposed by Alexander doesn't seem to cure it (patch > attached), outcome is the same Vineet - it's not the ldd/std that is necessarily buggy, it's the earlier tests of the address that guard that vectorized path. So your quoted parts of the code generation aren't necessarily the problematic ones. Did you actually test the code and check whether it has the same issue? Maybe it changed the address limit guards before that ldd/std? I also sent you a separate patch to test if just upgrading to a newer version of the zlib code helps. Although that may be buggy for other reasons, it's not like I actually tested the end result.. But it would be interesting to hear if that one works for you (again, ldd/std might be a valid end result of trying to vectorize that code assuming the aliasing tests are done correctly in the vectorized loop headers).
[Bug tree-optimization/100363] gcc generating wider load/store than warranted at -O3
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100363 --- Comment #13 from Vineet Gupta --- Sorry the workaround proposed by Alexander doesn't seem to cure it (patch attached), outcome is the same mov lp_count,r13;5 #, bnd.65 lp @.L201 ; lp_count:@.L50->@.L201#, .align 2 .L50: # ../lib/zlib_inflate/inffast.c:288: PUP(sout) = PUP(sfrom); ldd.a r18,[r21,8] # MEM[base: _496, offset: 0B], MEM[base: _496, offset: 0B] # ../lib/zlib_inflate/inffast.c:288: PUP(sout) = PUP(sfrom); std.ab r18,[r22,8] # MEM[base: vectp_prephitmp.73_741, offset: 0B], MEM[base: _496, offset: 0B] .align 2 .L201: ; ZOL_END, begins @.L50 #
[Bug tree-optimization/100363] gcc generating wider load/store than warranted at -O3
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100363 --- Comment #12 from Vineet Gupta --- Created attachment 50742 --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=50742&action=edit kernel patch as proposed on comment #7
[Bug tree-optimization/100363] gcc generating wider load/store than warranted at -O3
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100363 --- Comment #11 from Linus Torvalds --- (In reply to Linus Torvalds from comment #10) > > This particular code comes > from some old version of zlib, and I can't test because I don't have the ARC > background to make any sense of the generated code. Heh. We upgraded to a "recent version" of zlib back in 2006: "Upgrade the zlib_inflate implementation in the kernel from a patched version 1.1.3/4 to a patched 1.2.3" but it turns out that the "do things a 16-bit word at a time" was a kernel-local optimization for some very slow old PowerPC microcontroller. The code in upstream zlib actually looks rather better (which is not saying much, admittedly), doesn't have any 16-bit accesses, and we probably should just try to synchronize with that instead.
[Bug tree-optimization/100363] gcc generating wider load/store than warranted at -O3
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100363 --- Comment #10 from Linus Torvalds --- (In reply to Richard Biener from comment #9) > > Note alignment has nothing to do with strict-aliasing (-fno-strict-aliasing > you mean btw). I obviously meant -fno-strict-aliasing, yes. But I think it's actually essentially the same issue, just in a different guise: > One thing we do is (I'm not 50% sure this explains the observed issue) assume > that if you have two accesses with type 'short' and they are aligned > according to this type then they will not partly overlap. Note this has > nothing to do with C strict aliasing rules but is basic pointer math when > you know lower zero bits. Well, the thing is, you have two situations: (a) you can statically see that the two do not alias, because the offset arithmetic is either constant or you have some range logic that can tell that they are sufficiently far apart. (b) you can't. Now, everybody is ok with the static aliasing situation in (a). If you can tell that two addresses don't alias, your'e done, they are independent, there's no question about it. But that's not the situation here. So we're in (b). And what I find personally so annoying is that gcc has actually *done* that distance check, but apparently intentionally done it badly based on type information. And the reason I think this is similar to -fno-strict-aliasing is that it's that same (b) case, and it looks like a very similar "do a bad job of doing actual run-time alias analysis based on type information". It seems to be literally an off-by-one error, not because it generates better code, but because the compiler has decided to pointlessly make a bad range comparison based on type. But I've never worked with the gcc IR dumps, so Andrew Pinski's debug output in #c5 doesn't actually make me go "ahh, there". Maybe it's that 8 vs 6 that he pointed out. Did somebody notice that "offset > 8" was off-by-one, and should have been "offset >= 8"? And then changed it to "offset > 6" which is off-by-one in the other direction instead? > I suggest to try the fix suggested in comment#7 and report back if that > fixes the observed issue. Vineet? I still think gcc is doing the wrong thing, exactly because of that "pointlessly using the wrong range check" issue. This particular code comes from some old version of zlib, and I can't test because I don't have the ARC background to make any sense of the generated code.
[Bug tree-optimization/100363] gcc generating wider load/store than warranted at -O3
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100363 Richard Biener changed: What|Removed |Added CC||rguenth at gcc dot gnu.org --- Comment #9 from Richard Biener --- (In reply to Linus Torvalds from comment #8) > (In reply to Alexander Monakov from comment #7) > > > > Most likely the issue is that sout/sfrom are misaligned at runtime, while > > the vectorized code somewhere relies on them being sufficiently aligned for > > a 'short'. > > They absolutely are. > > And we build the kernel with -Wno-strict-aliasing exactly to make sure the > compiler doesn't think that "oh, I can make aliasing decisions based on type > information". > > Because we have those kinds of issues all over, and we know which > architectures support unaligned loads etc, and all the tricks with > "memcpy()" and unions make for entirely unreadable code. > > So please fix the aliasing logic to not be type-based when people explicitly > tell you not to do that. > > Linus Note alignment has nothing to do with strict-aliasing (-fno-strict-aliasing you mean btw). One thing we do is (I'm not 50% sure this explains the observed issue) assume that if you have two accesses with type 'short' and they are aligned according to this type then they will not partly overlap. Note this has nothing to do with C strict aliasing rules but is basic pointer math when you know lower zero bits. I suggest to try the fix suggested in comment#7 and report back if that fixes the observed issue.
[Bug tree-optimization/100363] gcc generating wider load/store than warranted at -O3
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100363 --- Comment #8 from Linus Torvalds --- (In reply to Alexander Monakov from comment #7) > > Most likely the issue is that sout/sfrom are misaligned at runtime, while > the vectorized code somewhere relies on them being sufficiently aligned for > a 'short'. They absolutely are. And we build the kernel with -Wno-strict-aliasing exactly to make sure the compiler doesn't think that "oh, I can make aliasing decisions based on type information". Because we have those kinds of issues all over, and we know which architectures support unaligned loads etc, and all the tricks with "memcpy()" and unions make for entirely unreadable code. So please fix the aliasing logic to not be type-based when people explicitly tell you not to do that. Linus
[Bug tree-optimization/100363] gcc generating wider load/store than warranted at -O3
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100363 Alexander Monakov changed: What|Removed |Added CC||amonakov at gcc dot gnu.org --- Comment #7 from Alexander Monakov --- The github issue has a more relevant code quote: #ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS <-- this is enabled for ARCv2 279:PUP(sout) = PUP(sfrom); #else PUP(sout) = UP_UNALIGNED(sfrom); #endif Most likely the issue is that sout/sfrom are misaligned at runtime, while the vectorized code somewhere relies on them being sufficiently aligned for a 'short'. It is unsafe to dereference a misaligned pointer. The pointed-to-type must have reduced alignment: typedef unsigned short u16_u __attribute__((aligned(1))); u16_u *sout = ... u16_u *sfrom = (void *)(from - OFF); (without -ffreestanding, memcpy/memmove is a portable way to express a misaligned access) https://trust-in-soft.com/blog/2020/04/06/gcc-always-assumes-aligned-pointer-accesses/
[Bug tree-optimization/100363] gcc generating wider load/store than warranted at -O3
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100363 --- Comment #6 from Vineet Gupta --- (In reply to Linus Torvalds from comment #4) > (In reply to Andrew Pinski from comment #1) > > The loop gets vectorized, I don't see the problem really. > > > See > > > https://github.com/foss-for-synopsys-dwc-arc-processors/toolchain/issues/372 > > and in particular the comment > >"In the first 8-byte copy, src and dst overlap" > > so apparently gcc has decided that they can't overlap, despite the two > pointers being literally generated from the same base pointer. Exactly: > But I don't real arc assembly, so I'll have to take Vineet's word for it. fwiw: LDD.a [base, off] is 8-byte load with pre-incr : eff addr = base + offset STD.ab [base, off] is 8-byte store with post-incr: eff addr = base > Vineet, have you been able to generate a smaller test-case? No I'm afraid not.
[Bug tree-optimization/100363] gcc generating wider load/store than warranted at -O3
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100363 Andrew Pinski changed: What|Removed |Added Status|WAITING |UNCONFIRMED Ever confirmed|1 |0
[Bug tree-optimization/100363] gcc generating wider load/store than warranted at -O3
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100363 Andrew Pinski changed: What|Removed |Added Component|middle-end |tree-optimization Keywords||wrong-code --- Comment #5 from Andrew Pinski --- On the trunk, on aarch64: There should be an aliasing check sfrom_289 = from_176 + 18446744073709551615; _871 = _843 + 18446744073709551615; _872 = _871 > 6; _873 = prephitmp_803 + 2; _874 = from_176 + 3; _875 = _873 - _874; _876 = (sizetype) _875; _877 = _876 > 12; _878 = _872 & _877; if (_878 != 0) goto ; [80.00%] else goto ; [20.00%] _873 is the sout In GCC 10 branch we get something similar: sfrom_289 = from_176 + 18446744073709551615; _859 = _823 + 18446744073709551615; _860 = _859 > 8; _861 = prephitmp_783 + 2; _862 = from_176 + 3; _863 = _861 - _862; _864 = (sizetype) _863; _865 = _864 > 12; _866 = _860 & _865; if (_866 != 0) goto ; [80.00%] else goto ; [20.00%] But I Notice 8 vs 6 here.
