[Bug tree-optimization/40679] Optimizer handles loops with volatiles and post-incr. wrong
--- Comment #8 from bastian dot schick at sciopta dot com 2009-07-08 13:06 --- (In reply to comment #6) > (In reply to comment #2) > > Replacing *tbl++ by tbl[i] gives this ARM code: > > .L2: > > mov r3, #10 > > str r3, [r2], #4 > > cmp r2, #0 > > bne .L2 > > bx lr > > > > See, gcc knows about the wrapping but still the *tbl++ version misses the > > end-condition which is the bug. > > The difference is that in the tbl[i] version there will not be a wraparound at > runtime because &tbl[i] for i == 64 is never computed, while in the *tbl++ > version the iteration with i == 63 will do tbl++ moving tbl from -4U to 0 > before the loop termination test, which triggers undefined behaviour. Ok fine, but why does it generate correct code if not using volatile for the pointer ?! mvn r2, #251 .L2: mov r3, #10 str r3, [r2, #-4] add r2, r2, #4 cmp r2, #4 bne .L2 bx lr Strange, no post-increment code is generated. The 68k version still uses post-increment and voilà, endless-loop. Also see the code for the tbl[i] version, the pointer still wraps. I suspect following: The test for 0 is removed maybe because the post-increment is defined to change flags( which it isn't). Since there is no test, the next optimization changes a "jump not zero" to an "jump always". -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40679
[Bug rtl-optimization/40679] Optimizer handles loops with volatiles and post-incr. wrong
--- Comment #4 from bastian dot schick at sciopta dot com 2009-07-08 09:06 --- (In reply to comment #3) > On trunk with -fno-tree-vrp I see the correct code being generated. It seems to be related to Bug #30785 (test for null pointer). -- http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40679
[Bug rtl-optimization/40679] Optimizer handles loops with volatiles and post-incr. wrong
--- Comment #2 from bastian dot schick at sciopta dot com 2009-07-08 08:24
---
(In reply to comment #1)
>
> Sent from my iPhone
Oh, dude (which one :-)
> On Jul 8, 2009, at 12:32 AM, "bastian dot schick at sciopta dot com"
> > wrote:
>
> > If the following code is compiled with -Os for ARM or ColdFire, the
> > exit
> > condition for the loop is removed.
> > Replacing *tbl++ with tbl[i] or using unsigned long instead of
> > volatile
> > unsigned long does not show the problem.
> > I suspect the post-increment optimization to be the problem, because
> > the
> > PowerPC version does not show the problem.
> > Also: Using a different start-address for tbl, does not show the
> > problem.
>
> It looks more like a wrapping issue. 4*64 = 256. So we go from -256u
> to 0 which causes wrapping of the pointer which is undefined and
If so, it should not be used, but it seems to be a new optimization as 3.4.4
does not use it.
> therefor I think gcc is doing the correct thing (If got my numbers
> correct).
Replacing *tbl++ by tbl[i] gives this ARM code:
.L2:
mov r3, #10
str r3, [r2], #4
cmp r2, #0
bne .L2
bx lr
See, gcc knows about the wrapping but still the *tbl++ version misses the
end-condition which is the bug.
Addenum: x86-gcc 4.3.3 (openSUSE 10.0) has the same problem.
Compiling with -O, gives correct code with *tbl++ :
.L2:
mov r3, #10
str r3, [r2], #4
cmp r2, #0
bne .L2
bx lr
--
bastian dot schick at sciopta dot com changed:
What|Removed |Added
----------------
CC| |bastian dot schick at
||sciopta dot com
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40679
[Bug rtl-optimization/40679] New: Optimizer handles loops with volatiles and post-incr. wrong
If the following code is compiled with -Os for ARM or ColdFire, the exit
condition for the loop is removed.
Replacing *tbl++ with tbl[i] or using unsigned long instead of volatile
unsigned long does not show the problem.
I suspect the post-increment optimization to be the problem, because the
PowerPC version does not show the problem.
Also: Using a different start-address for tbl, does not show the problem.
The problem has been reported also for 4.4.0.
typedef volatile unsigned long __vu32;
void bs()
{
int i;
__vu32 *tbl = (__vu32 *)0xff00;
for(i = 0; i < 64; ++i){
//-> tbl[i] = (__vu32)10;
*tbl++ = (__vu32)10;
}
}
Cmd-line: arm-none-eabi-gcc -S -Os t.c
Output:
.cpu arm7tdmi
.fpu softvfp
.eabi_attribute 20, 1
.eabi_attribute 21, 1
.eabi_attribute 23, 3
.eabi_attribute 24, 1
.eabi_attribute 25, 1
.eabi_attribute 26, 1
.eabi_attribute 30, 4
.eabi_attribute 18, 4
.file "t.c"
.text
.align 2
.global bs
.type bs, %function
bs:
@ Function supports interworking.
@ args = 0, pretend = 0, frame = 0
@ frame_needed = 0, uses_anonymous_args = 0
@ link register save eliminated.
mvn r2, #255
.L2:
mov r3, #10
str r3, [r2], #4
b .L2
.size bs, .-bs
.ident "GCC: (Sourcery G++ Lite 2008q3-39) 4.3.2"
--
Summary: Optimizer handles loops with volatiles and post-incr.
wrong
Product: gcc
Version: 4.3.2
Status: UNCONFIRMED
Severity: major
Priority: P3
Component: rtl-optimization
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: bastian dot schick at sciopta dot com
GCC build triplet: i686-pc-linux-gnu
GCC host triplet: i686-mingw32
GCC target triplet: i686-mingw32
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40679
