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* [Bug rtl-optimization/40679] New: Optimizer handles loops with volatiles and post-incr. wrong
@ 2009-07-08 7:33 bastian dot schick at sciopta dot com
2009-07-08 8:13 ` Andrew Pinski
` (9 more replies)
0 siblings, 10 replies; 11+ messages in thread
From: bastian dot schick at sciopta dot com @ 2009-07-08 7:33 UTC (permalink / raw)
To: gcc-bugs
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 *)0xffffff00;
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
^ permalink raw reply [flat|nested] 11+ messages in thread
* Re: [Bug rtl-optimization/40679] New: Optimizer handles loops with volatiles and post-incr. wrong
2009-07-08 7:33 [Bug rtl-optimization/40679] New: Optimizer handles loops with volatiles and post-incr. wrong bastian dot schick at sciopta dot com
@ 2009-07-08 8:13 ` Andrew Pinski
2009-07-08 8:14 ` [Bug rtl-optimization/40679] " pinskia at gmail dot com
` (8 subsequent siblings)
9 siblings, 0 replies; 11+ messages in thread
From: Andrew Pinski @ 2009-07-08 8:13 UTC (permalink / raw)
To: gcc-bugzilla; +Cc: gcc-bugs
Sent from my iPhone
On Jul 8, 2009, at 12:32 AM, "bastian dot schick at sciopta dot com" <gcc-bugzilla@gcc.gnu.org
> 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
therefor I think gcc is doing the correct thing (If got my numbers
correct).
>
> The problem has been reported also for 4.4.0.
>
> typedef volatile unsigned long __vu32;
> void bs()
> {
> int i;
> __vu32 *tbl = (__vu32 *)0xffffff00;
> 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
>
^ permalink raw reply [flat|nested] 11+ messages in thread
* [Bug rtl-optimization/40679] Optimizer handles loops with volatiles and post-incr. wrong
2009-07-08 7:33 [Bug rtl-optimization/40679] New: Optimizer handles loops with volatiles and post-incr. wrong bastian dot schick at sciopta dot com
2009-07-08 8:13 ` Andrew Pinski
@ 2009-07-08 8:14 ` pinskia at gmail dot com
2009-07-08 8:24 ` bastian dot schick at sciopta dot com
` (7 subsequent siblings)
9 siblings, 0 replies; 11+ messages in thread
From: pinskia at gmail dot com @ 2009-07-08 8:14 UTC (permalink / raw)
To: gcc-bugs
------- Comment #1 from pinskia at gmail dot com 2009-07-08 08:13 -------
Subject: Re: New: Optimizer handles loops with volatiles and post-incr. wrong
Sent from my iPhone
On Jul 8, 2009, at 12:32 AM, "bastian dot schick at sciopta dot com"
<gcc-bugzilla@gcc.gnu.org
> 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
therefor I think gcc is doing the correct thing (If got my numbers
correct).
>
> The problem has been reported also for 4.4.0.
>
> typedef volatile unsigned long __vu32;
> void bs()
> {
> int i;
> __vu32 *tbl = (__vu32 *)0xffffff00;
> 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
>
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40679
^ permalink raw reply [flat|nested] 11+ messages in thread
* [Bug rtl-optimization/40679] Optimizer handles loops with volatiles and post-incr. wrong
2009-07-08 7:33 [Bug rtl-optimization/40679] New: Optimizer handles loops with volatiles and post-incr. wrong bastian dot schick at sciopta dot com
2009-07-08 8:13 ` Andrew Pinski
2009-07-08 8:14 ` [Bug rtl-optimization/40679] " pinskia at gmail dot com
@ 2009-07-08 8:24 ` bastian dot schick at sciopta dot com
2009-07-08 8:49 ` ramana at gcc dot gnu dot org
` (6 subsequent siblings)
9 siblings, 0 replies; 11+ messages in thread
From: bastian dot schick at sciopta dot com @ 2009-07-08 8:24 UTC (permalink / raw)
To: gcc-bugs
------- 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"
> <gcc-bugzilla@gcc.gnu.org
> > 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
^ permalink raw reply [flat|nested] 11+ messages in thread
* [Bug rtl-optimization/40679] Optimizer handles loops with volatiles and post-incr. wrong
2009-07-08 7:33 [Bug rtl-optimization/40679] New: Optimizer handles loops with volatiles and post-incr. wrong bastian dot schick at sciopta dot com
` (2 preceding siblings ...)
2009-07-08 8:24 ` bastian dot schick at sciopta dot com
@ 2009-07-08 8:49 ` ramana at gcc dot gnu dot org
2009-07-08 9:07 ` bastian dot schick at sciopta dot com
` (5 subsequent siblings)
9 siblings, 0 replies; 11+ messages in thread
From: ramana at gcc dot gnu dot org @ 2009-07-08 8:49 UTC (permalink / raw)
To: gcc-bugs
------- Comment #3 from ramana at gcc dot gnu dot org 2009-07-08 08:49 -------
On trunk with -fno-tree-vrp I see the correct code being generated.
bs:
@ Function supports interworking.
@ args = 0, pretend = 0, frame = 0
@ frame_needed = 0, uses_anonymous_args = 0
@ link register save eliminated.
mvn r3, #255
mov r2, #10
.L2:
str r2, [r3], #4
cmp r3, #0
bne .L2
bx lr
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40679
^ permalink raw reply [flat|nested] 11+ messages in thread
* [Bug rtl-optimization/40679] Optimizer handles loops with volatiles and post-incr. wrong
2009-07-08 7:33 [Bug rtl-optimization/40679] New: Optimizer handles loops with volatiles and post-incr. wrong bastian dot schick at sciopta dot com
` (3 preceding siblings ...)
2009-07-08 8:49 ` ramana at gcc dot gnu dot org
@ 2009-07-08 9:07 ` bastian dot schick at sciopta dot com
2009-07-08 9:13 ` [Bug tree-optimization/40679] " ramana at gcc dot gnu dot org
` (4 subsequent siblings)
9 siblings, 0 replies; 11+ messages in thread
From: bastian dot schick at sciopta dot com @ 2009-07-08 9:07 UTC (permalink / raw)
To: gcc-bugs
------- 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
^ permalink raw reply [flat|nested] 11+ messages in thread
* [Bug tree-optimization/40679] Optimizer handles loops with volatiles and post-incr. wrong
2009-07-08 7:33 [Bug rtl-optimization/40679] New: Optimizer handles loops with volatiles and post-incr. wrong bastian dot schick at sciopta dot com
` (4 preceding siblings ...)
2009-07-08 9:07 ` bastian dot schick at sciopta dot com
@ 2009-07-08 9:13 ` ramana at gcc dot gnu dot org
2009-07-08 9:59 ` mikpe at it dot uu dot se
` (3 subsequent siblings)
9 siblings, 0 replies; 11+ messages in thread
From: ramana at gcc dot gnu dot org @ 2009-07-08 9:13 UTC (permalink / raw)
To: gcc-bugs
------- Comment #5 from ramana at gcc dot gnu dot org 2009-07-08 09:12 -------
Richi,
Can you comment on this one ?
Ramana
--
ramana at gcc dot gnu dot org changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |rguenth at gcc dot gnu dot
| |org
Component|rtl-optimization |tree-optimization
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40679
^ permalink raw reply [flat|nested] 11+ messages in thread
* [Bug tree-optimization/40679] Optimizer handles loops with volatiles and post-incr. wrong
2009-07-08 7:33 [Bug rtl-optimization/40679] New: Optimizer handles loops with volatiles and post-incr. wrong bastian dot schick at sciopta dot com
` (5 preceding siblings ...)
2009-07-08 9:13 ` [Bug tree-optimization/40679] " ramana at gcc dot gnu dot org
@ 2009-07-08 9:59 ` mikpe at it dot uu dot se
2009-07-08 10:10 ` rguenth at gcc dot gnu dot org
` (2 subsequent siblings)
9 siblings, 0 replies; 11+ messages in thread
From: mikpe at it dot uu dot se @ 2009-07-08 9:59 UTC (permalink / raw)
To: gcc-bugs
------- Comment #6 from mikpe at it dot uu dot se 2009-07-08 09:59 -------
(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.
And the issue is not ARM, the same infinite loop occurs for e.g. target i686.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40679
^ permalink raw reply [flat|nested] 11+ messages in thread
* [Bug tree-optimization/40679] Optimizer handles loops with volatiles and post-incr. wrong
2009-07-08 7:33 [Bug rtl-optimization/40679] New: Optimizer handles loops with volatiles and post-incr. wrong bastian dot schick at sciopta dot com
` (6 preceding siblings ...)
2009-07-08 9:59 ` mikpe at it dot uu dot se
@ 2009-07-08 10:10 ` rguenth at gcc dot gnu dot org
2009-07-08 13:06 ` bastian dot schick at sciopta dot com
2009-07-08 13:11 ` rguenth at gcc dot gnu dot org
9 siblings, 0 replies; 11+ messages in thread
From: rguenth at gcc dot gnu dot org @ 2009-07-08 10:10 UTC (permalink / raw)
To: gcc-bugs
------- Comment #7 from rguenth at gcc dot gnu dot org 2009-07-08 10:10 -------
Indeed the overflow invokes undefined behavior.
--
rguenth at gcc dot gnu dot org changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |RESOLVED
Resolution| |INVALID
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40679
^ permalink raw reply [flat|nested] 11+ messages in thread
* [Bug tree-optimization/40679] Optimizer handles loops with volatiles and post-incr. wrong
2009-07-08 7:33 [Bug rtl-optimization/40679] New: Optimizer handles loops with volatiles and post-incr. wrong bastian dot schick at sciopta dot com
` (7 preceding siblings ...)
2009-07-08 10:10 ` rguenth at gcc dot gnu dot org
@ 2009-07-08 13:06 ` bastian dot schick at sciopta dot com
2009-07-08 13:11 ` rguenth at gcc dot gnu dot org
9 siblings, 0 replies; 11+ messages in thread
From: bastian dot schick at sciopta dot com @ 2009-07-08 13:06 UTC (permalink / raw)
To: gcc-bugs
[-- Warning: decoded text below may be mangled, UTF-8 assumed --]
[-- Attachment #1: Type: text/plain, Size: 1488 bytes --]
------- 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
^ permalink raw reply [flat|nested] 11+ messages in thread
* [Bug tree-optimization/40679] Optimizer handles loops with volatiles and post-incr. wrong
2009-07-08 7:33 [Bug rtl-optimization/40679] New: Optimizer handles loops with volatiles and post-incr. wrong bastian dot schick at sciopta dot com
` (8 preceding siblings ...)
2009-07-08 13:06 ` bastian dot schick at sciopta dot com
@ 2009-07-08 13:11 ` rguenth at gcc dot gnu dot org
9 siblings, 0 replies; 11+ messages in thread
From: rguenth at gcc dot gnu dot org @ 2009-07-08 13:11 UTC (permalink / raw)
To: gcc-bugs
------- Comment #9 from rguenth at gcc dot gnu dot org 2009-07-08 13:11 -------
induction variable optimization is different w/o volatile.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40679
^ permalink raw reply [flat|nested] 11+ messages in thread
end of thread, other threads:[~2009-07-08 13:11 UTC | newest]
Thread overview: 11+ messages (download: mbox.gz / follow: Atom feed)
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2009-07-08 7:33 [Bug rtl-optimization/40679] New: Optimizer handles loops with volatiles and post-incr. wrong bastian dot schick at sciopta dot com
2009-07-08 8:13 ` Andrew Pinski
2009-07-08 8:14 ` [Bug rtl-optimization/40679] " pinskia at gmail dot com
2009-07-08 8:24 ` bastian dot schick at sciopta dot com
2009-07-08 8:49 ` ramana at gcc dot gnu dot org
2009-07-08 9:07 ` bastian dot schick at sciopta dot com
2009-07-08 9:13 ` [Bug tree-optimization/40679] " ramana at gcc dot gnu dot org
2009-07-08 9:59 ` mikpe at it dot uu dot se
2009-07-08 10:10 ` rguenth at gcc dot gnu dot org
2009-07-08 13:06 ` bastian dot schick at sciopta dot com
2009-07-08 13:11 ` rguenth at gcc dot gnu dot org
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