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* [Bug tree-optimization/40844] New: O2 optimizes out assignment to bitfield
@ 2009-07-24 5:22 jim at bodwin dot us
2009-07-24 5:54 ` Andrew Pinski
` (2 more replies)
0 siblings, 3 replies; 4+ messages in thread
From: jim at bodwin dot us @ 2009-07-24 5:22 UTC (permalink / raw)
To: gcc-bugs
Incorrect code is produced for the following source with the O2 option. In
particular, the assignment to the bitfield field2 is optimized out of the code
entirely and regImage is left all zero. Correct code is produced with the O1
option and (at least) with gcc version 4.3.2.
==================== Source Code
typedef struct hwreg {
short field1 : 4;
short field2 : 2;
short field3 : 10;
} hwreg_t;
void foo(volatile short *hwPtr, short f2) {
hwreg_t regImage; // Place to build reg image bit-by-bit
*(short *) ®Image = 0;
regImage.field2 = f2;
*hwPtr = *((short *) ®Image);
}
============================ Object dump
/tmp/foo.o: file format elf32-littlearm
Disassembly of section .text:
00000000 <foo>:
0: e3a03000 mov r3, #0 ; 0x0
4: e24dd004 sub sp, sp, #4 ; 0x4
8: e1c030b0 strh r3, [r0]
c: e28dd004 add sp, sp, #4 ; 0x4
10: e12fff1e bx lr
============================ gcc -v output
Configured with: ../gcc-4.4.1/configure --target=arm-elf --disable-__cxa_atexit
--with-newlib --enable-languages=c,c++ --with-gnu-as --with-gnu-ld
--with-gxx-include-dir=/home/jmb/arm-elf/include --prefix=/home/jmb/arm-elf :
(reconfigured) ../gcc-4.4.1/configure --target=arm-elf --disable-__cxa_atexit
--with-newlib --enable-languages=c,c++ --with-gnu-as --with-gnu-ld
--with-gxx-include-dir=/home/jmb/arm-elf/include --prefix=/home/jmb/arm-elf
Thread model: single
gcc version 4.4.1 (GCC)
--
Summary: O2 optimizes out assignment to bitfield
Product: gcc
Version: 4.4.1
Status: UNCONFIRMED
Severity: critical
Priority: P3
Component: tree-optimization
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: jim at bodwin dot us
GCC build triplet: i486-pc-linux-gnu
GCC host triplet: i486-pc-linux-gnu
GCC target triplet: arm-elf
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40844
^ permalink raw reply [flat|nested] 4+ messages in thread* Re: [Bug tree-optimization/40844] New: O2 optimizes out assignment to bitfield
2009-07-24 5:22 [Bug tree-optimization/40844] New: O2 optimizes out assignment to bitfield jim at bodwin dot us
@ 2009-07-24 5:54 ` Andrew Pinski
2009-07-24 5:54 ` [Bug tree-optimization/40844] " pinskia at gmail dot com
2009-07-24 12:26 ` rguenth at gcc dot gnu dot org
2 siblings, 0 replies; 4+ messages in thread
From: Andrew Pinski @ 2009-07-24 5:54 UTC (permalink / raw)
To: gcc-bugzilla; +Cc: gcc-bugs
Sent from my iPhone
On Jul 23, 2009, at 10:22 PM, "jim at bodwin dot us" <gcc-bugzilla@gcc.gnu.org
> wrote:
> Incorrect code is produced for the following source with the O2
> option. In
> particular, the assignment to the bitfield field2 is optimized out
> of the code
> entirely and regImage is left all zero. Correct code is produced
> with the O1
> option and (at least) with gcc version 4.3.2.
>
> ==================== Source Code
>
> typedef struct hwreg {
> short field1 : 4;
> short field2 : 2;
> short field3 : 10;
> } hwreg_t;
>
> void foo(volatile short *hwPtr, short f2) {
> hwreg_t regImage; // Place to build reg image bit-by-bit
> *(short *) ®Image = 0;
>
> regImage.field2 = f2;
>
> *hwPtr = *((short *) ®Image);
> }
I think you are violating c/c++ aliasing rules here.
>
> ============================ Object dump
>
>
> /tmp/foo.o: file format elf32-littlearm
>
>
> Disassembly of section .text:
>
> 00000000 <foo>:
> 0: e3a03000 mov r3, #0 ; 0x0
> 4: e24dd004 sub sp, sp, #4 ; 0x4
> 8: e1c030b0 strh r3, [r0]
> c: e28dd004 add sp, sp, #4 ; 0x4
> 10: e12fff1e bx lr
>
> ============================ gcc -v output
>
> Configured with: ../gcc-4.4.1/configure --target=arm-elf --disable-
> __cxa_atexit
> --with-newlib --enable-languages=c,c++ --with-gnu-as --with-gnu-ld
> --with-gxx-include-dir=/home/jmb/arm-elf/include --prefix=/home/jmb/
> arm-elf :
> (reconfigured) ../gcc-4.4.1/configure --target=arm-elf --disable-
> __cxa_atexit
> --with-newlib --enable-languages=c,c++ --with-gnu-as --with-gnu-ld
> --with-gxx-include-dir=/home/jmb/arm-elf/include --prefix=/home/jmb/
> arm-elf
> Thread model: single
> gcc version 4.4.1 (GCC)
>
>
> --
> Summary: O2 optimizes out assignment to bitfield
> Product: gcc
> Version: 4.4.1
> Status: UNCONFIRMED
> Severity: critical
> Priority: P3
> Component: tree-optimization
> AssignedTo: unassigned at gcc dot gnu dot org
> ReportedBy: jim at bodwin dot us
> GCC build triplet: i486-pc-linux-gnu
> GCC host triplet: i486-pc-linux-gnu
> GCC target triplet: arm-elf
>
>
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40844
>
^ permalink raw reply [flat|nested] 4+ messages in thread* [Bug tree-optimization/40844] O2 optimizes out assignment to bitfield
2009-07-24 5:22 [Bug tree-optimization/40844] New: O2 optimizes out assignment to bitfield jim at bodwin dot us
2009-07-24 5:54 ` Andrew Pinski
@ 2009-07-24 5:54 ` pinskia at gmail dot com
2009-07-24 12:26 ` rguenth at gcc dot gnu dot org
2 siblings, 0 replies; 4+ messages in thread
From: pinskia at gmail dot com @ 2009-07-24 5:54 UTC (permalink / raw)
To: gcc-bugs
------- Comment #1 from pinskia at gmail dot com 2009-07-24 05:54 -------
Subject: Re: New: O2 optimizes out assignment to bitfield
Sent from my iPhone
On Jul 23, 2009, at 10:22 PM, "jim at bodwin dot us" <gcc-bugzilla@gcc.gnu.org
> wrote:
> Incorrect code is produced for the following source with the O2
> option. In
> particular, the assignment to the bitfield field2 is optimized out
> of the code
> entirely and regImage is left all zero. Correct code is produced
> with the O1
> option and (at least) with gcc version 4.3.2.
>
> ==================== Source Code
>
> typedef struct hwreg {
> short field1 : 4;
> short field2 : 2;
> short field3 : 10;
> } hwreg_t;
>
> void foo(volatile short *hwPtr, short f2) {
> hwreg_t regImage; // Place to build reg image bit-by-bit
> *(short *) ®Image = 0;
>
> regImage.field2 = f2;
>
> *hwPtr = *((short *) ®Image);
> }
I think you are violating c/c++ aliasing rules here.
>
> ============================ Object dump
>
>
> /tmp/foo.o: file format elf32-littlearm
>
>
> Disassembly of section .text:
>
> 00000000 <foo>:
> 0: e3a03000 mov r3, #0 ; 0x0
> 4: e24dd004 sub sp, sp, #4 ; 0x4
> 8: e1c030b0 strh r3, [r0]
> c: e28dd004 add sp, sp, #4 ; 0x4
> 10: e12fff1e bx lr
>
> ============================ gcc -v output
>
> Configured with: ../gcc-4.4.1/configure --target=arm-elf --disable-
> __cxa_atexit
> --with-newlib --enable-languages=c,c++ --with-gnu-as --with-gnu-ld
> --with-gxx-include-dir=/home/jmb/arm-elf/include --prefix=/home/jmb/
> arm-elf :
> (reconfigured) ../gcc-4.4.1/configure --target=arm-elf --disable-
> __cxa_atexit
> --with-newlib --enable-languages=c,c++ --with-gnu-as --with-gnu-ld
> --with-gxx-include-dir=/home/jmb/arm-elf/include --prefix=/home/jmb/
> arm-elf
> Thread model: single
> gcc version 4.4.1 (GCC)
>
>
> --
> Summary: O2 optimizes out assignment to bitfield
> Product: gcc
> Version: 4.4.1
> Status: UNCONFIRMED
> Severity: critical
> Priority: P3
> Component: tree-optimization
> AssignedTo: unassigned at gcc dot gnu dot org
> ReportedBy: jim at bodwin dot us
> GCC build triplet: i486-pc-linux-gnu
> GCC host triplet: i486-pc-linux-gnu
> GCC target triplet: arm-elf
>
>
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40844
>
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40844
^ permalink raw reply [flat|nested] 4+ messages in thread* [Bug tree-optimization/40844] O2 optimizes out assignment to bitfield
2009-07-24 5:22 [Bug tree-optimization/40844] New: O2 optimizes out assignment to bitfield jim at bodwin dot us
2009-07-24 5:54 ` Andrew Pinski
2009-07-24 5:54 ` [Bug tree-optimization/40844] " pinskia at gmail dot com
@ 2009-07-24 12:26 ` rguenth at gcc dot gnu dot org
2 siblings, 0 replies; 4+ messages in thread
From: rguenth at gcc dot gnu dot org @ 2009-07-24 12:26 UTC (permalink / raw)
To: gcc-bugs
------- Comment #2 from rguenth at gcc dot gnu dot org 2009-07-24 12:26 -------
indeed.
--
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=40844
^ permalink raw reply [flat|nested] 4+ messages in thread
end of thread, other threads:[~2009-07-24 12:26 UTC | newest]
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2009-07-24 5:22 [Bug tree-optimization/40844] New: O2 optimizes out assignment to bitfield jim at bodwin dot us
2009-07-24 5:54 ` Andrew Pinski
2009-07-24 5:54 ` [Bug tree-optimization/40844] " pinskia at gmail dot com
2009-07-24 12:26 ` rguenth at gcc dot gnu dot org
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