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* [Bug c/63991] New: redundant read when write to volatile member of packed structure
@ 2014-11-20 11:07 potrepalov@asc-ural.ru
2014-11-20 12:06 ` [Bug middle-end/63991] " rguenth at gcc dot gnu.org
2014-11-22 11:58 ` ebotcazou at gcc dot gnu.org
0 siblings, 2 replies; 3+ messages in thread
From: potrepalov@asc-ural.ru @ 2014-11-20 11:07 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63991
Bug ID: 63991
Summary: redundant read when write to volatile member of packed
structure
Product: gcc
Version: 4.8.3
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: potrepalov@asc-ural.ru
There is redundant read when write to volatile member of packed structure:
exti.c:
---------------
typedef struct
{
volatile unsigned int IMR;
volatile unsigned int EMR;
volatile unsigned int RTSR;
volatile unsigned int FTSR;
volatile unsigned int SWIER;
volatile unsigned int PR;
} EXTI_TypeDef;
void
EXTI_5_IRQHandler( EXTI_TypeDef * exti ) { exti->PR = 1 << 5; }
---------------
make.bat:
---------------
arm-none-eabi-gcc -mthumb -march=armv7e-m -fpack-struct -O2 -c exti.c -o
exti-bug.o
arm-none-eabi-gcc -mthumb -march=armv7e-m -O2 -c exti.c -o exti.o
arm-none-eabi-objdump -a -S exti.o > exti.lst
arm-none-eabi-objdump -a -S exti-bug.o > exti-bug.lst
---------------
exti.lst
---------------
exti.o: file format elf32-littlearm
exti.o
Disassembly of section .text:
00000000 <EXTI_5_IRQHandler>:
0: 2320 movs r3, #32
2: 6143 str r3, [r0, #20]
4: 4770 bx lr
6: bf00 nop
----------------
exti-bug.lst
----------------
exti-bug.o: file format elf32-littlearm
exti-bug.o
Disassembly of section .text:
00000000 <EXTI_5_IRQHandler>:
0: 2320 movs r3, #32
2: 6942 ldr r2, [r0, #20]
4: 6143 str r3, [r0, #20]
6: 4770 bx lr
----------------
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug middle-end/63991] redundant read when write to volatile member of packed structure
2014-11-20 11:07 [Bug c/63991] New: redundant read when write to volatile member of packed structure potrepalov@asc-ural.ru
@ 2014-11-20 12:06 ` rguenth at gcc dot gnu.org
2014-11-22 11:58 ` ebotcazou at gcc dot gnu.org
1 sibling, 0 replies; 3+ messages in thread
From: rguenth at gcc dot gnu.org @ 2014-11-20 12:06 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63991
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Target| |arm-none-eabi
Status|UNCONFIRMED |NEW
Last reconfirmed| |2014-11-20
CC| |ebotcazou at gcc dot gnu.org
Component|c |middle-end
Ever confirmed|0 |1
Known to fail| |5.0
--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
That's indeed a bug as if the structure is unaligned (which is what happens
with -fpack-struct). Correct code looks like
mov r2, #32
mov r3, #0
ldrb r1, [r0, #20] @ zero_extendqisi2
strb r2, [r0, #20]
ldrb r2, [r0, #21] @ zero_extendqisi2
strb r3, [r0, #21]
ldrb r2, [r0, #22] @ zero_extendqisi2
strb r3, [r0, #22]
ldrb r2, [r0, #23] @ zero_extendqisi2
strb r3, [r0, #23]
but still for some reason we create volatile unused loads at expansion time
(and those prevail and don't get DCEd, obviously).
Probably happens on all strict-alignment targets.
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug middle-end/63991] redundant read when write to volatile member of packed structure
2014-11-20 11:07 [Bug c/63991] New: redundant read when write to volatile member of packed structure potrepalov@asc-ural.ru
2014-11-20 12:06 ` [Bug middle-end/63991] " rguenth at gcc dot gnu.org
@ 2014-11-22 11:58 ` ebotcazou at gcc dot gnu.org
1 sibling, 0 replies; 3+ messages in thread
From: ebotcazou at gcc dot gnu.org @ 2014-11-22 11:58 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63991
--- Comment #2 from Eric Botcazou <ebotcazou at gcc dot gnu.org> ---
It looks like -fpack-struct cannot be used when -fstrict-volatile-bitfields is
in effect, i.e. on ARM EABI. As for unaligned volatile fields on
strict-alignment
targets, they clearly ask for trouble. Simply do not use -fpack-struct.
^ permalink raw reply [flat|nested] 3+ messages in thread
end of thread, other threads:[~2014-11-22 11:58 UTC | newest]
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2014-11-20 11:07 [Bug c/63991] New: redundant read when write to volatile member of packed structure potrepalov@asc-ural.ru
2014-11-20 12:06 ` [Bug middle-end/63991] " rguenth at gcc dot gnu.org
2014-11-22 11:58 ` ebotcazou at gcc dot gnu.org
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