Hi,
> Surely in code like that, you would make "x" volatile? Memory clobbers
> are not a substitute for correct use of volatile accesses.
No,
It is as I wrote, a memory clobber is the only way to guarantee that
the asm statement is not move somewhere else.
I changed the example to use volatile and compiled it
with gcc-Version 4.6.3 (Ubuntu/Linaro 4.6.3-1ubuntu5)
volatile int x;
void
test()
{
x = 1;
asm volatile("nop");
x = 0;
}
gcc -S -O2 test.c gives:
test:
.LFB0:
.cfi_startproc
movl $1, x(%rip)
movl $0, x(%rip)
#APP
# 6 "test.c" 1
nop
# 0 "" 2
#NO_APP
ret
.cfi_endproc
While it works with asm volatile("nop" ::: "memory").
Likewise for "cli" and "sti" if you try to implement critical sections.
Although, these instructions do not touch any memory, we
need the memory clobber to prevent code motion.
Bernd.
