Konstantin Vladimirov schrieb:
Hi,
This problem is backend independent, so I build reproduction in x86
backend. Consider code:
int func(int x);
int test(int x, int *data)
{
int retval;
register int *buffer asm ("eax");
buffer = data;
retval = func(x);
__asm__ __volatile__ (".internal_label _t." "4096"
".%0.%1.%2:;nop"::"ri" (buffer), "ri" (4137), "ri" (4));
return retval;
}
here nop will be replaced for something, that essentially uses %eax,
but it is not actually required for minimal reproduction.
being compiled with gcc (SUSE Linux) 4.5.0 20100604 [gcc-4_5-branch
revision 160292]
with following options:
gcc -m32 -O2 -S -fomit-frame-pointer reprox86.c
yields assembler:
.type test, @function
test:
subl $28, %esp
movl 32(%esp), %eax
movl %eax, (%esp)
call func // <--- here eax is clobbered
.internal_label _t.4096.%eax.$4137.$4:;nop 0
addl $28, %esp
ret
That looks quite odd. I have a question -- why compiler doesn't save &
restore buffer (i.e. %eax) value around function call, when we
specifically pointed to %eax usage in the inline assembler
instruction?
Thanks for responses in advance.
There was quite long discussion on the topic last August starting at
http://gcc.gnu.org/ml/gcc/2011-08/msg00023.html
with resonable conclusions like
http://gcc.gnu.org/ml/gcc/2011-08/msg00067.html
I don't know what the current state of implementation is or if anyone is
working on it or planned/plans to improve the matter.
Johann
