On 11/20/2015 2:17 AM, Andrew Haley wrote:
On 20/11/15 01:23, David Wohlferd wrote:
I tried to picture the most basic case I can think of that uses
something clobber-able:
for (int x=0; x < 1000; x++)
asm("#stuff");
This generates very simple and highly performant code:
movl $1000, %eax
.L2:
#stuff
subl $1, %eax
jne .L2
Using extended asm to simulate the clobberall gives:
movl $1000, 44(%rsp)
.L2:
#stuff
subl $1, 44(%rsp)
jne .L2
It allocates an extra 4 bytes, and changed everything to memory accesses
instead of using a register.
Can you show us your code? I get
xx:
movl $1000, %eax
.L2:
#stuff
subl $1, %eax
jne .L2
rep; ret
for
void xx() {
for (int x=0; x < 1000; x++)
asm volatile("#stuff" : : : "memory");
}
What you're describing looks like a bug: x doesn't have its address
taken.
The intent for 24414 is to change basic asm such that it will become
(quoting jeff) "an opaque blob that read/write/clobber any register or
memory location." Such being the case, "memory" is not sufficient:
#define CLOBBERALL "eax", "ebx", "ecx", "edx", "r8", "r9", "r10", "r11",
"r12", "r13", "r14", "r15", "edi", "esi", "ebp", "cc", "memory"
int main()
{
for (int x=0; x < 1000; x++)
asm("#":::CLOBBERALL);
}
dw
