https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68084
Bug ID: 68084
Summary: Inverted conditions generated for x86 inline assembly
"flag output constraints"
Product: gcc
Version: 6.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: inline-asm
Assignee: unassigned at gcc dot gnu.org
Reporter: richardpku at gmail dot com
Target Milestone: ---
I have been trying out the new "flag output constraints" feature in inline
assembly ("=@ccCC"), and have found GCC sometimes incorrectly generates
inverted conditions in assembler code.
The minimal test case is as follows:
_Bool dec_ref(unsigned *p)
{
_Bool r;
asm ("cmp %2, %1" : "=@ccae"(r) : "m"(*p), "ri"(2));
if (r)
return __atomic_sub_fetch(p, 1, __ATOMIC_RELEASE) == 0;
return 1;
}
GCC (6.0.0 20151024) generates the following assembler code:
dec_ref:
cmp $2, (%rdi)
movl $1, %eax
jc .L7
rep ret
.p2align 4,,10
.p2align 3
.L7:
lock subl $1, (%rdi)
sete %al
ret
where "jc" is incorrect. It should be "jnc".
Interestingly, GCC generates the expected code if "=@ccae" is written as
"=@ccnb" (ae and nb are synonymous).
