https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102566
--- Comment #9 from Thiago Macieira <thiago at kde dot org> ---
Looks like it doesn't work for the sign bit.
$ cat /tmp/test.cpp
#include <atomic>
bool tbit(std::atomic<int> &i)
{
return i.fetch_or(CONSTANT, std::memory_order_relaxed) & CONSTANT;
}
$ ~/dev/gcc/bin/gcc -DCONSTANT='(1<<30)' -S -o - -O2 /tmp/test.cpp | sed -n
'/startproc/,/endproc/p'
.cfi_startproc
lock btsl $30, (%rdi)
setc %al
ret
.cfi_endproc
$ ~/dev/gcc/bin/gcc -DCONSTANT='(1<<31)' -S -o - -O2 /tmp/test.cpp | sed -n
'/startproc/,/endproc/p'
.cfi_startproc
movl (%rdi), %eax
.L2:
movl %eax, %ecx
movl %eax, %edx
orl $-2147483648, %ecx
lock cmpxchgl %ecx, (%rdi)
jne .L2
shrl $31, %edx
movl %edx, %eax
ret
.cfi_endproc
Changing to std::atomic<unsigned> makes no difference.
