Hi Alex ,
Agree that float division don't touch memory ,but fdiv result (stack
register ) is stored back to a memory i.e fResult .
So compiler barrier in the inline asm i.e ::memory should prevent the
shrinkage of instructions like "fstps fResult(%rip)" behind the
fence ?
BTW ,if we make fDivident and fResult = 0.0f gloabls,the code
emitted looks ok i.e
#gcc -S test.c -O3 -mmmx -mno-sse
flds .LC0(%rip)
fsts fDivident(%rip)
fdivs .LC1(%rip)
fstps fResult(%rip)
#APP
# 10 "test.c" 1
mfence
# 0 "" 2
#NO_APP
flds fResult(%rip)
movl $.LC2, %edi
xorl %eax, %eax
fstpl (%rsp)
call printf
So i strongly believe that ,its compiler issue and please feel free
correct me in any case.
Thank you and waiting for your reply.
~Umesh
On Fri, Apr 13, 2018 at 5:58 PM, Alexander Monakov <amona...@ispras.ru> wrote:
> On Fri, 13 Apr 2018, Vivek Kinhekar wrote:
>> The mfence instruction with memory clobber asm instruction should create a
>> barrier between division and printf instructions.
>
> No, floating-point division does not touch memory, so the asm does not (and
> need not) restrict its motion.
>
> Alexander
