On Friday, 7 February 2014 at 15:42:06 UTC, Sean Kelly wrote:
Oops. I thought that since Intel has officially defined loads
as having acquire semantics, I had eliminated the barrier
requirement there. But I guess not. I suppose it's an issue
worth discussing. Does anyone know offhand what C++0x
implementations do for load acquires on x86?
Offhand - no. But who forbids empirical tests? :)
--8<-- main.cpp
#include <atomic>
#include <cstdint>
#include <iostream>
int test32() {
std::atomic<int> ai(0xfacefeed);
return ai.load(std::memory_order_acquire);
}
int64_t test64() {
std::atomic<int64_t> ai(0xbadface00badface);
return ai.load(std::memory_order_acquire);
}
int main(int argc, char** argv) {
auto i1 = test32();
auto i2 = test64();
// Prevent dead code optimization
std::cout << i1 << " " << i2 << std::endl;
}
-->8--
I've pulled the atomic ops into separate functions to try and
prevent the compiler from being too clever.
I'm using --std=c++11 but --std=c++0x would work as well.
$ g++ -Ofast -m32 --std=c++11 main.cpp
$ objdump -d -w -r -C --no-show-raw-insn
--disassembler-options=intel a.out | less -S
--8<--
08048830 <test32()>:
8048830: sub esp,0x10
8048833: mov DWORD PTR [esp+0xc],0xfacefeed
804883b: mov eax,DWORD PTR [esp+0xc]
804883f: add esp,0x10
8048842: ret
8048843: lea esi,[esi+0x0]
8048849: lea edi,[edi+eiz*1+0x0]
08048850 <test64()>:
8048850: sub esp,0x1c
8048853: mov DWORD PTR [esp+0x10],0xbadface
804885b: mov DWORD PTR [esp+0x14],0xbadface0
8048863: fild QWORD PTR [esp+0x10]
8048867: fistp QWORD PTR [esp]
804886a: mov eax,DWORD PTR [esp]
804886d: mov edx,DWORD PTR [esp+0x4]
8048871: add esp,0x1c
8048874: ret
8048875: xchg ax,ax
8048877: xchg ax,ax
8048879: xchg ax,ax
804887b: xchg ax,ax
804887d: xchg ax,ax
804887f: nop
-->8--
$ g++ -Ofast -m64 --std=c++11 main.cpp
$ objdump -d -w -r -C --no-show-raw-insn
--disassembler-options=intel a.out | less -S
--8<--
0000000000400950 <test32()>:
400950: mov DWORD PTR [rsp-0x18],0xfacefeed
400958: mov eax,DWORD PTR [rsp-0x18]
40095c: ret
40095d: nop DWORD PTR [rax]
0000000000400960 <test64()>:
400960: movabs rax,0xbadface00badface
40096a: mov QWORD PTR [rsp-0x18],rax
40096f: mov rax,QWORD PTR [rsp-0x18]
400974: ret
400975: nop WORD PTR cs:[rax+rax*1+0x0]
40097f: nop
-->8--
No barriers in sight.
