考虑以下程序:
#include <iostream>
#include <cmath>
#include <cstring>
#include <xmmintrin.h>
using namespace std;
int main()
{
// 4 float32s.
__m128 nans;
// Set them all to 0xffffffff which should be NaN.
memset(&nans, 0xff, 4*4);
// cmpord should return a mask of 0xffffffff for any non-NaNs, and 0x00000000 for NaNs.
__m128 mask = _mm_cmpord_ps(nans, nans);
// AND the mask with nans to zero any of the nans. The result should be 0x00000000 for every component.
__m128 z = _mm_and_ps(mask, nans);
cout << z[0] << " " << z[1] << " " << z[2] << " " << z[3] << endl;
return 0;
}
如果我使用Apple Clang 7.0.2编译,有时加上-ffast-math
,有时不加,我会得到预期的输出0 0 0 0
:
$ clang --version
Apple LLVM version 7.0.2 (clang-700.1.81)
Target: x86_64-apple-darwin14.5.0
Thread model: posix
$ clang test.cpp -o test
$ ./test
0 0 0 0
$ clang test.cpp -ffast-math -o test
$ ./test
0 0 0 0
然而,在更新到8.1.0之后(很抱歉,我不知道这实际上对应哪个版本的Clang - Apple不再发布相关信息),-ffast-math
似乎破坏了此功能:
$ clang --version
Apple LLVM version 8.1.0 (clang-802.0.42)
Target: x86_64-apple-darwin16.6.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin
$ clang test.cpp -o test
$ ./test
0 0 0 0
$ clang test.cpp -ffast-math -o test
$ ./test
nan nan nan nan
我怀疑这是由于严格别名规则之类的原因。有人可以解释一下这种行为吗?
编辑:我忘记提到如果你这样做 nans = { std::nanf(nullptr), ...
它能正常工作。
另外,在 godbolt 上查看,似乎 Clang 3.8.1 和 Clang 3.9 之间的行为发生了变化 - 后者删除了 cmpordps
指令。GCC 7.1 似乎保留它。