IEEE 754：带有 fesetround() 的 sqrtf()：编译器之间的不同结果：0x42440a72 与 0x42440a73

Question

#include <stdio.h>
#include <stdint.h>
#include <fenv.h>
#include <math.h>

int main()
{
    typedef union { uint32_t u; float f; } ufloat;
    ufloat val;
    float arg = 2401.999999;
    int r;
    r = fesetround(FE_DOWNWARD);
    val.f = sqrtf(arg);
    printf ("FE_DOWNWARD   %22.13a [0x%x] %d\n", val.f, val.u, r);
    r = fesetround(FE_TONEAREST);
    val.f = sqrtf(arg);
    printf ("FE_TONEAREST  %22.13a [0x%x] %d\n", val.f, val.u, r);
    r = fesetround(FE_TOWARDZERO);
    val.f = sqrtf(arg);
    printf ("FE_TOWARDZERO %22.13a [0x%x] %d\n", val.f, val.u, r);
    r = fesetround(FE_UPWARD);
    val.f = sqrtf(arg);
    printf ("FE_UPWARD     %22.13a [0x%x] %d\n", val.f, val.u, r);
    return 0;
}

主机：Win10 x64。

结果：

案例一

$ clang t2.c -o t2.clang.exe && ./t2.clang.exe
FE_DOWNWARD     0x1.8814e60000000p+5 [0x42440a73] 0
FE_TONEAREST    0x1.8814e60000000p+5 [0x42440a73] 0
FE_TOWARDZERO   0x1.8814e60000000p+5 [0x42440a73] 0
FE_UPWARD       0x1.8814e60000000p+5 [0x42440a73] 0
 
$ clang --version
clang version 8.0.1 (tags/RELEASE_801/final)

案例二

$ gcc t2.c -o t2.gcc.exe && ./t2.gcc.exe
FE_DOWNWARD     0x1.8814e60000000p+5 [0x42440a73] 0
FE_TONEAREST    0x1.8814e60000000p+5 [0x42440a73] 0
FE_TOWARDZERO   0x1.8814e60000000p+5 [0x42440a73] 0
FE_UPWARD       0x1.8814e60000000p+5 [0x42440a73] 0
 
$ gcc --version
gcc (GCC) 10.2.0

案例三

cl t2.c && t2
Microsoft (R) C/C++ Optimizing Compiler Version 19.25.28611 for x64
...
FE_DOWNWARD     0x1.8814e40000000p+5 [0x42440a72] 0
FE_TONEAREST    0x1.8814e60000000p+5 [0x42440a73] 0
FE_TOWARDZERO   0x1.8814e40000000p+5 [0x42440a72] 0
FE_UPWARD       0x1.8814e60000000p+5 [0x42440a73] 0

案例4.

cl t2.c && t2
Microsoft (R) C/C++ Optimizing Compiler Version 19.25.28611 for x86
...
The system cannot execute the specified program.
Pop-up window appears: "Virus & thread protection: Windows Defender Antivirus found threats. Get details."

案例5.

wandbox.org: gcc HEAD 11.0.0 20200
$ gcc prog.c -Wall -Wextra -std=c99 "-lm"
FE_DOWNWARD     0x1.8814e40000000p+5 [0x42440a72] 0
FE_TONEAREST    0x1.8814e60000000p+5 [0x42440a73] 0
FE_TOWARDZERO   0x1.8814e40000000p+5 [0x42440a72] 0
FE_UPWARD       0x1.8814e60000000p+5 [0x42440a73] 0

案例 6.

wandbox.org: clang HEAD 12.0.0
$ clang prog.c -Wall -Wextra -std=gnu11 "-lm"
FE_DOWNWARD     0x1.8814e40000000p+5 [0x42440a72] 0
FE_TONEAREST    0x1.8814e60000000p+5 [0x42440a73] 0
FE_TOWARDZERO   0x1.8814e40000000p+5 [0x42440a72] 0
FE_UPWARD       0x1.8814e60000000p+5 [0x42440a73] 0

问题：

为什么编译器之间的结果不同（0x42440a72 与 0x42440a73）？
如何在编译器之间获得相同的结果？

Answer 1

Why there are different results (0x42440a72 vs. 0x42440a73) between compilers?

至少对于较旧的 gcc，<fenv.h> 不需要支持。

见floating-point environment access in <fenv.h>, Library feature, no compiler support required.

当我使用“GNU C11 (GCC) 版本 9.3.0 (x86_64-pc-cygwin)”编译时，以下

#include <fenv.h>
#pragma STDC FENV_ACCESS ON

我收到以下信息：

warning: ignoring #pragma STDC FENV_ACCESS [-Wunknown-pragmas]

另见 , pragma STDC FENV_ACCESS ON is not supported

How to get the same results between compilers?

没有太大帮助，但不要使用 fenv.h 的可选功能或避免使用 select 编译器。

可能存在 gcc 编译时标志可以提供帮助，但我不知道。

也用#pragma STDC FENV_ACCESS ON。这可能无法解决此问题，但可以防止相关问题。

IEEE 754：带有 fesetround() 的 sqrtf()：编译器之间的不同结果：0x42440a72 与 0x42440a73

IEEE 754: sqrtf() with fesetround(): different results between compilers: 0x42440a72 vs. 0x42440a73

c

floating-point

rounding

ieee-754

cl