有什么方法可以确保浮点运算结果在linux和windows中相同 [英] Is there any way to make sure the floating point arithmetic result the same in both linux and windows

问题描述

我的程序在 linux 和 windows 上都运行，我必须确保浮点运算在不同的操作系统中得到相同的结果.

My programe runs both in linux and windows, I have to make sure the floating point arithmetic get the same result in different OS.

代码如下:

for (int i = 0; i < 100000; ++i)
{
    float d_value = 10.0f / float(i);
    float p_value = 0.01f * float(i) + 100.0f;
}

我使用g++ -m32 -c -static -g -O0 -ffloat-store"在 linux 中构建代码.我使用 "/fp:precise/O2" 在 windows 中使用 vs2005 构建代码.

I use "g++ -m32 -c -static -g -O0 -ffloat-store" to build the code in linux. I use "/fp:precise /O2" to build the code in windows with vs2005.

当我打印d_value"和p_value"时，d_value"在 linux 和 windows 中都是一样的.但是p_value"有时会有所不同.例如，以十六进制格式打印p_value":

When I printf the "d_value" and the "p_value", the "d_value" is all the same both in linux and windows. But the "p_value" is different sometimes. For exsample, print the "p_value" with hexadecimal format:

windows:  42d5d1eb
linux:    42d5d1ec

为什么会发生这种情况?

Why dose this happen?

我的g++版本是

Configured with: ../src/configure -v --with-pkgversion='Debian 4.4.5-8' --with-bugurl=file:///usr/share/doc/gcc-4.4/README.Bugs --enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr --program-suffix=-4.4 --enable-shared --enable-multiarch --enable-linker-build-id --with-system-zlib --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.4 --libdir=/usr/lib --enable-nls --enable-clocale=gnu --enable-libstdcxx-debug --enable-objc-gc --enable-targets=all --with-arch-32=i586 --with-tune=generic --enable-checking=release --build=i486-linux-gnu --host=i486-linux-gnu --target=i486-linux-gnu
Thread model: posix
gcc version 4.4.5 (Debian 4.4.5-8)

我使用标志 -ffloat-store，因为这里有人建议:Linux、Mac OS X 和 Windows 之间的不同数学舍入行为

I use the flag -ffloat-store, because of someone's suggestion here: Different math rounding behaviour between Linux, Mac OS X and Windows

推荐答案

在 Windows 上使用 /fp:strict 告诉编译器生成严格遵循 IEEE 754 的代码，gcc -msse2 -mfpmath=sse 在 Linux 上获得相同的行为.

Use /fp:strict on Windows to tell the compiler to produce code that strictly follows IEEE 754, and gcc -msse2 -mfpmath=sse on Linux to obtain the same behavior there.

您所看到的差异的原因已在 StackOverflow 上讨论过，但最好的调查是 David Monniaux 的 文章.

The reasons for the differences you are seeing have been discussed in spots on StackOverflow, but the best survey is David Monniaux's article.

我使用 gcc -msse2 -mpfmath=sse 编译时得到的汇编指令如下.指令 cvtsi2ssq、divss、mulss、addss 是要使用的正确指令，它们会导致程序在p_value 在某一点包含 42d5d1ec.

The assembly instructions I obtain when compiling with gcc -msse2 -mpfmath=sse are as follow. Instructions cvtsi2ssq, divss, mulss, addss are the correct instructions to use, and they result in a program where p_value contains at one point 42d5d1ec.

    .globl  _main
    .align  4, 0x90
_main:                                  ## @main
    .cfi_startproc
## BB#0:
    pushq   %rbp
Ltmp2:
    .cfi_def_cfa_offset 16
Ltmp3:
    .cfi_offset %rbp, -16
    movq    %rsp, %rbp
Ltmp4:
    .cfi_def_cfa_register %rbp
    subq    $32, %rsp
    movl    $0, -4(%rbp)
    movl    $0, -8(%rbp)
LBB0_1:                                 ## =>This Inner Loop Header: Depth=1
    cmpl    $100000, -8(%rbp)       ## imm = 0x186A0
    jge LBB0_4
## BB#2:                                ##   in Loop: Header=BB0_1 Depth=1
    movq    _p_value@GOTPCREL(%rip), %rax
    movabsq $100, %rcx
    cvtsi2ssq   %rcx, %xmm0
    movss   LCPI0_0(%rip), %xmm1
    movabsq $10, %rcx
    cvtsi2ssq   %rcx, %xmm2
    cvtsi2ss    -8(%rbp), %xmm3
    divss   %xmm3, %xmm2
    movss   %xmm2, -12(%rbp)
    cvtsi2ss    -8(%rbp), %xmm2
    mulss   %xmm2, %xmm1
    addss   %xmm0, %xmm1
    movss   %xmm1, (%rax)
    movl    (%rax), %edx
    movl    %edx, -16(%rbp)
    leaq    L_.str(%rip), %rdi
    movl    -16(%rbp), %esi
    movb    $0, %al
    callq   _printf
    movl    %eax, -20(%rbp)         ## 4-byte Spill
## BB#3:                                ##   in Loop: Header=BB0_1 Depth=1
    movl    -8(%rbp), %eax
    addl    $1, %eax
    movl    %eax, -8(%rbp)
    jmp LBB0_1
LBB0_4:
    movl    -4(%rbp), %eax
    addq    $32, %rsp
    popq    %rbp
    ret

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