gcc -Ofast - complete list of limitations

问题

I'm using -Ofast gcc option in my program cause latency requirements. I wrote simple test program:

#include <iostream>
#include <math.h>

static double quiet_NaN = std::numeric_limits<double>::quiet_NaN();

int main()
{
    double newValue = 130000; 
    double curValue = quiet_NaN; 
    printf("newValue = %f\n", newValue); 
    printf("curValue = %f\n", curValue); 
    printf("isnan(newValue) = %d\n", isnan(newValue)); 
    printf("isnan(curValue) = %d\n", isnan(curValue)); 
    printf("newValue == curValue %d\n", (newValue == curValue)); 
    printf("newValue != curValue %d\n", (newValue != curValue)); 
}

I've tried to run it with default flags and with -Ofast:

$ g++ TestPointer.cpp 
$./a.out 
newValue = 130000.000000
curValue = nan
isnan(newValue) = 0
isnan(curValue) = 1
newValue == curValue 0
newValue != curValue 1

$ g++ -Ofast TestPointer.cpp 
$ ./a.out 
newValue = 130000.000000
curValue = nan
isnan(newValue) = 0
isnan(curValue) = 1
newValue == curValue 1
newValue != curValue 0

So the result of != and == can not be trusted. Does it mean that I should == and != only when both values are known to be not nan, otherwise I should test with isnan before?

Is it guaranteed that isnan works correctly with -Ofast? How correctly == and != works for double with -Ofast? Can someone provide complete list of limitations added by -Ofast?

回答1:

You're observing the effects of -ffast-math.

From the docs:

-Ofast

Disregard strict standards compliance. -Ofast enables all -O3 optimizations. It also enables optimizations that are not valid for all standard-compliant programs. It turns on -ffast-math and the Fortran-specific -fno-protect-parens and -fstack-arrays.

and

-ffast-math

Sets -fno-math-errno, -funsafe-math-optimizations, -fno-trapping-math, -ffinite-math-only, -fno-rounding-math, -fno-signaling-nans and fcx-limited-range.

and

-ffinite-math-only

Allow optimizations for floating-point arithmetic that assume that arguments and results are not NaNs or +-Infs.

There are several gcc bug reports for this marked invalid.

Problems with -ffast-math and isnan

Additionally, comparison of strict IEEE floating points always results in false.

Checking if a double (or float) is NaN in C++

This doesn't necessarily apply for -ffast-math but it explains what you show.

gcc does not describe a formal standard for how -ffast-math floats work so you'll just have to work out the details empirically if you must and not assume consistency between versions of gcc. Better yet, completely avoid the combination of NaN and -ffast-math.

来源：https://stackoverflow.com/questions/27315941/gcc-ofast-complete-list-of-limitations

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