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Re: Linking Program with Lib That Uses -fwrapv or '-fno-strict-overflow'


On Mon, Jul 25, 2011 at 8:43 PM, Ian Lance Taylor <iant@google.com> wrote:
> Jeffrey Walton <noloader@gmail.com> writes:
>
>> Is it possible to build a library which specifies -fwrapv or
>> '-fno-strict-overflow' but does not penalize the final executable
>> (with respect to optimizations) after final linking?
>>
>> The question arises from a library which detects wrap/carry/overflow
>> and might need to specify -fwrapv or '-fno-strict-overflow' to take
>> advantage of 2s compliment math used on many modern architectures.
>
> I'm not sure I understand the question. ?The -fwrapv and
> -fno-strict-overflow options only affect the compilation of the code
> with which they are used. ?They do not affect code which is compiled
> without those options. ?They do not affect the linker. ?So the answer to
> your question may be that you should just use the options with the
> specific code that needs them. ?If the answer doesn't make sense, can
> you restate the question?
>

A simplified example (I know you gave a previous work around) assuming
32 bit ints:

// liboverflow, compiled with -fwrapv
int my_abs(int x, int* r)
{
    int y = abs(x);
    if (y < 0)
        return 0;

    *r = y;
    return 1;
}

// myprogram, compiled with -O2 and linked against liboverflow
int x = 0x80000000, r = 0;
if(my_abs(x, &r))
    printf(...)

Will the resulting program enjoy O2 and/or produce incorrect results?

Jeff


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