On Mon, Jul 25, 2011 at 8:43 PM, Ian Lance Taylor <iant@xxxxxxxxxx> wrote:
> Jeffrey Walton <noloader@xxxxxxxxx> 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
