Re: [PATCH] intprops: add WRAPV and const flavors for GCC 5

[Pádraig Brady]

On 17/12/15 21:02, Paul Eggert wrote:
> On 12/17/2015 06:19 AM, Pádraig Brady wrote:
>> -     long int result = INT_MULTIPLY_WRAPV (a, b);
>> +     long int result = a * b;
> 
> This doesn't look right, because a * b could overflow which would lead 
> to undefined behavior. INT_MULTIPLY_WRAPV does not rely on undefined 
> behavior due to integer overflow, which is the point of that example.

Right. The issue I have with the example is that INT_MULTIPLY_WRAPV()
takes 3 arguments, while only 2 are provided.

> I tracked this down to what appears to be a compiler bug in GCC, filed a 
> bug report with the GCC folks (GCC bug 68971), and installed the 
> attached patch instead.

Right. I see the undefined code is never called or added to the translation 
unit.
In case gcc decides to continue to warn in this case, I suppose we could avoid
the undefined code by promoting to a wider type and explicitly truncating
the result (though that might need __int128 if available for your new 64 bit 
case).

thanks,
Pádraig.