[PATCH][4.3] Deprecate -ftrapv

[Robert Dewar]

Joseph S. Myers wrote:
> On Fri, 29 Feb 2008, Robert Dewar wrote:
> 
>> Well no one "needs" anything in this sense, any interfaced is Turing
>> complete so there is *always* a work around deficiencies in the back end. But
>> right now, the "do this itself" is really horribly inefficient:
> 
> You can see the C code in libgcc2.c (__addvSI3 etc.) used by -ftrapv (that 
> code depends on signed overflow wrapping as has been noted before, it 
> should of course be written with unsigned types).  It does use 
> double-width types for multiplication (and some more complicated code for 
> multiplication of types that are already double-width).
> 
> If it were done at gimplification time I imagine something like the libgcc 
> code would be used, but with conversions to/from unsigned inserted as 
> needed.  It would be possible to do optimizations at gimplification time 
> if one argument is constant (converting to a range check).

Well presumably one would want to use target dependent stuff for
detecting overflow where it exists (sticky overflow bits on
power, O flag on PC, trapping add on MIPS etc).
>