[PATCH][4.3] Deprecate -ftrapv

[Joseph S. Myers]

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).

-- 
Joseph S. Myers
joseph@codesourcery.com