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).
