RFA (convert): PATCH for c++/69631 (wrong overflow error with -fwrapv)

[Richard Biener]

On Fri, Feb 5, 2016 at 9:34 PM, Jason Merrill <jason@redhat.com> wrote:
> The problem here was that the call to convert_to_integer_nofold was still
> pushing the conversion down into the multiply expression, and when we do the
> multiplication in unsigned short it overflows.  This patch fixes
> convert_to_integer_1 to not do any truncation distribution when !dofold.
> But that broke several testcases, because fold() doesn't ever try to do that
> distribution, and so was missing some optimizations revealed by the
> distribution.  So this patch changes cp_fold to redo the conversion with
> dofold enabled.  I also change the C++ convert() entry point to do folding
> conversion again, since a few places in convert_to_integer_1, and probably
> elsewhere in the back end, expect that.
>
> For this email I didn't re-indent the truncation distribution code in
> convert_to_integer_1 to make it easier to read; for what I check in should I
> leave it like this, re-indent it, or do something else like a goto to avoid
> the need for re-indentation?

re-indent is fine but you could also simply tail-duplicate (optimized)

      /* When parsing long initializers, we might end up with a lot of casts.
         Shortcut this.  */
      if (TREE_CODE (expr) == INTEGER_CST)
        return fold_convert (type, expr);
      return build1 (CONVERT_EXPR, type, expr);

and thus do

   if (!dofold)
     return ...;

> Tested x86_64-pc-linux-gnu, OK for trunk?

Ok.

Note that I think we should move all the truncation distribution code elsewhere
(match.pd).  But that's probably for GCC 7.

Richard.