[Bug middle-end/48580] missed optimization: integer overflow checks

[jules at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=48580

jules at gcc dot gnu.org changed:

           What    |Removed                     |Added
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                 CC|                            |jules at gcc dot gnu.org

--- Comment #12 from jules at gcc dot gnu.org 2011-10-05 12:41:55 UTC ---
I don't much like the idea of using builtins for operations as fundamental as
integer arithmetic. How about this as a straw-man suggestion: adding new
qualifiers for "fat" integers-with-flags, somewhat in the spirit of the
embedded-C fractional/saturating types? So you might have:

int x, y;

void foo (void)
{
  _Flagged int res;

  res = (_Flagged int) x + y;

  if (_Carry (res))
    printf ("sum carried\n");

  if (_Overflow (res))
    printf ("sum overflowed\n");
}

this avoids problems with global state, and allows for the programming style
which (I vaguely get the impression that) people seem to want -- performing a
"normal" integer operation, then querying for carry/overflow flags afterwards.

These types wouldn't be allowed to cross ABI boundaries (although of course you
could use the magic builtins _Carry/_Overflow to extract values to pass to
functions), and "_Overflow" would only be allowed for signed types. You could
also have "_Borrow" for subtraction maybe (whose meaning would be the inverse
of _Carry).

Signalling variants could look like, e.g.:

void bar (void)
{
  int res;

  res = (_Signalling _Flagged int) x + y;
}

Things would get awkward if you tried to use these new constructs in
more-complicated expressions, I suppose. Anyway, just an idea.