[Bug middle-end/53100] Optimize __int128 with range information

["marc.glisse at normalesup dot org" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53100

--- Comment #1 from Marc Glisse <marc.glisse at normalesup dot org> 2012-04-29 08:05:59 UTC ---
(In reply to comment #0)
> It would be convenient if I
> could just write the whole code with __int128 and let the compiler do the
> optimization by tracking the range of numbers.

The transformation from an __int128 to a pair of long happens extremely late
(optabs.c), so we can't count on tree-vrp to notice that one of them is always
zero (and actually it is either 0 or -1, as a sign extension, which would make
this hard). On the other hand, tree-vrp does have the information that the
differences are in [-4294967295, 4294967295], which comfortably fits in a type
half the size of __int128. It seems a possible strategy would be to have
tree-vrp mark variables that fit in a type half their size (only for TImode?),
try and preserve that information along the way, and finally use it in
expand_doubleword_mult. But that seems to imply storing the information in an
rtx, and rtx seems a bit too densely packed to add this.

Better ideas?