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From: "glisse at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/68131] missed optimization and warning for broken overflow check
Date: Wed, 28 Oct 2015 12:59:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68131

--- Comment #3 from Marc Glisse <glisse at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #2)
> We indeed do not have this optimization, only A + CST CMP CST to A CMP CST'
> and related.  Note that _9 might be negative so we also need range info
> for this.  VRPs symbolic range stuff isn't good enough to simplify this.

If we want to simplify directly to false, I would expect a match.pd pattern
using tree_expr_nonnegative_p to work in this case (conversion from a smaller
unsigned). By the way, maybe tree_single_nonnegative_warnv_p could try looking
at get_range_info before forwarding to gimple_stmt_nonnegative_warnv_p.

But the much simpler transformation: a+b<a => b<0 does not require any VRP info
and would let the next VRP pass finish the work.