On Jan 4, 2007, at 13:34, Segher Boessenkool wrote:
The "signed wrap is undefined" thing doesn't fit in this category
though:
-- It is an important optimisation for loops with a signed
induction variable;
It certainly isn't that important. Even SpecINT compiled with
-O3 and top-of-tree GCC *improves* 1% by adding -fwrapv.
If the compiler itself can rely on wrap-around semantics and
doesn't have to worry about introducing overflows between
optimization passes, it can reorder simple chains of additions.
This is more important for many real-world applications than
being able to perform some complex loop-interchange.
Compiler developers always make the mistake of overrating
their optimizations.
If GCC does really poorly on a few important loops that matter,
that issue is easily addressed. If GCC generates unreliable
code for millions of boring lines of important real-world C,
the compiler is worthless.
-Geert
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