Bug in loop optimize (invalid postinc to preinc transformation)

[Andreas Schwab]

Jamie Lokier <egcs@tantalophile.demon.co.uk> writes:

|> Alexandre Oliva wrote:
|> > > Compiling without optimization indicates that the compiler is
|> > > transforming (p++ < x) into (++p < (x+1)), even when not optimizing.
|> > > This transformation is incorrect because x+1 wraps around.
|> > 
|> > Overflow invokes undefined behavior.  Since incrementing p in this
|> > case involves overflow, I think the transformation is ok, as far as
|> > undefined behavior goes.
|> 
|> Overflow of unsigned integers has defined behaviour -- they wrap.  The
|> question is, what rule applies to pointer comparisons?

Comparing pointers that don't point to the same (part of) object is
undefined.

Andreas.

-- 
Andreas Schwab                                  "And now for something
SuSE Labs                                        completely different."
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