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Re: [C PATCH] Discard P - (P + CST) optimization in pointer_diff (PR c/61240)


On Wed, Aug 06, 2014 at 10:22:19AM +0200, Marek Polacek wrote:
> On Tue, Aug 05, 2014 at 02:14:21PM -0600, Jeff Law wrote:
> > My concern is the code we're removing discusses the need to simplify when
> > these expressions are in static initializers.  What's going to ensure that
> > we're still simplifying instances which appear in static initializers?  I
> > don't see anything which tests that.   And does it still work for targets
> > which utilize PSImode?
> 
> Aw nuts.  So with the patch we'd start erroring out on
> static __PTRDIFF_TYPE__ d1 = p - (p + 1);
> static __PTRDIFF_TYPE__ d2 = p - (p - 1);
> (it's nowhere in the testsuite/code base - and I hadn't noticed that
> until today :()
> while we'd still accept
> static __PTRDIFF_TYPE__ d5 = (p - 1) - p;
> static __PTRDIFF_TYPE__ d6 = (p + 1) - p;
> (Those are not constant expression according to ISO C.)
> 
> The reason is that fold_build can fold
> "(long int) (p + 4) - (long int) p" to 4, but not
> "(long int) p - (long int) (p + 4)".
> 
> That means we have to have a way how to fold the latter, but only in
> static initializers.  So I guess I need to implement this in
> fold-const.c...   Oh well.
> 
> Nevertheless, I'd guess the fwprop bits could go in separately (it's
> beneficial for C++).

Well, if you are going to implement it in fwprop AND fold-const, then the
natural place to fix that would be in *.pd on the match-and-simplify branch.

	Jakub


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