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Re: extension dependant givs
- To: Richard Henderson <rth at cygnus dot com>
- Subject: Re: extension dependant givs
- From: Toon Moene <toon at moene dot indiv dot nluug dot nl>
- Date: Fri, 08 Sep 2000 15:41:06 +0200
- CC: gcc-patches at gcc dot gnu dot org
- Organization: Moene Computational Physics, Maartensdijk, The Netherlands
- References: <20000907155208.A3507@cygnus.com>
Richard Henderson wrote:
> This is a variant of a patch I did once upon a time for the new ia32
> backend. The object is to recognize cases in which a sign or zero-extend
> of a BIV does not affect the outcome, and thus could be folded in to
> the initialization of a GIV.
> At the time it was put aside, since the conditions that give rise to the
> problem didn't happen often enough to be worth the effort. However, it
> does happen often on IA-64.
[ Score:1 Interesting ]
Why is this different on the IA-64 vs. Alpha - they're both 32-bit int,
64 bit pointer machines, no ?
> This optimization unfortunately does not trigger on Alpha at all,
> because there we are more apt to see
> (zero_extend:DI (subreg:SI (reg:DI) 0))
> and this code does not handle nested extensions. Another thing
> to fix, perhaps.
Sure, you added special code to the Fortran Frontend in '97 to get it to
generate reasonable code vis a vis this problem - would be nice if we
could remove it (and not have to add it to g95) ...
> The next thing to do is to recognize that in
> int i, start, end;
> for (i = start; i < end; ++i)
> "i" cannot overflow if "end" is loop invariant since we're protected
> by the initial comparison, and the +1 ensures we will not pass "end".
> This appears to be very common, particularly when start == 0.
Is this also a problem with:
int i, start, end;
for (i = end - start; i >=0 ; --i)
where start and end are untouched by the loop ? (This is how Fortran
loops are encoded).
Ceterum censeo "dependent" scribendum esse.
Toon Moene - mailto:email@example.com - phoneto: +31 346 214290
Saturnushof 14, 3738 XG Maartensdijk, The Netherlands
GNU Fortran 77: http://gcc.gnu.org/onlinedocs/g77_news.html
GNU Fortran 95: http://g95.sourceforge.net/ (under construction)