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Re: Optimizations and the -pedantic option

From: Ian Lance Taylor <iant at google dot com>
To: "Dario Saccavino" <kathoum at gmail dot com>
Cc: gcc-help at gcc dot gnu dot org
Date: 29 Feb 2008 07:13:16 -0800
Subject: Re: Optimizations and the -pedantic option
References: <b6621a6e0802261312r40bc6204wd965f38cca5477da@mail.gmail.com> <m3mypnw5l3.fsf@localhost.localdomain> <b6621a6e0802290159v6241f7efoc4d148a8f8723f12@mail.gmail.com>

"Dario Saccavino" <kathoum@gmail.com> writes:

> It seems that a transformation that is supposed to help optimization
> (seeing j as a constant value instead of a variable) has a negative
> effect.

Yes.  It's because of this comment in tree-tailcall.c:

      /* There should be no previous addition.  TODO -- it should be fairly
	 straightforward to lift this restriction -- just allow storing
	 more complicated expressions in *A, and gimplify it in
	 adjust_accumulator_values.  */

> I've just finished compiling gcc 4.2.3, and I'm very pleased to
> announce that all of the snippets posted to this thread are properly
> optimized (with -O2). Hail to the gcc team!

Interestingly, this was fixed not by fixing the above case, but by
disabling a different, incorrect, optimization.  The patch which
re-enables the tailcall optimization is the patch for
http://gcc.gnu.org/PR30364 .

Ian

References:
- Optimizations and the -pedantic option
  - From: Dario Saccavino
- Re: Optimizations and the -pedantic option
  - From: Ian Lance Taylor
- Re: Optimizations and the -pedantic option
  - From: Dario Saccavino

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