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Re: [PATCH] Avoid useless REG_EQUAL notes


On Thu, 20 Feb 2003, Joern Rennecke wrote:
> The REG_EQUAL note is valuable if the set is changed later,
> e.g. by cse or gcse, to use a register instead.
>
> To avoid run-time performance regressions, you patch should add
> REG_EQUAL notes in all the cases where a SET is modified so that
> the constant is not explicit anymore.

We already have code that adds REG_EQUAL notes when making substitutions
in GCSE/CSE.  My first patch to avoid redundant REG_EQUAL notes fixed it:
http://gcc.gnu.org/ml/gcc-patches/2003-01/msg02173.html

I'm actually working on improvements to GCSE, CSE and combine that'll
catch more optimization opportunities using REG_EQUAL notes, but unless
the number of redundant notes is kept under control, the benefits will
be adversely affected by the poor signal-to-noise ratio.

For an example of this class of improvements, see Kazu's recent patch:
http://gcc.gnu.org/ml/gcc-patches/2003-02/msg00144.html.

Its still waiting to be reviewed...

Roger
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