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Re: RFC: allowing fwprop to propagate subregs
- From: kenner at vlsi1 dot ultra dot nyu dot edu (Richard Kenner)
- To: uweigand at de dot ibm dot com
- Cc: bonzini at gnu dot org, gcc-patches at gcc dot gnu dot org, richard dot sandiford at linaro dot org
- Date: Tue, 17 Jan 2012 14:58:50 EST
- Subject: Re: RFC: allowing fwprop to propagate subregs
- References: <OF5CC97609.F2965202-ONC1257988.006AA629@de.ibm.com>
> I tried to implement that suggestion, but interestingly enough I cannot
> really test it since I was unable to find any single case where that
> SUBREG case in apply_distributive_law actually causes any difference
> whatsoever in generated code.
>
> Do you have any further suggestion of how to find a testcase (some
> particular source code and/or architecture)?
No. It may well be irrelevant now and was only relevant in cases where
we just had SImode and not the narrower modes on some of the older
architectures such as a29k. It may also be that the optimizations that
this caught are now done at tree level, though I think the former is
the more likely.
> Given the current set of results, since I do not have any way to verify
> whether my simplify_set changes would actually trigger correctly, I'd
> rather propose to just remove the SUBREG case in apply_distributive_law
> (i.e. only apply the first patch below).
>
> Thoughts?
I think that's reasonable. But I'd replace it with a comment saying
what used to be there and why it was removed.