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Re: [tree-ssa] Eliminate more dead PHIs
- From: Jan Hubicka <hubicka at ucw dot cz>
- To: law at redhat dot com
- Cc: gcc-patches at gcc dot gnu dot org
- Date: Wed, 7 Jan 2004 23:16:48 +0100
- Subject: Re: [tree-ssa] Eliminate more dead PHIs
- References: <200401072211.i07MBX4e031068@speedy.slc.redhat.com>
>
> A few weeks ago I noticed that we were leaving PHI nodes for virtual
> variables in the IL when the result of the PHI was never used.
>
> I finally looked at it in more depth today. Would you believe that
> more than 80% of the PHI nodes in the dce1 dumpfile for alias.c are
> are for dead virtual variables! Egad.
Duh!
>
> Zdenek's changes help, but only slightly.
>
> We never need to consider a PHI for a virtual variable inherently
> necessary -- and that little trick eliminates a ton of totally
> useless PHI nodes. Whee.
Sounds very cool!
Any performance numbers by any chance?
Won't we end up re-discovering them by subsequent SSA passes?
Honza