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Re: [PATCH] Check rtx_costs in combine.c's try_combine
- From: kenner at vlsi1 dot ultra dot nyu dot edu (Richard Kenner)
- To: roger at eyesopen dot com
- Cc: gcc-patches at gcc dot gnu dot org
- Date: Mon, 28 Jun 04 07:20:38 EDT
- Subject: Re: [PATCH] Check rtx_costs in combine.c's try_combine
The following patch adds an additional test to combine.c's try_combine
to only allow instructions to be merged/combined if the backend's
TARGET_RTX_COST macro indictates that the replacement is atleast as
cheap as the original. Historically, GCC's "combine" pass has used
a greedy algorithm that always attempts to build complex composite
instructions that are recognized by the back-end independent of any
cost metric. Increasingly with modern processors, the more complex
instructions supported are an ISA are not automatically better than
their component parts.
Yes, but in such a case, those complex instructions should not in the MD
file as there is no situation in which they should be used.
I disagree with this patch.