[PATCH] Check rtx_costs in combine.c's try_combine

Richard Kenner kenner@vlsi1.ultra.nyu.edu
Mon Jun 28 13:49:00 GMT 2004


    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.



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