[PATCH] D133902: X86: Stop assigning register costs for longer encodings

[Simon Pilgrim via Phabricator via llvm-commits]

RKSimon added a comment.

Early x86-64 CPUs (often with weak frontend/decoders) might benefit from avoiding REX encodings (I know when I was doing exegesis testing for the atom/silvermont sched models avoiding REX registers gave much more consistent backend numbers), although I'm not certain how much work adding CPU Feature specific costs would be (or how much benefit they'd give for most people).



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Comment at: llvm/lib/Target/X86/X86RegisterInfo.td:128
 
-// X86-64 only, requires REX.
-let SubRegIndices = [sub_8bit, sub_8bit_hi_phony], CostPerUse = [1],
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Keep the comments?


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Comment at: llvm/lib/Target/X86/X86RegisterInfo.td:221
 
-// X86-64 only
-let CostPerUse = [1] in {
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This comment is still useful


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