[PATCH] D122891: [X86] Fold AND(SRL(X,Y),1) -> SETCC(BT(X,Y))

[Phoebe Wang via Phabricator via llvm-commits]

pengfei added a comment.

> As noticed on PR39174, if we're extracting a single non-constant bit, then try to use BT+SETCC instead to avoid messing around moving the shift amount to the ECX register, using slow x86 shift ops etc.

Why using ECX register is a slow shift?


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