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

[Simon Pilgrim via Phabricator via llvm-commits]

RKSimon added a comment.

In D122891#3422324 <https://reviews.llvm.org/D122891#3422324>, @pengfei wrote:

>> 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?

Just that we have to move the shift amount to ECX for the shift ops, which can have side effects on register pressure/allocation.


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