Only match BMI (BLSR, BLSI, BLSMSK) if the add/sub op is single use
If the add/sub is not single use, it will need to be materialized later, in which case using the BMI instruction is a de-optimization in terms of code-size and throughput. i.e: ``` // Good leal -1(%rdi), %eax andl %eax, %eax xorl %eax, %esi ... ``` ``` // Unecessary BMI (lower throughput, larger code size) leal -1(%rdi), %eax blsr %edi, %eax xorl %eax, %esi ... ``` Note, this may cause more `mov` instructions to be emitted sometimes because BMI instructions only have 1 src and write-only to dst. A better approach may be to only avoid BMI for (and/xor X, (add/sub 0/-1, X)) if this is the last use of X but NOT the last use of (add/sub 0/-1, X). Reviewed By: RKSimon Differential Revision: https://reviews.llvm.org/D141180
Loading
Please sign in to comment