tree 7673dcb53c81b46a39fb23d77f6d3baedd08235a
parent 94dc7cfcd18b507815b910b3102173323f176b00
author Phoebe Wang <phoebe.wang@intel.com> 1659664714 +0800
committer Copybara-Service <copybara-worker@google.com> 1659666780 -0700

Reland "[X86][MC] Always emit `rep` prefix for `bsf`"

`BMI` new instruction `tzcnt` has better performance than `bsf` on new
processors. Its encoding has a mandatory prefix '0xf3' compared to
`bsf`. If we force emit `rep` prefix for `bsf`, we will gain better
performance when the same code run on new processors.

GCC has already done this way: https://c.godbolt.org/z/6xere6fs1

Fixes #34191

Reviewed By: craig.topper, skan

Differential Revision: https://reviews.llvm.org/D130956

GitOrigin-RevId: 7f648d27a85a98fa077f0968dea081821627d477