[llvm] [AMDGPU] Simplify WaitcntBrackets::getRegInterval with getPhysRegBaseClass (PR #74087)

[Jay Foad via llvm-commits]

jayfoad wrote:

> You should absolutely try to make use of the MCInstrDesc. Looking up the physical register class is extremely expensive. At one point the waitcnt pass was one of the most expensive backend passes, spending most of its time looking up physical register classes

That is the whole point of getPhysRegBaseClass - it is just two table lookups so should be fast: https://reviews.llvm.org/D139616

https://github.com/llvm/llvm-project/pull/74087