[all-commits] [llvm/llvm-project] cd4007: [RegisterPressure] NFC: Clean up RP handling for i...

[Jeffrey Byrnes via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: cd40070bfc77ab2641e69d3e749206ee5c5bbb42
      https://github.com/llvm/llvm-project/commit/cd40070bfc77ab2641e69d3e749206ee5c5bbb42
  Author: Jeffrey Byrnes <jeffrey.byrnes at amd.com>
  Date:   2024-10-01 (Tue, 01 Oct 2024)

  Changed paths:
    M llvm/lib/CodeGen/RegisterPressure.cpp

  Log Message:
  -----------
  [RegisterPressure] NFC: Clean up RP handling for instructions with overlapping Def/Use (#109875)

The current RP handling for uses of an MI that overlap with defs is
confusing and unnecessary. Moreover, the lane masks do not accurately
model the liveness behavior of the subregs. This cleans things up a bit
and more accurately models subreg lane liveness by sinking the use
handling into subsent Uses loop.

The effect of this PR is to replace

A. `increaseRegPressure(Reg, LiveAfter, ~LiveAfter & LiveBefore)`

with 

B. `increaseRegPressure(Reg, LiveAfter, LiveBefore)`

Note that A (Defs loop) and B (Uses loop) have different definitions of
LiveBefore

A. `LiveBefore = (LiveAfter & ~DefLanes) | UseLanes`

and 

B. `LiveBefore =  LiveAfter | UseLanes`

Also note, `increaseRegPressure` will exit if `PrevMask` (`LiveAfter`
for both A/B) has any active lanes, thus these calls will only have an
effect if `LiveAfter` is 0.


A. NewMask = ~LiveAfter & ((LiveAfter & ~DefLanes) | UseLanes) => (1 &
UseLanes) => UseLanes = (0 | UseLanes) => (LiveAfter | UseLanes) =
NewMask B.



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