[PATCH] D105192: [AMDGPU] Optimize VGPR LiveRange in waterfall loops

[Ruiling, Song via Phabricator via llvm-commits]

ruiling added inline comments.


================
Comment at: llvm/lib/Target/AMDGPU/SIOptimizeVGPRLiveRange.cpp:309
+      if (MO.readsReg() && IndRegs.count(MOReg) == 0) {
+        // Do not collect the induction variable
+        if (MI.getOpcode() == AMDGPU::V_READFIRSTLANE_B32) {
----------------
why the source operand of v_readfirstlane cannot be optimized?


================
Comment at: llvm/lib/Target/AMDGPU/SIOptimizeVGPRLiveRange.cpp:507
+  // The optimized Reg is not alive through the Loop block anymore.
+  OldVarInfo.AliveBlocks.reset(Loop->getNumber());
+  NewVarInfo.AliveBlocks.set(Loop->getNumber());
----------------
As you just replace the uses of OldReg within the waterfall loop, the OldReg may still be possibly alive in the waterfall loop. Consider the cases:
1. OldReg defined outside of a outer_loop that encloses waterfall loop.

```
def(OldReg)

outer_loop:
  use(OldReg)   // a use here require us to keep the OldReg alive through the whole range of outer_loop.
  br waterfall_loop

waterfall_loop:
  ...
  brcond waterfall_loop, outer_loop_end_bb

outer_loop_end_bb:
  ...
  brcond outer_loop
```

2. the OldReg is still used after the waterfall loop:

```
  def(OldReg)
  ...

waterfall_loop:
  ...
  brcond waterfall_loop

after_waterfall_loop_bb:
  use(OldReg)
```

In the above two listed cases, the OldReg should still be alive through the waterfall loop.
I think we cannot reuse the physical register of OldReg in the waterfall loop. Or did I miss something?



Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D105192/new/

https://reviews.llvm.org/D105192