[PATCH] D78206: [Target][ARM] Make Low Overhead Loops coexist with VPT blocks

[Sam Parker via Phabricator via llvm-commits]

samparker added inline comments.


================
Comment at: llvm/lib/Target/ARM/ARMLowOverheadLoops.cpp:263
+          if (Def->getParent() == MI->getParent())
+            if (getVPTInstrPredicate(*Def) == ARMVCC::None)
+              return false;
----------------
I feel like this will only be safe if we're still guaranteeing that any predicated instruction is predicated upon the VCTP. I'm concerned about some code that may look llike this:


```
loop.ph:
  a = ...
  b = ...
  c = ...
  n = ...
  z = ...
  DLS

loop:
  VCTP
  VPTT z, n
  VSTRT
  VPTTTT a, b
  VLDRT d
  VLDRT e
  VADDT z, e, d
  LE loop
```

Is it possible now to have a VCTP in the loop with nothing actually predicated upon it?


================
Comment at: llvm/lib/Target/ARM/ARMLowOverheadLoops.cpp:859
 
-  // Start a new vpt block when we discover a vpt.
-  if (MI->getOpcode() == ARM::MVE_VPST) {
     VPTBlocks.emplace_back(MI, CurrentPredicate);
     CurrentBlock = &VPTBlocks.back();
----------------
I doubt CurrentPredicate is correct here as a VPT would be generating its own, maybe it should be cleared..?


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D78206/new/

https://reviews.llvm.org/D78206