[PATCH] D152706: [AMDGPU] Use SSAUpdater in PromoteAlloca

[Pierre van Houtryve via Phabricator via llvm-commits]

Pierre-vh added inline comments.


================
Comment at: llvm/lib/Target/AMDGPU/AMDGPUPromoteAlloca.cpp:341-358
+  bool HasSeenBB = false;
+  BasicBlock *BB = I->getParent();
+  while (InsertIt != WorkList.end()) {
+    // -> Instruction is in the same basic block. If the new instruction comes
+    // before it, insert here.
+    //
+    // -> Instruction is in a different basic block: If we've already seen the
----------------
arsenm wrote:
> This whole thing feels wrong to me. You shouldn't need to figure out any ordering, much less by using comesBefore repeatedly
I think that the issue was introduced because I'm looking through things such as bitcasts, which means that we can have IR like this:
```
bb1:
  %x = alloca ...
  %cast.x = cast %x to ...

bb2: 
  store ... %x
  store ... %cast.x
```

Then, when I iterate the users of the alloca and build the worklist, the worklist may have the store to %cast.x before the store to %x, so I need to sort the worklist to use SSAAUpdater.

I'm not sure how `LoadStorePromoter` would help, I've looked at it but it seems to be for a different purpose


Repository:
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CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D152706/new/

https://reviews.llvm.org/D152706