[PATCH] D136169: [AMDGPU] Avoid SCC clobbering before S_CSELECT_B32

[Alexander via Phabricator via llvm-commits]

alex-t added inline comments.


================
Comment at: llvm/lib/Target/AMDGPU/SIRegisterInfo.cpp:2216-2224
+          while (I != MBB->end()) {
+            if (*I != MI && I->readsRegister(AMDGPU::SCC)) {
+              NeedSaveSCC = true;
+              break;
+            }
+            if (*I != MI && I->definesRegister(AMDGPU::SCC))
+              break;
----------------
foad wrote:
> What is this loop for? Isn't the scavenger already supposed to know whether SCC is live?
> What is this loop for? Isn't the scavenger already supposed to know whether SCC is live?

The scavenger's knowledge depends on the accurate dead/kill flags insertion. This loop decreases the number of false-positive "live" SCC and accordingly unnecessary SCC saves/restores.

As Matt rightly pointed out, the dead/kill correct placement is out of this change scope.
It also is going to be a complex task.

Should I add a corresponding TODO here?


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D136169/new/

https://reviews.llvm.org/D136169