[AArch64] Run CSE after conversion from flag-setting variants to non-flag setting variants

[AZero13]

During comparison optimization, many subs become sub, which is good. However, this means that now there are new cse opportunities that can be exploited because now, NZCV is no longer a use in those instructions.

This will come into play more as I set ADDS to be the default rather than add for CSE purposes.

[llvmbot]

@llvm/pr-subscribers-backend-aarch64

Author: AZero13 (AZero13)

Changes

During comparison optimization, many subs become sub, which is good. However, this means that now there are new cse opportunities that can be exploited because now, NZCV is no longer a use in those instructions.

This will come into play more as I set ADDS to be the default rather than add for CSE purposes.

---

Full diff: https://github.com/llvm/llvm-project/pull/153316.diff

1 Files Affected:

• (modified) llvm/lib/Target/AArch64/AArch64TargetMachine.cpp (+6)

diff --git a/llvm/lib/Target/AArch64/AArch64TargetMachine.cpp b/llvm/lib/Target/AArch64/AArch64TargetMachine.cpp
index 95eab16511e5a..3e44d9efe7ed3 100644
--- a/llvm/lib/Target/AArch64/AArch64TargetMachine.cpp
+++ b/llvm/lib/Target/AArch64/AArch64TargetMachine.cpp
@@ -790,6 +790,12 @@ void AArch64PassConfig::addMachineSSAOptimization() {
 
   if (TM->getOptLevel() != CodeGenOptLevel::None)
     addPass(createAArch64MIPeepholeOptPass());
+
+  // Clean-up any last code that can be eliminated
+  // Due to the fact that the demotion of some instructions
+  // can result in the removal of instructions previously unable to be removed
+  addPass(&MachineCSELegacyID);
+  addPass(&DeadMachineInstructionElimID);
 }
 
 bool AArch64PassConfig::addILPOpts() {