[llvm] Optimize AggressiveAntiDepBreaker.cpp::StartBlock() (PR #119951)

[via llvm-commits]

https://github.com/xjtuhu created https://github.com/llvm/llvm-project/pull/119951

Hoist BB->size(), it does not make sense to call it in the inner for loop. 
State->UnionGroups(Reg, 0) can be eliminated, because the logic in the constructor of AggressiveAntiDepState is equivalent to unifying all registers to 0.

>From 9fd1e76e34e38481c825c7b4b5cfebfaf01c417a Mon Sep 17 00:00:00 2001
From: Zhengping-amd <luck60 at 126.com>
Date: Sat, 14 Dec 2024 14:25:02 +0800
Subject: [PATCH]  Optimize AggressiveAntiDepBreaker.cpp::StartBlock()

Hoist BB->size() and eliminate State->UnionGroups(Reg, 0) in the inner for loop, because the logic in the constructor of AggressiveAntiDepState is equivalent to unifying all registers to 0.
---
 llvm/lib/CodeGen/AggressiveAntiDepBreaker.cpp | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/llvm/lib/CodeGen/AggressiveAntiDepBreaker.cpp b/llvm/lib/CodeGen/AggressiveAntiDepBreaker.cpp
index e40248197c7c7c..30bbe20d518276 100644
--- a/llvm/lib/CodeGen/AggressiveAntiDepBreaker.cpp
+++ b/llvm/lib/CodeGen/AggressiveAntiDepBreaker.cpp
@@ -152,12 +152,12 @@ void AggressiveAntiDepBreaker::StartBlock(MachineBasicBlock *BB) {
   std::vector<unsigned> &DefIndices = State->GetDefIndices();
 
   // Examine the live-in regs of all successors.
+  unsigned BBSize = BB->size();
   for (MachineBasicBlock *Succ : BB->successors())
     for (const auto &LI : Succ->liveins()) {
       for (MCRegAliasIterator AI(LI.PhysReg, TRI, true); AI.isValid(); ++AI) {
         unsigned Reg = *AI;
-        State->UnionGroups(Reg, 0);
-        KillIndices[Reg] = BB->size();
+        KillIndices[Reg] = BBSize;
         DefIndices[Reg] = ~0u;
       }
     }
@@ -174,8 +174,7 @@ void AggressiveAntiDepBreaker::StartBlock(MachineBasicBlock *BB) {
       continue;
     for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI) {
       unsigned AliasReg = *AI;
-      State->UnionGroups(AliasReg, 0);
-      KillIndices[AliasReg] = BB->size();
+      KillIndices[AliasReg] = BBSize;
       DefIndices[AliasReg] = ~0u;
     }
   }