[llvm] [GVN] lazily update dominator tree when merge unconditional branches in function (PR #88477)

[via llvm-commits]

https://github.com/Enna1 created https://github.com/llvm/llvm-project/pull/88477

With this change, the wall time for of GVN pass decreased from 873,745.492 ms to 367,375.304 ms in an our internal testcase.

>From 157c8d4cc74f6b3faf17e195a1f58cbc0ec56a6d Mon Sep 17 00:00:00 2001
From: "xumingjie.enna1" <xumingjie.enna1 at bytedance.com>
Date: Fri, 12 Apr 2024 11:04:16 +0800
Subject: [PATCH] [GVN] lazily update dominator tree when merge unconditional
 branches in function

With this change, the wall time for of GVN pass decreased from 873,745.492 ms to 367,375.304 ms in an our internal testcase.
---
 llvm/lib/Transforms/Scalar/GVN.cpp | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/llvm/lib/Transforms/Scalar/GVN.cpp b/llvm/lib/Transforms/Scalar/GVN.cpp
index 67fb2a5da3bb71..86d5c9909f3dc5 100644
--- a/llvm/lib/Transforms/Scalar/GVN.cpp
+++ b/llvm/lib/Transforms/Scalar/GVN.cpp
@@ -2708,7 +2708,7 @@ bool GVNPass::runImpl(Function &F, AssumptionCache &RunAC, DominatorTree &RunDT,
   bool Changed = false;
   bool ShouldContinue = true;
 
-  DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Eager);
+  DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Lazy);
   // Merge unconditional branches, allowing PRE to catch more
   // optimization opportunities.
   for (BasicBlock &BB : llvm::make_early_inc_range(F)) {
@@ -2718,6 +2718,7 @@ bool GVNPass::runImpl(Function &F, AssumptionCache &RunAC, DominatorTree &RunDT,
 
     Changed |= removedBlock;
   }
+  DTU.flush();
 
   unsigned Iteration = 0;
   while (ShouldContinue) {