[llvm] Perform mergefunc after constmerge (PR #92498)

[YAMAMOTO Takashi via llvm-commits]

https://github.com/yamt created https://github.com/llvm/llvm-project/pull/92498

Constmerge can fold switch jump tables, possibly making functions identical again. It can help mergefunc.
On the otherhand, the opposite seems unlikely.

Fixes https://github.com/llvm/llvm-project/issues/92201

>From ebc89c7f173c05c5c3a6e0f88b6854b984ad4a46 Mon Sep 17 00:00:00 2001
From: YAMAMOTO Takashi <yamamoto at midokura.com>
Date: Fri, 17 May 2024 14:31:17 +0900
Subject: [PATCH] Perform mergefunc after constmerge

Constmerge can fold switch jump tables, possibly making functions
identical again. It can help mergefunc.
On the otherhand, the opposite seems unlikely.

Fixes https://github.com/llvm/llvm-project/issues/92201
---
 llvm/lib/Passes/PassBuilderPipelines.cpp | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/llvm/lib/Passes/PassBuilderPipelines.cpp b/llvm/lib/Passes/PassBuilderPipelines.cpp
index 1892e16a06528..bc7be57284ad8 100644
--- a/llvm/lib/Passes/PassBuilderPipelines.cpp
+++ b/llvm/lib/Passes/PassBuilderPipelines.cpp
@@ -1525,10 +1525,6 @@ PassBuilder::buildModuleOptimizationPipeline(OptimizationLevel Level,
   if (EnableIROutliner)
     MPM.addPass(IROutlinerPass());
 
-  // Merge functions if requested.
-  if (PTO.MergeFunctions)
-    MPM.addPass(MergeFunctionsPass());
-
   // Now we need to do some global optimization transforms.
   // FIXME: It would seem like these should come first in the optimization
   // pipeline and maybe be the bottom of the canonicalization pipeline? Weird
@@ -1536,6 +1532,11 @@ PassBuilder::buildModuleOptimizationPipeline(OptimizationLevel Level,
   MPM.addPass(GlobalDCEPass());
   MPM.addPass(ConstantMergePass());
 
+  // Merge functions if requested. It has a better chance to merge functions
+  // after ConstantMerge folded jump tables.
+  if (PTO.MergeFunctions)
+    MPM.addPass(MergeFunctionsPass());
+
   if (PTO.CallGraphProfile && !LTOPreLink)
     MPM.addPass(CGProfilePass(LTOPhase == ThinOrFullLTOPhase::FullLTOPostLink ||
                               LTOPhase == ThinOrFullLTOPhase::ThinLTOPostLink));