From c3999ab5f3f0dc6031dc0e46b5f5a00df88e917e Mon Sep 17 00:00:00 2001
From: Peter Collingbourne <peter@pcc.me.uk>
Date: Thu, 19 Mar 2015 18:23:29 +0000
Subject: [PATCH] GlobalDCE: Improve performance for large modules containing
 comdats.

When we encounter a global with a comdat, rather than iterating over
every global in the module to find globals in the same comdat, store the
members in a multimap. This effectively lowers the complexity to O(N log N),
improving performance significantly for large modules such as might be
encountered during LTO.

It looks like we used to do something like this until r219191.

No functional change.

Differential Revision: http://reviews.llvm.org/D8431

llvm-svn: 232743
---
 lib/Transforms/IPO/GlobalDCE.cpp | 26 ++++++++++++++++----------
 1 file changed, 16 insertions(+), 10 deletions(-)

diff --git a/lib/Transforms/IPO/GlobalDCE.cpp b/lib/Transforms/IPO/GlobalDCE.cpp
index 0c844fe7065..ba04c80508c 100644
--- a/lib/Transforms/IPO/GlobalDCE.cpp
+++ b/lib/Transforms/IPO/GlobalDCE.cpp
@@ -24,6 +24,7 @@
 #include "llvm/Transforms/Utils/CtorUtils.h"
 #include "llvm/Transforms/Utils/GlobalStatus.h"
 #include "llvm/Pass.h"
+#include <unordered_map>
 using namespace llvm;
 
 #define DEBUG_TYPE "globaldce"
@@ -47,6 +48,7 @@ namespace {
   private:
     SmallPtrSet<GlobalValue*, 32> AliveGlobals;
     SmallPtrSet<Constant *, 8> SeenConstants;
+    std::unordered_multimap<Comdat *, GlobalValue *> ComdatMembers;
 
     /// GlobalIsNeeded - mark the specific global value as needed, and
     /// recursively mark anything that it uses as also needed.
@@ -78,6 +80,17 @@ bool GlobalDCE::runOnModule(Module &M) {
   // Remove empty functions from the global ctors list.
   Changed |= optimizeGlobalCtorsList(M, isEmptyFunction);
 
+  // Collect the set of members for each comdat.
+  for (Function &F : M)
+    if (Comdat *C = F.getComdat())
+      ComdatMembers.insert(std::make_pair(C, &F));
+  for (GlobalVariable &GV : M.globals())
+    if (Comdat *C = GV.getComdat())
+      ComdatMembers.insert(std::make_pair(C, &GV));
+  for (GlobalAlias &GA : M.aliases())
+    if (Comdat *C = GA.getComdat())
+      ComdatMembers.insert(std::make_pair(C, &GA));
+
   // Loop over the module, adding globals which are obviously necessary.
   for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) {
     Changed |= RemoveUnusedGlobalValue(*I);
@@ -177,6 +190,7 @@ bool GlobalDCE::runOnModule(Module &M) {
   // Make sure that all memory is released
   AliveGlobals.clear();
   SeenConstants.clear();
+  ComdatMembers.clear();
 
   return Changed;
 }
@@ -188,17 +202,9 @@ void GlobalDCE::GlobalIsNeeded(GlobalValue *G) {
   if (!AliveGlobals.insert(G).second)
     return;
 
-  Module *M = G->getParent();
   if (Comdat *C = G->getComdat()) {
-    for (Function &F : *M)
-      if (F.getComdat() == C)
-        GlobalIsNeeded(&F);
-    for (GlobalVariable &GV : M->globals())
-      if (GV.getComdat() == C)
-        GlobalIsNeeded(&GV);
-    for (GlobalAlias &GA : M->aliases())
-      if (GA.getComdat() == C)
-        GlobalIsNeeded(&GA);
+    for (auto &&CM : make_range(ComdatMembers.equal_range(C)))
+      GlobalIsNeeded(CM.second);
   }
 
   if (GlobalVariable *GV = dyn_cast<GlobalVariable>(G)) {