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

diff --git a/llvm/lib/Transforms/IPO/GlobalDCE.cpp b/llvm/lib/Transforms/IPO/GlobalDCE.cpp
index 0c844fe..ba04c80 100644 (file)
--- a/llvm/lib/Transforms/IPO/GlobalDCE.cpp
+++ b/llvm/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)) {