[PATCH] D38916: [GlobalDCE] Use DenseMap instead of unordered_multimap for GVDependencies.

[Michael Zolotukhin via Phabricator via llvm-commits]

mzolotukhin created this revision.

std::unordered_multimap happens to be very slow when the number of elements
grows large. On one of our internal applications we observed a 17x compile time
improvement from changing it to DenseMap.


https://reviews.llvm.org/D38916

Files:
  include/llvm/Transforms/IPO/GlobalDCE.h
  lib/Transforms/IPO/GlobalDCE.cpp


Index: lib/Transforms/IPO/GlobalDCE.cpp
===================================================================
--- lib/Transforms/IPO/GlobalDCE.cpp
+++ lib/Transforms/IPO/GlobalDCE.cpp
@@ -115,7 +115,7 @@
     ComputeDependencies(User, Deps);
   Deps.erase(&GV); // Remove self-reference.
   for (GlobalValue *GVU : Deps) {
-    GVDependencies.insert(std::make_pair(GVU, &GV));
+    GVDependencies[GVU].insert(&GV);
   }
 }
 
@@ -199,8 +199,8 @@
                                            AliveGlobals.end()};
   while (!NewLiveGVs.empty()) {
     GlobalValue *LGV = NewLiveGVs.pop_back_val();
-    for (auto &&GVD : make_range(GVDependencies.equal_range(LGV)))
-      MarkLive(*GVD.second, &NewLiveGVs);
+    for (auto *GVD : GVDependencies[LGV])
+      MarkLive(*GVD, &NewLiveGVs);
   }
 
   // Now that all globals which are needed are in the AliveGlobals set, we loop
Index: include/llvm/Transforms/IPO/GlobalDCE.h
===================================================================
--- include/llvm/Transforms/IPO/GlobalDCE.h
+++ include/llvm/Transforms/IPO/GlobalDCE.h
@@ -35,7 +35,7 @@
   SmallPtrSet<GlobalValue*, 32> AliveGlobals;
 
   /// Global -> Global that uses this global.
-  std::unordered_multimap<GlobalValue *, GlobalValue *> GVDependencies;
+  DenseMap<GlobalValue *, SmallPtrSet<GlobalValue *, 4>> GVDependencies;
 
   /// Constant -> Globals that use this global cache.
   std::unordered_map<Constant *, SmallPtrSet<GlobalValue *, 8>>


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