[GlobalDCE] Use DenseMap instead of unordered_multimap for GVDependencies.

Summary:
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.

Reviewers: mehdi_amini, serge-sans-paille, davide

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D38916

llvm-svn: 316045

1 files changed, 1 insertions, 1 deletions

diff --git a/llvm/include/llvm/Transforms/IPO/GlobalDCE.h b/llvm/include/llvm/Transforms/IPO/GlobalDCE.h
index 9ca939c15b6..7ca241f4645 100644
--- a/llvm/include/llvm/Transforms/IPO/GlobalDCE.h
+++ b/llvm/include/llvm/Transforms/IPO/GlobalDCE.h
@@ -35,7 +35,7 @@ private:
   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>>