[PATCH] [ScalarEvolution] Change data structure to memorize computed result

[Wan, Xiaofei]

Hi atrick,

In ScalarEvolution, the computed loop disposition and block disposition are stored in a map to avoid repeated calculation.

/// LoopDispositions - Memoized computeLoopDisposition results.
DenseMap<const SCEV *, std::map<const Loop *, LoopDisposition> > LoopDispositions;
/// BlockDispositions - Memoized computeBlockDisposition results.
DenseMap<const SCEV *, std::map<const BasicBlock *, BlockDisposition> > BlockDispositions;

The profiling data shows It is expensive to look up a result in such two-dimensional map container, even more expensive than calculating them directly each time.

There are two solutions to cost-down the look-up.
1. Usually, a SCEV belongs to very little loops/blocks, so in the upper container, std::map only has 1-2 elements, linear search is faster than map here.
    DenseMap<const SCEV *, SmallVector<std::pair<const Loop *, LoopDisposition>, 2> > LoopDispositions;
    DenseMap<const SCEV *, SmallVector<std::pair<const BasicBlock *, BlockDisposition>, 2> > BlockDispositions;
2. Remove the memorized container and calculate them directly each time.

Both solutions could introduce 1~3% improvement for cases with many scalar expressions (470.lbm in CPUSpec2006); Solution 2 has better improvement than solution1, but I prefer solution 1 since it may benefit some extreme cases.


http://llvm-reviews.chandlerc.com/D2117

Files:
  lib/Analysis/ScalarEvolution.cpp
  include/llvm/Analysis/ScalarEvolution.h

Index: lib/Analysis/ScalarEvolution.cpp
===================================================================
--- lib/Analysis/ScalarEvolution.cpp
+++ lib/Analysis/ScalarEvolution.cpp
@@ -5059,15 +5059,15 @@
 /// original value V is returned.
 const SCEV *ScalarEvolution::getSCEVAtScope(const SCEV *V, const Loop *L) {
   // Check to see if we've folded this expression at this loop before.
-  std::map<const Loop *, const SCEV *> &Values = ValuesAtScopes[V];
-  std::pair<std::map<const Loop *, const SCEV *>::iterator, bool> Pair =
-    Values.insert(std::make_pair(L, static_cast<const SCEV *>(0)));
-  if (!Pair.second)
-    return Pair.first->second ? Pair.first->second : V;
+  SmallVector<std::pair<const Loop *, const SCEV *>, 2> &Values = ValuesAtScopes[V];
+  for (unsigned u = 0; u < Values.size(); u++) {
+    if (Values[u].first == L)
+      return Values[u].second ? Values[u].second : V;
+  }
 
   // Otherwise compute it.
   const SCEV *C = computeSCEVAtScope(V, L);
-  ValuesAtScopes[V][L] = C;
+  ValuesAtScopes[V].push_back(std::make_pair(L, C));
   return C;
 }
 
@@ -6727,7 +6727,7 @@
 //===----------------------------------------------------------------------===//
 
 ScalarEvolution::ScalarEvolution()
-  : FunctionPass(ID), FirstUnknown(0) {
+  : FunctionPass(ID), ValuesAtScopes(64), LoopDispositions(64), BlockDispositions(64), FirstUnknown(0) {
   initializeScalarEvolutionPass(*PassRegistry::getPassRegistry());
 }
 
@@ -6865,14 +6865,14 @@
 
 ScalarEvolution::LoopDisposition
 ScalarEvolution::getLoopDisposition(const SCEV *S, const Loop *L) {
-  std::map<const Loop *, LoopDisposition> &Values = LoopDispositions[S];
-  std::pair<std::map<const Loop *, LoopDisposition>::iterator, bool> Pair =
-    Values.insert(std::make_pair(L, LoopVariant));
-  if (!Pair.second)
-    return Pair.first->second;
-
+  SmallVector<std::pair<const Loop *, LoopDisposition>, 2> &Values = LoopDispositions[S];
+  for (unsigned u = 0; u < Values.size(); u++) {
+    if (Values[u].first == L)
+      return Values[u].second;
+  }
   LoopDisposition D = computeLoopDisposition(S, L);
-  return LoopDispositions[S][L] = D;
+  LoopDispositions[S].push_back(std::make_pair(L, D));
+  return D;
 }
 
 ScalarEvolution::LoopDisposition
@@ -6964,14 +6964,14 @@
 
 ScalarEvolution::BlockDisposition
 ScalarEvolution::getBlockDisposition(const SCEV *S, const BasicBlock *BB) {
-  std::map<const BasicBlock *, BlockDisposition> &Values = BlockDispositions[S];
-  std::pair<std::map<const BasicBlock *, BlockDisposition>::iterator, bool>
-    Pair = Values.insert(std::make_pair(BB, DoesNotDominateBlock));
-  if (!Pair.second)
-    return Pair.first->second;
-
+  SmallVector<std::pair<const BasicBlock *, BlockDisposition>, 2> &Values = BlockDispositions[S];
+  for (unsigned u = 0; u < Values.size(); u++) {
+    if (Values[u].first == BB)
+      return Values[u].second;
+  }
   BlockDisposition D = computeBlockDisposition(S, BB);
-  return BlockDispositions[S][BB] = D;
+  BlockDispositions[S].push_back(std::make_pair(BB, D));
+  return D;
 }
 
 ScalarEvolution::BlockDisposition
Index: include/llvm/Analysis/ScalarEvolution.h
===================================================================
--- include/llvm/Analysis/ScalarEvolution.h
+++ include/llvm/Analysis/ScalarEvolution.h
@@ -361,18 +361,18 @@
     /// that we attempt to compute getSCEVAtScope information for, which can
     /// be expensive in extreme cases.
     DenseMap<const SCEV *,
-             std::map<const Loop *, const SCEV *> > ValuesAtScopes;
+             SmallVector<std::pair<const Loop *, const SCEV *>, 2> > ValuesAtScopes;
 
     /// LoopDispositions - Memoized computeLoopDisposition results.
     DenseMap<const SCEV *,
-             std::map<const Loop *, LoopDisposition> > LoopDispositions;
+             SmallVector<std::pair<const Loop *, LoopDisposition>, 2> > LoopDispositions;
 
     /// computeLoopDisposition - Compute a LoopDisposition value.
     LoopDisposition computeLoopDisposition(const SCEV *S, const Loop *L);
 
     /// BlockDispositions - Memoized computeBlockDisposition results.
     DenseMap<const SCEV *,
-             std::map<const BasicBlock *, BlockDisposition> > BlockDispositions;
+             SmallVector<std::pair<const BasicBlock *, BlockDisposition>, 2> > BlockDispositions;
 
     /// computeBlockDisposition - Compute a BlockDisposition value.
     BlockDisposition computeBlockDisposition(const SCEV *S, const BasicBlock *BB);
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