[PATCH] D51715: [LICM] Avoid duplicate work during building AliasSetTracker

[Serguei Katkov via Phabricator via llvm-commits]

skatkov created this revision.
skatkov added reviewers: mkazantsev, reames, kariddi, anna.

Currently we re-use cached info from sub loops or traverse them
to populate AliasSetTracker. But after that we traverse all basic blocks
from the current loop. This is redundant work.

All what we need is traversing the all basic blocks from the loop except
those which are used to get the data from the cache.

This should improve compile time only.


https://reviews.llvm.org/D51715

Files:
  lib/Transforms/Scalar/LICM.cpp


Index: lib/Transforms/Scalar/LICM.cpp
===================================================================
--- lib/Transforms/Scalar/LICM.cpp
+++ lib/Transforms/Scalar/LICM.cpp
@@ -1542,21 +1542,17 @@
 LoopInvariantCodeMotion::collectAliasInfoForLoop(Loop *L, LoopInfo *LI,
                                                  AliasAnalysis *AA) {
   std::unique_ptr<AliasSetTracker> CurAST;
-  SmallVector<Loop *, 4> RecomputeLoops;
-  auto mergeLoop = [&CurAST](Loop *L) {
-    // Loop over the body of this loop, looking for calls, invokes, and stores.
-    for (BasicBlock *BB : L->blocks())
-      CurAST->add(*BB); // Incorporate the specified basic block
-  };
+  DenseSet<BasicBlock *> HandledBBs;
+
+  // Try to re-use cached info from sub loops.
   for (Loop *InnerL : L->getSubLoops()) {
     auto MapI = LoopToAliasSetMap.find(InnerL);
     // If the AST for this inner loop is missing it may have been merged into
     // some other loop's AST and then that loop unrolled, and so we need to
     // recompute it.
-    if (MapI == LoopToAliasSetMap.end()) {
-      RecomputeLoops.push_back(InnerL);
+    if (MapI == LoopToAliasSetMap.end())
       continue;
-    }
+
     std::unique_ptr<AliasSetTracker> InnerAST = std::move(MapI->second);
 
     if (CurAST) {
@@ -1568,16 +1564,17 @@
       CurAST = std::move(InnerAST);
     }
     LoopToAliasSetMap.erase(MapI);
+    // Inner loop has been handled, so no need to process its BBs again.
+    HandledBBs.insert(InnerL->block_begin(), InnerL->block_end());
   }
   if (!CurAST)
     CurAST = make_unique<AliasSetTracker>(*AA);
 
-  // Add everything from the sub loops that are no longer directly available.
-  for (Loop *InnerL : RecomputeLoops)
-    mergeLoop(InnerL);
-
-  // And merge in this loop.
-  mergeLoop(L);
+  // Loop over the body of this loop, looking for calls, invokes, and stores.
+  // Do not traverse already handled BBs.
+  for (BasicBlock *BB : L->blocks())
+    if (!HandledBBs.count(BB))
+      CurAST->add(*BB); // Incorporate the specified basic block
 
   return CurAST;
 }


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