PR14448 - prevent the loop vectorizer from vectorizing the same loop twice.

The LoopVectorizer often runs multiple times on the same function due to inlining.
When this happens the loop vectorizer often vectorizes the same loops multiple times, increasing code size and adding unneeded branches.
With this patch, the vectorizer during vectorization puts metadata on scalar loops and marks them as 'already vectorized' so that it knows to ignore them when it sees them a second time.

PR14448.

llvm-svn: 176399

1 files changed, 18 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
index ef9c7c9db2e..0d113728084 100644
--- a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
+++ b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
@@ -116,6 +116,12 @@ static const unsigned TinyTripCountUnrollThreshold = 128;
 /// number of pointers. Notice that the check is quadratic!
 static const unsigned RuntimeMemoryCheckThreshold = 4;
 
+/// We use a metadata with this name  to indicate that a scalar loop was
+/// vectorized and that we don't need to re-vectorize it if we run into it
+/// again.
+static const char*
+AlreadyVectorizedMDName = "llvm.vectorizer.already_vectorized";
+
 namespace {
 
 // Forward declarations.
@@ -1159,6 +1165,11 @@ InnerLoopVectorizer::createEmptyLoop(LoopVectorizationLegality *Legal) {
   BasicBlock *ExitBlock = OrigLoop->getExitBlock();
   assert(ExitBlock && "Must have an exit block");
 
+  // Mark the old scalar loop with metadata that tells us not to vectorize this
+  // loop again if we run into it.
+  MDNode *MD = MDNode::get(OldBasicBlock->getContext(), ArrayRef<Value*>());
+  OldBasicBlock->getTerminator()->setMetadata(AlreadyVectorizedMDName, MD);
+
   // Some loops have a single integer induction variable, while other loops
   // don't. One example is c++ iterators that often have multiple pointer
   // induction variables. In the code below we also support a case where we
@@ -2224,6 +2235,13 @@ bool LoopVectorizationLegality::canVectorizeInstrs() {
   BasicBlock *PreHeader = TheLoop->getLoopPreheader();
   BasicBlock *Header = TheLoop->getHeader();
 
+  // If we marked the scalar loop as "already vectorized" then no need
+  // to vectorize it again.
+  if (Header->getTerminator()->getMetadata(AlreadyVectorizedMDName)) {
+    DEBUG(dbgs() << "LV: This loop was vectorized before\n");
+    return false;
+  }
+
   // For each block in the loop.
   for (Loop::block_iterator bb = TheLoop->block_begin(),
        be = TheLoop->block_end(); bb != be; ++bb) {