[PATCH] [LoopAccesses] Add canAnalyzeLoop

[Adam Nemet]

Hi hfinkel, aschwaighofer, nadav,

This allows the analysis to be attempted with any loop.  This feature
will be used with -analysis.  (LV only requests the analysis on loops
that have already satisfied these tests.)

This is part of the patchset that converts LoopAccessAnalysis into an
actual analysis pass.

http://reviews.llvm.org/D7688

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

Index: include/llvm/Analysis/LoopAccessAnalysis.h
===================================================================
--- include/llvm/Analysis/LoopAccessAnalysis.h
+++ include/llvm/Analysis/LoopAccessAnalysis.h
@@ -131,6 +131,10 @@
   /// no memory dependence cycles.
   bool canVectorizeMemory() { return CanVecMem; }
 
+  /// \brief Check if the structure of the loop allows it to be analyzed by this
+  /// pass.
+  bool canAnalyzeLoop();
+
   RuntimePointerCheck *getRuntimePointerCheck() { return &PtrRtCheck; }
 
   /// Return true if the block BB needs to be predicated in order for the loop
Index: lib/Analysis/LoopAccessAnalysis.cpp
===================================================================
--- lib/Analysis/LoopAccessAnalysis.cpp
+++ lib/Analysis/LoopAccessAnalysis.cpp
@@ -829,6 +829,55 @@
   return true;
 }
 
+bool LoopAccessInfo::canAnalyzeLoop() {
+    // We can only vectorize innermost loops.
+  if (!TheLoop->empty()) {
+    emitAnalysis(VectorizationReport() << "loop is not the innermost loop");
+    return false;
+  }
+
+  // We must have a single backedge.
+  if (TheLoop->getNumBackEdges() != 1) {
+    emitAnalysis(
+        VectorizationReport() <<
+        "loop control flow is not understood by vectorizer");
+    return false;
+  }
+
+  // We must have a single exiting block.
+  if (!TheLoop->getExitingBlock()) {
+    emitAnalysis(
+        VectorizationReport() <<
+        "loop control flow is not understood by vectorizer");
+    return false;
+  }
+
+  // We only handle bottom-tested loops, i.e. loop in which the condition is
+  // checked at the end of each iteration. With that we can assume that all
+  // instructions in the loop are executed the same number of times.
+  if (TheLoop->getExitingBlock() != TheLoop->getLoopLatch()) {
+    emitAnalysis(
+        VectorizationReport() <<
+        "loop control flow is not understood by vectorizer");
+    return false;
+  }
+
+  // We need to have a loop header.
+  DEBUG(dbgs() << "LAA: Found a loop: " <<
+        TheLoop->getHeader()->getName() << '\n');
+
+  // ScalarEvolution needs to be able to find the exit count.
+  const SCEV *ExitCount = SE->getBackedgeTakenCount(TheLoop);
+  if (ExitCount == SE->getCouldNotCompute()) {
+    emitAnalysis(VectorizationReport() <<
+                 "could not determine number of loop iterations");
+    DEBUG(dbgs() << "LAA: SCEV could not compute the loop exit count.\n");
+    return false;
+  }
+
+  return true;
+}
+
 void LoopAccessInfo::analyzeLoop(ValueToValueMap &Strides) {
 
   typedef SmallVector<Value*, 16> ValueVector;
@@ -1220,7 +1269,8 @@
 
   if (!LAI) {
     LAI = make_unique<LoopAccessInfo>(L, SE, DL, TLI, AA, DT);
-    LAI->analyzeLoop(Strides);
+    if (LAI->canAnalyzeLoop())
+      LAI->analyzeLoop(Strides);
 #ifndef NDEBUG
     LAI->NumSymbolicStrides = Strides.size();
 #endif

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