[LV] Report multiple reasons for not vectorizing under allowExtraAnalysis

The default behavior of -Rpass-analysis=loop-vectorizer is to report only the
first reason encountered for not vectorizing, if one is found, at which time the
vectorizer aborts its handling of the loop. This patch allows multiple reasons
for not vectorizing to be identified and reported, at the potential expense of
additional compile-time, under allowExtraAnalysis which can currently be turned
on by Clang's -fsave-optimization-record and opt's -pass-remarks-missed.

Removed from LoopVectorizationLegality::canVectorize() the redundant checking
and reporting if we CantComputeNumberOfIterations, as LAI::canAnalyzeLoop() also
does that. This redundancy is caught by a lit test once multiple reasons are
reported.

Patch initially developed by Dror Barak.

Differential Revision: https://reviews.llvm.org/D33396

llvm-svn: 303613

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -5086,12 +5086,18 @@ bool LoopVectorizationLegality::canVectorizeWithIfConvert() {
 }
 
 bool LoopVectorizationLegality::canVectorize() {
+  // Store the result and return it at the end instead of exiting early, in case
+  // allowExtraAnalysis is used to report multiple reasons for not vectorizing.
+  bool Result = true;
   // We must have a loop in canonical form. Loops with indirectbr in them cannot
   // be canonicalized.
   if (!TheLoop->getLoopPreheader()) {
     ORE->emit(createMissedAnalysis("CFGNotUnderstood")
               << "loop control flow is not understood by vectorizer");
-    return false;
+    if (ORE->allowExtraAnalysis())
+      Result = false;
+    else
+      return false;
   }
 
   // FIXME: The code is currently dead, since the loop gets sent to
@@ -5101,21 +5107,30 @@ bool LoopVectorizationLegality::canVectorize() {
   if (!TheLoop->empty()) {
     ORE->emit(createMissedAnalysis("NotInnermostLoop")
               << "loop is not the innermost loop");
-    return false;
+    if (ORE->allowExtraAnalysis())
+      Result = false;
+    else
+      return false;
   }
 
   // We must have a single backedge.
   if (TheLoop->getNumBackEdges() != 1) {
     ORE->emit(createMissedAnalysis("CFGNotUnderstood")
               << "loop control flow is not understood by vectorizer");
-    return false;
+    if (ORE->allowExtraAnalysis())
+      Result = false;
+    else
+      return false;
   }
 
   // We must have a single exiting block.
   if (!TheLoop->getExitingBlock()) {
     ORE->emit(createMissedAnalysis("CFGNotUnderstood")
               << "loop control flow is not understood by vectorizer");
-    return false;
+    if (ORE->allowExtraAnalysis())
+      Result = false;
+    else
+      return false;
   }
 
   // We only handle bottom-tested loops, i.e. loop in which the condition is
@@ -5124,7 +5139,10 @@ bool LoopVectorizationLegality::canVectorize() {
   if (TheLoop->getExitingBlock() != TheLoop->getLoopLatch()) {
     ORE->emit(createMissedAnalysis("CFGNotUnderstood")
               << "loop control flow is not understood by vectorizer");
-    return false;
+    if (ORE->allowExtraAnalysis())
+      Result = false;
+    else
+      return false;
   }
 
   // We need to have a loop header.
@@ -5135,28 +5153,28 @@ bool LoopVectorizationLegality::canVectorize() {
   unsigned NumBlocks = TheLoop->getNumBlocks();
   if (NumBlocks != 1 && !canVectorizeWithIfConvert()) {
     DEBUG(dbgs() << "LV: Can't if-convert the loop.\n");
-    return false;
-  }
-
-  // ScalarEvolution needs to be able to find the exit count.
-  const SCEV *ExitCount = PSE.getBackedgeTakenCount();
-  if (ExitCount == PSE.getSE()->getCouldNotCompute()) {
-    ORE->emit(createMissedAnalysis("CantComputeNumberOfIterations")
-              << "could not determine number of loop iterations");
-    DEBUG(dbgs() << "LV: SCEV could not compute the loop exit count.\n");
-    return false;
+    if (ORE->allowExtraAnalysis())
+      Result = false;
+    else
+      return false;
   }
 
   // Check if we can vectorize the instructions and CFG in this loop.
   if (!canVectorizeInstrs()) {
     DEBUG(dbgs() << "LV: Can't vectorize the instructions or CFG\n");
-    return false;
+    if (ORE->allowExtraAnalysis())
+      Result = false;
+    else
+      return false;
   }
 
   // Go over each instruction and look at memory deps.
   if (!canVectorizeMemory()) {
     DEBUG(dbgs() << "LV: Can't vectorize due to memory conflicts\n");
-    return false;
+    if (ORE->allowExtraAnalysis())
+      Result = false;
+    else
+      return false;
   }
 
   DEBUG(dbgs() << "LV: We can vectorize this loop"
@@ -5184,13 +5202,17 @@ bool LoopVectorizationLegality::canVectorize() {
               << "Too many SCEV assumptions need to be made and checked "
               << "at runtime");
     DEBUG(dbgs() << "LV: Too many SCEV checks needed.\n");
-    return false;
+    if (ORE->allowExtraAnalysis())
+      Result = false;
+    else
+      return false;
   }
 
-  // Okay! We can vectorize. At this point we don't have any other mem analysis
+  // Okay! We've done all the tests. If any have failed, return false. Otherwise
+  // we can vectorize, and at this point we don't have any other mem analysis
   // which may limit our maximum vectorization factor, so just return true with
   // no restrictions.
-  return true;
+  return Result;
 }
 
 static Type *convertPointerToIntegerType(const DataLayout &DL, Type *Ty) {