LoopVectorize: MaxVF should not be larger than the loop trip count

Summary:
Improve how MaxVF is computed while taking into account that MaxVF should not be larger than the loop's trip count.

Other than saving on compile-time by pruning the possible MaxVF candidates, this patch fixes pr34438 which exposed the following flow:
1. Short trip count identified -> Don't bail out, set OptForSize:=True to avoid tail-loop and runtime checks.
2. Compute MaxVF returned 16 on a target supporting AVX512.
3. OptForSize -> choose VF:=MaxVF.
4. Bail out because TripCount = 8, VF = 16, TripCount % VF !=0 means we need a tail loop.

With this patch step 2. will choose MaxVF=8 based on TripCount.

Reviewers: Ayal, dorit, mkuper, hfinkel

Reviewed By: hfinkel

Subscribers: hfinkel, llvm-commits

Differential Revision: https://reviews.llvm.org/D37425

llvm-svn: 312472

1 files changed, 8 insertions, 4 deletions

diff --git a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
index d7fdb666880..fac76ba643c 100644
--- a/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
+++ b/llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
@@ -1960,7 +1960,7 @@ public:
 private:
   /// \return An upper bound for the vectorization factor, larger than zero.
   /// One is returned if vectorization should best be avoided due to cost.
-  unsigned computeFeasibleMaxVF(bool OptForSize);
+  unsigned computeFeasibleMaxVF(bool OptForSize, unsigned ConstTripCount = 0);
 
   /// The vectorization cost is a combination of the cost itself and a boolean
   /// indicating whether any of the contributing operations will actually
@@ -6187,7 +6187,7 @@ Optional<unsigned> LoopVectorizationCostModel::computeMaxVF(bool OptForSize) {
     return None;
   }
 
-  unsigned MaxVF = computeFeasibleMaxVF(OptForSize);
+  unsigned MaxVF = computeFeasibleMaxVF(OptForSize, TC);
 
   if (TC % MaxVF != 0) {
     // If the trip count that we found modulo the vectorization factor is not
@@ -6208,7 +6208,9 @@ Optional<unsigned> LoopVectorizationCostModel::computeMaxVF(bool OptForSize) {
   return MaxVF;
 }
 
-unsigned LoopVectorizationCostModel::computeFeasibleMaxVF(bool OptForSize) {
+unsigned
+LoopVectorizationCostModel::computeFeasibleMaxVF(bool OptForSize,
+                                                 unsigned ConstTripCount) {
   MinBWs = computeMinimumValueSizes(TheLoop->getBlocks(), *DB, &TTI);
   unsigned SmallestType, WidestType;
   std::tie(SmallestType, WidestType) = getSmallestAndWidestTypes();
@@ -6237,7 +6239,9 @@ unsigned LoopVectorizationCostModel::computeFeasibleMaxVF(bool OptForSize) {
   if (MaxVectorSize == 0) {
     DEBUG(dbgs() << "LV: The target has no vector registers.\n");
     MaxVectorSize = 1;
-  }
+  } else if (ConstTripCount && ConstTripCount < MaxVectorSize &&
+             isPowerOf2_32(ConstTripCount))
+    MaxVectorSize = ConstTripCount;
 
   assert(MaxVectorSize <= 64 && "Did not expect to pack so many elements"
                                 " into one vector!");