[PATCH] D27690: [LV] Don't vectorize when we have a static bound on trip count

[Michael Kuperstein via Phabricator via llvm-commits]

mkuper created this revision.
mkuper added reviewers: mssimpso, gilr.
mkuper added a subscriber: llvm-commits.
Herald added a subscriber: mzolotukhin.

We check if the exact trip count is known and is smaller than the "tiny loop" bound. We should be checking the maximum bound on the trip count instead.

Note that right now this probably won't do a lot of good, since getSmallConstantMaxTripCount() seems to choke on the interesting cases, I hope to fix this in follow-ups.


https://reviews.llvm.org/D27690

Files:
  lib/Transforms/Vectorize/LoopVectorize.cpp
  test/Transforms/LoopVectorize/small-loop.ll


Index: test/Transforms/LoopVectorize/small-loop.ll
===================================================================
--- test/Transforms/LoopVectorize/small-loop.ll
+++ test/Transforms/LoopVectorize/small-loop.ll
@@ -30,3 +30,28 @@
   ret void
 }
 
+;CHECK-LABEL: @bound1(
+;CHECK-NOT: load <4 x i32>
+;CHECK: ret void
+define void @bound1(i32 %k) nounwind uwtable ssp {
+  br label %1
+
+; <label>:1                                       ; preds = %1, %0
+  %indvars.iv = phi i64 [ 0, %0 ], [ %indvars.iv.next, %1 ]
+  %2 = getelementptr inbounds [2048 x i32], [2048 x i32]* @b, i64 0, i64 %indvars.iv
+  %3 = load i32, i32* %2, align 4
+  %4 = getelementptr inbounds [2048 x i32], [2048 x i32]* @c, i64 0, i64 %indvars.iv
+  %5 = load i32, i32* %4, align 4
+  %6 = add nsw i32 %5, %3
+  %7 = getelementptr inbounds [2048 x i32], [2048 x i32]* @a, i64 0, i64 %indvars.iv
+  store i32 %6, i32* %7, align 4
+  %indvars.iv.next = add i64 %indvars.iv, 1
+  %lftr.wideiv = trunc i64 %indvars.iv.next to i32
+  %large = icmp sge i32 %lftr.wideiv, 8
+  %exitcond = icmp eq i32 %lftr.wideiv, %k
+  %realexit = or i1 %large, %exitcond 
+  br i1 %realexit, label %8, label %1
+
+; <label>:8                                       ; preds = %1
+  ret void
+}
Index: lib/Transforms/Vectorize/LoopVectorize.cpp
===================================================================
--- lib/Transforms/Vectorize/LoopVectorize.cpp
+++ lib/Transforms/Vectorize/LoopVectorize.cpp
@@ -7382,8 +7382,8 @@
 
   // Check the loop for a trip count threshold:
   // do not vectorize loops with a tiny trip count.
-  const unsigned TC = SE->getSmallConstantTripCount(L);
-  if (TC > 0u && TC < TinyTripCountVectorThreshold) {
+  const unsigned MaxTC = SE->getSmallConstantMaxTripCount(L);
+  if (MaxTC > 0u && MaxTC < TinyTripCountVectorThreshold) {
     DEBUG(dbgs() << "LV: Found a loop with a very small trip count. "
                  << "This loop is not worth vectorizing.");
     if (Hints.getForce() == LoopVectorizeHints::FK_Enabled)


-------------- next part --------------
A non-text attachment was scrubbed...
Name: D27690.81155.patch
Type: text/x-patch
Size: 2017 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20161212/b4ebe565/attachment.bin>