[LV] Fix bug when unrolling (only) a loop with non-latch exit

If we unroll a loop in the vectorizer (without vectorizing), and the cost model requires a epilogue be generated for correctness, the code generation must actually do so.

The included test case on an unmodified opt will access memory one past the expected bound.  As a result, this patch is fixing a latent miscompile.

Differential Revision: https://reviews.llvm.org/D103700

Author: preames

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -1566,14 +1566,14 @@ class LoopVectorizationCostModel {
 
   /// Returns true if we're required to use a scalar epilogue for at least
   /// the final iteration of the original loop.
-  bool requiresScalarEpilogue() const {
+  bool requiresScalarEpilogue(ElementCount VF) const {
     if (!isScalarEpilogueAllowed())
       return false;
     // If we might exit from anywhere but the latch, must run the exiting
     // iteration in scalar form.
     if (TheLoop->getExitingBlock() != TheLoop->getLoopLatch())
       return true;
-    return InterleaveInfo.requiresScalarEpilogue();
+    return VF.isVector() && InterleaveInfo.requiresScalarEpilogue();
   }
 
   /// Returns true if a scalar epilogue is not allowed due to optsize or a
@@ -3181,18 +3181,13 @@ Value *InnerLoopVectorizer::getOrCreateVectorTripCount(Loop *L) {
   // unroll factor (number of SIMD instructions).
   Value *R = Builder.CreateURem(TC, Step, "n.mod.vf");
 
-  // There are two cases where we need to ensure (at least) the last iteration
-  // runs in the scalar remainder loop. Thus, if the step evenly divides
-  // the trip count, we set the remainder to be equal to the step. If the step
-  // does not evenly divide the trip count, no adjustment is necessary since
-  // there will already be scalar iterations. Note that the minimum iterations
-  // check ensures that N >= Step. The cases are:
-  // 1) If there is a non-reversed interleaved group that may speculatively
-  //    access memory out-of-bounds.
-  // 2) If any instruction may follow a conditionally taken exit. That is, if
-  //    the loop contains multiple exiting blocks, or a single exiting block
-  //    which is not the latch.
-  if (VF.isVector() && Cost->requiresScalarEpilogue()) {
+  // There are cases where we *must* run at least one iteration in the remainder
+  // loop.  See the cost model for when this can happen.  If the step evenly
+  // divides the trip count, we set the remainder to be equal to the step. If
+  // the step does not evenly divide the trip count, no adjustment is necessary
+  // since there will already be scalar iterations. Note that the minimum
+  // iterations check ensures that N >= Step.
+  if (Cost->requiresScalarEpilogue(VF)) {
     auto *IsZero = Builder.CreateICmpEQ(R, ConstantInt::get(R->getType(), 0));
     R = Builder.CreateSelect(IsZero, Step, R);
   }
@@ -3246,8 +3241,8 @@ void InnerLoopVectorizer::emitMinimumIterationCountCheck(Loop *L,
   // vector trip count is zero. This check also covers the case where adding one
   // to the backedge-taken count overflowed leading to an incorrect trip count
   // of zero. In this case we will also jump to the scalar loop.
-  auto P = Cost->requiresScalarEpilogue() ? ICmpInst::ICMP_ULE
-                                          : ICmpInst::ICMP_ULT;
+  auto P = Cost->requiresScalarEpilogue(VF) ? ICmpInst::ICMP_ULE
+                                            : ICmpInst::ICMP_ULT;
 
   // If tail is to be folded, vector loop takes care of all iterations.
   Value *CheckMinIters = Builder.getFalse();
@@ -8323,8 +8318,8 @@ BasicBlock *EpilogueVectorizerMainLoop::emitMinimumIterationCountCheck(
 
   // Generate code to check if the loop's trip count is less than VF * UF of the
   // main vector loop.
-  auto P =
-      Cost->requiresScalarEpilogue() ? ICmpInst::ICMP_ULE : ICmpInst::ICMP_ULT;
+  auto P = Cost->requiresScalarEpilogue(ForEpilogue ? EPI.EpilogueVF : VF) ?
+      ICmpInst::ICMP_ULE : ICmpInst::ICMP_ULT;
 
   Value *CheckMinIters = Builder.CreateICmp(
       P, Count, ConstantInt::get(Count->getType(), VFactor * UFactor),
@@ -8467,8 +8462,8 @@ EpilogueVectorizerEpilogueLoop::emitMinimumVectorEpilogueIterCountCheck(
 
   // Generate code to check if the loop's trip count is less than VF * UF of the
   // vector epilogue loop.
-  auto P =
-      Cost->requiresScalarEpilogue() ? ICmpInst::ICMP_ULE : ICmpInst::ICMP_ULT;
+  auto P = Cost->requiresScalarEpilogue(EPI.EpilogueVF) ?
+      ICmpInst::ICMP_ULE : ICmpInst::ICMP_ULT;
 
   Value *CheckMinIters = Builder.CreateICmp(
       P, Count,

llvm/test/Transforms/LoopVectorize/unroll_nonlatch.ll

@@ -2,11 +2,11 @@
 ; RUN: opt %s -S -loop-vectorize -force-vector-interleave=2 | FileCheck %s
 
 ; Demonstrate a case where we unroll a loop, but don't vectorize it.
-; This currently reveals a miscompile.  The original loop runs stores in
-; the latch block on iterations 0 to 1022, and exits when %indvars.iv = 1023.
-; Currently, the unrolled loop produced by the vectorizer runs the iteration
-; where %indvar.iv = 1023 in the vector.body loop before exiting.  This results
-; in an out of bounds access..
+; The original loop runs stores in the latch block on iterations 0 to 1022,
+; and exits when %indvars.iv = 1023. (That is, it actually runs the stores
+; for an odd number of iterations.)  If we unroll by two in the "vector.body"
+; loop, we must exit to the epilogue on iteration with %indvars.iv = 1022 to
+; avoid an out of bounds access.
 
 define void @test(double* %data) {
 ; CHECK-LABEL: @test(
@@ -31,13 +31,13 @@ define void @test(double* %data) {
 ; CHECK-NEXT:    store double [[TMP8]], double* [[TMP4]], align 8
 ; CHECK-NEXT:    store double [[TMP9]], double* [[TMP5]], align 8
 ; CHECK-NEXT:    [[INDEX_NEXT]] = add nuw i64 [[INDEX]], 2
-; CHECK-NEXT:    [[TMP10:%.*]] = icmp eq i64 [[INDEX_NEXT]], 1024
+; CHECK-NEXT:    [[TMP10:%.*]] = icmp eq i64 [[INDEX_NEXT]], 1022
 ; CHECK-NEXT:    br i1 [[TMP10]], label [[MIDDLE_BLOCK:%.*]], label [[VECTOR_BODY]], !llvm.loop [[LOOP0:![0-9]+]]
 ; CHECK:       middle.block:
-; CHECK-NEXT:    [[CMP_N:%.*]] = icmp eq i64 1024, 1024
+; CHECK-NEXT:    [[CMP_N:%.*]] = icmp eq i64 1024, 1022
 ; CHECK-NEXT:    br i1 [[CMP_N]], label [[FOR_END:%.*]], label [[SCALAR_PH]]
 ; CHECK:       scalar.ph:
-; CHECK-NEXT:    [[BC_RESUME_VAL:%.*]] = phi i64 [ 1024, [[MIDDLE_BLOCK]] ], [ 0, [[ENTRY:%.*]] ]
+; CHECK-NEXT:    [[BC_RESUME_VAL:%.*]] = phi i64 [ 1022, [[MIDDLE_BLOCK]] ], [ 0, [[ENTRY:%.*]] ]
 ; CHECK-NEXT:    br label [[FOR_BODY:%.*]]
 ; CHECK:       for.body:
 ; CHECK-NEXT:    [[INDVARS_IV:%.*]] = phi i64 [ [[BC_RESUME_VAL]], [[SCALAR_PH]] ], [ [[INDVARS_IV_NEXT:%.*]], [[FOR_LATCH:%.*]] ]