[LoopVectorizer] Don't create unused block masks for reductions. NFC

[dmgreen]

Summary

This is pulled out of D75069, where I somewhat accidentally removed some unneeded block masks when we don't have any reductions. This is the same thing as a separate patch, and shouldn't have any effect on codegen.

[dmgreen]

ping

[SjoerdMeijer]

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

1307

Quick question on terminology and "inside" and "outside".
getReductionVars() returns "reduction variables found in the loop". Here you're actually checking if there are inside loop reductions. But having inside reductions means there's a statement/loop after the vectorised loop that sums the partial reductions/sums, correct? If so, it's probably worth clarifying this a bit.

[dmgreen]

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

1307

Ideally this would go with D75069, where inloop vs outside loop reductions are introduced. Until then this might not be the best language on it's own, but I was planning to commit the two close together in any case.

Outside loop reductions are what you would expect from reductions right now:

loop:
  l = load
  a = add a, l
vecreduce(a)

Inside loop reductions put the reduction into the loop:

loop:
  l = load
  a = a + vecreduce(l)

So in the short term, this should probably be called "hasReductions()". But with D75069 should make more sense.

[SjoerdMeijer]

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

1307

Thanks for clarifying. I got it, and yes, together with D75069 it makes sense.
"in-loop reduction" is descriptive, while I am not entirely sold on "outside loop reductions". I was used to terms "partial reductions" and "final reduction", where the latter collects the partial values, which is done after/outside the loop. But we are possibly entering bikeshedding territory here. This looks like a good fix, with hasOutOfLoopReductions renamed to hasReductions like you suggested, or possibly hasFinalReduction if that makes sense.

[dmgreen]

Update this to be a little simpler, not including hasOutOfLoopReductions which turned out to not be needed in the followup.

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

public:
   bool foldTailByMasking() const { return FoldTailByMasking; }
 
   bool blockNeedsPredication(BasicBlock *BB) {
     return foldTailByMasking() || Legal->blockNeedsPredication(BB);
   }
 
   /// Estimate cost of an intrinsic call instruction CI if it were vectorized
   /// with factor VF.  Return the cost of the instruction, including
   /// scalarization overhead if it's needed.
   unsigned getVectorIntrinsicCost(CallInst *CI, unsigned VF);
 
   /// Estimate cost of a call instruction CI if it were vectorized with factor
   /// VF. Return the cost of the instruction, including scalarization overhead
   /// if it's needed. The flag NeedToScalarize shows if the call needs to be
   /// scalarized -
   /// i.e. either vector version isn't available, or is too expensive.
   unsigned getVectorCallCost(CallInst *CI, unsigned VF, bool &NeedToScalarize);
for (auto IG : InterleaveGroups) {
     for (unsigned i = 0; i < IG->getFactor(); ++i)
       if (Instruction *Member = IG->getMember(i)) {
         RecipeBuilder.getRecipe(Member)->eraseFromParent();
       }
   }
 
   // Finally, if tail is folded by masking, introduce selects between the phi
   // and the live-out instruction of each reduction, at the end of the latch.
-  if (CM.foldTailByMasking()) {
+  if (CM.foldTailByMasking() && !Legal->getReductionVars().empty()) {
     Builder.setInsertPoint(VPBB);
     auto *Cond = RecipeBuilder.createBlockInMask(OrigLoop->getHeader(), Plan);
     for (auto &Reduction : Legal->getReductionVars()) {
       VPValue *Phi = Plan->getVPValue(Reduction.first);
       VPValue *Red = Plan->getVPValue(Reduction.second.getLoopExitInstr());
       Builder.createNaryOp(Instruction::Select, {Cond, Red, Phi});
     }
   }

llvm/test/Transforms/LoopVectorize/icmp-uniforms.ll

for.end:
   ret i32 %tmp4
 }
 
 ; Check for crash exposed by D76992.
 ; CHECK:       N0 [label =
 ; CHECK-NEXT:    "loop:\n" +
 ; CHECK-NEXT:      "WIDEN-INDUCTION %iv = phi 0, %iv.next\l" +
 ; CHECK-NEXT:      "WIDEN\l""  %cond0 = icmp %iv, 13\l" +
-; CHECK-NEXT:      "WIDEN-SELECT%s = select %cond0, 10, 20\l" +
-; CHECK-NEXT:      "EMIT vp<%1> = icmp ule ir<%iv> vp<%0>\l"
+; CHECK-NEXT:      "WIDEN-SELECT%s = select %cond0, 10, 20\l"
 ; CHECK-NEXT:  ]
 define void @test() {
 entry:
   br label %loop
 
 loop:                       ; preds = %loop, %entry
   %iv = phi i64 [ 0, %entry ], [ %iv.next, %loop ]
   %cond0 = icmp ult i64 %iv, 13
   %s = select i1 %cond0, i32 10, i32 20
   %iv.next = add nuw nsw i64 %iv, 1
   %exitcond = icmp eq i64 %iv.next, 14
   br i1 %exitcond, label %exit, label %loop
 
 exit:           ; preds = %loop
   ret void
 }

llvm/test/Transforms/LoopVectorize/tail-folding-vectorization-factor-1.ll

 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
 ; RUN: opt < %s  -loop-vectorize -force-vector-interleave=4 -pass-remarks='loop-vectorize' -disable-output -S 2>&1 | FileCheck %s --check-prefix=CHECK-REMARKS
 ; RUN: opt < %s  -loop-vectorize -force-vector-interleave=4 -S | FileCheck %s
 
 ; These tests are to check that fold-tail procedure produces correct scalar code when
 ; loop-vectorization is only unrolling but not vectorizing.
 
 ; CHECK-REMARKS:      remark: {{.*}} interleaved loop (interleaved count: 4)
 ; CHECK-REMARKS-NEXT: remark: {{.*}} interleaved loop (interleaved count: 4)
 ; CHECK-REMARKS-NOT:  remark: {{.*}} vectorized loop
 
 define void @VF1-VPlanExe() {
-; CHECK-LABEL: @VF1-VPlanExe
+; CHECK-LABEL: @VF1-VPlanExe(
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    br i1 false, label [[SCALAR_PH:%.*]], label [[VECTOR_PH:%.*]]
 ; CHECK:       vector.ph:
 ; CHECK-NEXT:    br label [[VECTOR_BODY:%.*]]
 ; CHECK:       vector.body:
 ; CHECK-NEXT:    [[INDEX:%.*]] = phi i64 [ 0, [[VECTOR_PH]] ], [ [[INDEX_NEXT:%.*]], [[VECTOR_BODY]] ]
 ; CHECK-NEXT:    [[INDUCTION:%.*]] = add i64 [[INDEX]], 0
 ; CHECK-NEXT:    [[INDUCTION1:%.*]] = add i64 [[INDEX]], 1
 ; CHECK-NEXT:    [[INDUCTION2:%.*]] = add i64 [[INDEX]], 2
 ; CHECK-NEXT:    [[INDUCTION3:%.*]] = add i64 [[INDEX]], 3
-; CHECK-NEXT:    [[TMP0:%.*]] = icmp ule i64 [[INDUCTION]], 14
-; CHECK-NEXT:    [[TMP1:%.*]] = icmp ule i64 [[INDUCTION1]], 14
-; CHECK-NEXT:    [[TMP2:%.*]] = icmp ule i64 [[INDUCTION2]], 14
-; CHECK-NEXT:    [[TMP3:%.*]] = icmp ule i64 [[INDUCTION3]], 14
 ; CHECK-NEXT:    [[INDEX_NEXT]] = add i64 [[INDEX]], 4
-; CHECK-NEXT:    [[TMP4:%.*]] = icmp eq i64 [[INDEX_NEXT]], 16
-; CHECK-NEXT:    br i1 [[TMP4]], label [[MIDDLE_BLOCK:%.*]], label [[VECTOR_BODY]], !llvm.loop !0
+; CHECK-NEXT:    [[TMP0:%.*]] = icmp eq i64 [[INDEX_NEXT]], 16
+; CHECK-NEXT:    br i1 [[TMP0]], label [[MIDDLE_BLOCK:%.*]], label [[VECTOR_BODY]], !llvm.loop !0
 ; CHECK:       middle.block:
 ; CHECK-NEXT:    br i1 true, label [[FOR_COND_CLEANUP:%.*]], label [[SCALAR_PH]]
 ; CHECK:       scalar.ph:
 ; CHECK-NEXT:    [[BC_RESUME_VAL:%.*]] = phi i64 [ 16, [[MIDDLE_BLOCK]] ], [ 0, [[ENTRY:%.*]] ]
 ; CHECK-NEXT:    br label [[FOR_BODY:%.*]]
 ; CHECK:       for.cond.cleanup:
 ; CHECK-NEXT:    ret void
 ; CHECK:       for.body:

 for.body:
   %indvars.iv = phi i64 [ 0, %entry ], [ %indvars.iv.next, %for.body ]
   %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
   %exitcond = icmp eq i64 %indvars.iv.next, 15
   br i1 %exitcond, label %for.cond.cleanup, label %for.body
 }
 
 define void @VF1-VPWidenCanonicalIVRecipeExe(double* %ptr1) {
-; CHECK-LABEL: @VF1-VPWidenCanonicalIVRecipeExe
+; CHECK-LABEL: @VF1-VPWidenCanonicalIVRecipeExe(
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    [[PTR2:%.*]] = getelementptr inbounds double, double* [[PTR1:%.*]], i64 15
 ; CHECK-NEXT:    br i1 false, label [[SCALAR_PH:%.*]], label [[VECTOR_PH:%.*]]
 ; CHECK:       vector.ph:
 ; CHECK-NEXT:    [[IND_END:%.*]] = getelementptr double, double* [[PTR1]], i64 16
 ; CHECK-NEXT:    br label [[VECTOR_BODY:%.*]]
 ; CHECK:       vector.body:
 ; CHECK-NEXT:    [[INDEX:%.*]] = phi i64 [ 0, [[VECTOR_PH]] ], [ [[INDEX_NEXT:%.*]], [[VECTOR_BODY]] ]
 ; CHECK-NEXT:    [[TMP0:%.*]] = add i64 [[INDEX]], 0
 ; CHECK-NEXT:    [[NEXT_GEP:%.*]] = getelementptr double, double* [[PTR1]], i64 [[TMP0]]
 ; CHECK-NEXT:    [[TMP1:%.*]] = add i64 [[INDEX]], 1
 ; CHECK-NEXT:    [[NEXT_GEP1:%.*]] = getelementptr double, double* [[PTR1]], i64 [[TMP1]]
 ; CHECK-NEXT:    [[TMP2:%.*]] = add i64 [[INDEX]], 2
 ; CHECK-NEXT:    [[NEXT_GEP2:%.*]] = getelementptr double, double* [[PTR1]], i64 [[TMP2]]
 ; CHECK-NEXT:    [[TMP3:%.*]] = add i64 [[INDEX]], 3
 ; CHECK-NEXT:    [[NEXT_GEP3:%.*]] = getelementptr double, double* [[PTR1]], i64 [[TMP3]]
-; CHECK-NEXT:    [[VEC_IV:%.*]] = add i64 [[INDEX]], 0
-; CHECK-NEXT:    [[VEC_IV4:%.*]] = add i64 [[INDEX]], 1
-; CHECK-NEXT:    [[VEC_IV5:%.*]] = add i64 [[INDEX]], 2
-; CHECK-NEXT:    [[VEC_IV6:%.*]] = add i64 [[INDEX]], 3
-; CHECK-NEXT:    [[TMP4:%.*]] = icmp ule i64 [[VEC_IV]], 14
-; CHECK-NEXT:    [[TMP5:%.*]] = icmp ule i64 [[VEC_IV4]], 14
-; CHECK-NEXT:    [[TMP6:%.*]] = icmp ule i64 [[VEC_IV5]], 14
-; CHECK-NEXT:    [[TMP7:%.*]] = icmp ule i64 [[VEC_IV6]], 14
 ; CHECK-NEXT:    [[INDEX_NEXT]] = add i64 [[INDEX]], 4
-; CHECK-NEXT:    [[TMP8:%.*]] = icmp eq i64 [[INDEX_NEXT]], 16
-; CHECK-NEXT:    br i1 [[TMP8]], label [[MIDDLE_BLOCK:%.*]], label [[VECTOR_BODY]], !llvm.loop !3
+; CHECK-NEXT:    [[TMP4:%.*]] = icmp eq i64 [[INDEX_NEXT]], 16
+; CHECK-NEXT:    br i1 [[TMP4]], label [[MIDDLE_BLOCK:%.*]], label [[VECTOR_BODY]], !llvm.loop !3
 ; CHECK:       middle.block:
 ; CHECK-NEXT:    br i1 true, label [[FOR_COND_CLEANUP:%.*]], label [[SCALAR_PH]]
 ; CHECK:       scalar.ph:
 ; CHECK-NEXT:    [[BC_RESUME_VAL:%.*]] = phi double* [ [[IND_END]], [[MIDDLE_BLOCK]] ], [ [[PTR1]], [[ENTRY:%.*]] ]
 ; CHECK-NEXT:    br label [[FOR_BODY:%.*]]
 ; CHECK:       for.cond.cleanup:
 ; CHECK-NEXT:    ret void
 ; CHECK:       for.body: