[VPlan] Return cost of 0 for VPWidenCastRecipe without underlying value.

In some cases, VPWidenCastRecipes are created but not considered in the
legacy cost model, including truncates/extends when evaluating a reduction
in a smaller type. Return 0 for such casts for now, to avoid divergences
between VPlan and legacy cost models.

Fixes #113526.

Author: fhahn

llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp

@@ -1524,6 +1524,11 @@ void VPWidenCastRecipe::execute(VPTransformState &State) {
 
 InstructionCost VPWidenCastRecipe::computeCost(ElementCount VF,
                                                VPCostContext &Ctx) const {
+  // TODO: In some cases, VPWidenCastRecipes are created but not considered in
+  // the legacy cost model, including truncates/extends when evaluating a
+  // reduction in a smaller type.
+  if (!getUnderlyingValue())
+    return 0;
   // Computes the CastContextHint from a recipes that may access memory.
   auto ComputeCCH = [&](const VPRecipeBase *R) -> TTI::CastContextHint {
     if (VF.isScalar())

llvm/test/Transforms/LoopVectorize/X86/cost-model.ll

@@ -1037,6 +1037,71 @@ exit:
   ret i64 %red.mul
 }
 
+; Test case for https://github.com/llvm/llvm-project/issues/113526.
+define i32 @narrowed_reduction(ptr %a, i1 %cmp) #0 {
+; CHECK-LABEL: @narrowed_reduction(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[CONV:%.*]] = zext i1 [[CMP:%.*]] to i32
+; CHECK-NEXT:    br i1 true, label [[SCALAR_PH:%.*]], label [[VECTOR_PH:%.*]]
+; CHECK:       vector.ph:
+; CHECK-NEXT:    [[BROADCAST_SPLATINSERT:%.*]] = insertelement <16 x i32> poison, i32 [[CONV]], i64 0
+; CHECK-NEXT:    [[BROADCAST_SPLAT:%.*]] = shufflevector <16 x i32> [[BROADCAST_SPLATINSERT]], <16 x i32> poison, <16 x i32> zeroinitializer
+; CHECK-NEXT:    br label [[VECTOR_BODY:%.*]]
+; CHECK:       vector.body:
+; CHECK-NEXT:    [[INDEX:%.*]] = phi i32 [ 0, [[VECTOR_PH]] ], [ [[INDEX_NEXT:%.*]], [[VECTOR_BODY]] ]
+; CHECK-NEXT:    [[VEC_PHI:%.*]] = phi <16 x i32> [ zeroinitializer, [[VECTOR_PH]] ], [ [[TMP6:%.*]], [[VECTOR_BODY]] ]
+; CHECK-NEXT:    [[VEC_PHI1:%.*]] = phi <16 x i32> [ zeroinitializer, [[VECTOR_PH]] ], [ [[TMP7:%.*]], [[VECTOR_BODY]] ]
+; CHECK-NEXT:    [[TMP0:%.*]] = and <16 x i32> [[VEC_PHI]], <i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1>
+; CHECK-NEXT:    [[TMP1:%.*]] = and <16 x i32> [[VEC_PHI1]], <i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32 1>
+; CHECK-NEXT:    [[TMP2:%.*]] = or <16 x i32> [[TMP0]], [[BROADCAST_SPLAT]]
+; CHECK-NEXT:    [[TMP3:%.*]] = or <16 x i32> [[TMP1]], [[BROADCAST_SPLAT]]
+; CHECK-NEXT:    [[TMP4:%.*]] = trunc <16 x i32> [[TMP2]] to <16 x i1>
+; CHECK-NEXT:    [[TMP5:%.*]] = trunc <16 x i32> [[TMP3]] to <16 x i1>
+; CHECK-NEXT:    [[TMP6]] = zext <16 x i1> [[TMP4]] to <16 x i32>
+; CHECK-NEXT:    [[TMP7]] = zext <16 x i1> [[TMP5]] to <16 x i32>
+; CHECK-NEXT:    [[INDEX_NEXT]] = add nuw i32 [[INDEX]], 32
+; CHECK-NEXT:    [[TMP8:%.*]] = icmp eq i32 [[INDEX_NEXT]], 0
+; CHECK-NEXT:    br i1 [[TMP8]], label [[MIDDLE_BLOCK:%.*]], label [[VECTOR_BODY]], !llvm.loop [[LOOP26:![0-9]+]]
+; CHECK:       middle.block:
+; CHECK-NEXT:    [[TMP9:%.*]] = trunc <16 x i32> [[TMP6]] to <16 x i1>
+; CHECK-NEXT:    [[TMP10:%.*]] = trunc <16 x i32> [[TMP7]] to <16 x i1>
+; CHECK-NEXT:    [[BIN_RDX:%.*]] = or <16 x i1> [[TMP10]], [[TMP9]]
+; CHECK-NEXT:    [[TMP11:%.*]] = call i1 @llvm.vector.reduce.or.v16i1(<16 x i1> [[BIN_RDX]])
+; CHECK-NEXT:    [[TMP12:%.*]] = zext i1 [[TMP11]] to i32
+; CHECK-NEXT:    br i1 true, label [[EXIT:%.*]], label [[SCALAR_PH]]
+; CHECK:       scalar.ph:
+; CHECK-NEXT:    [[BC_RESUME_VAL:%.*]] = phi i32 [ 1, [[MIDDLE_BLOCK]] ], [ 1, [[ENTRY:%.*]] ]
+; CHECK-NEXT:    [[BC_MERGE_RDX:%.*]] = phi i32 [ [[TMP12]], [[MIDDLE_BLOCK]] ], [ 0, [[ENTRY]] ]
+; CHECK-NEXT:    br label [[LOOP:%.*]]
+; CHECK:       loop:
+; CHECK-NEXT:    [[IV:%.*]] = phi i32 [ [[BC_RESUME_VAL]], [[SCALAR_PH]] ], [ [[INC:%.*]], [[LOOP]] ]
+; CHECK-NEXT:    [[OR13:%.*]] = phi i32 [ [[BC_MERGE_RDX]], [[SCALAR_PH]] ], [ [[OR:%.*]], [[LOOP]] ]
+; CHECK-NEXT:    [[AND:%.*]] = and i32 [[OR13]], 1
+; CHECK-NEXT:    [[OR]] = or i32 [[AND]], [[CONV]]
+; CHECK-NEXT:    [[INC]] = add i32 [[IV]], 1
+; CHECK-NEXT:    [[EC:%.*]] = icmp eq i32 [[IV]], 0
+; CHECK-NEXT:    br i1 [[EC]], label [[EXIT]], label [[LOOP]], !llvm.loop [[LOOP27:![0-9]+]]
+; CHECK:       exit:
+; CHECK-NEXT:    [[OR_LCSSA:%.*]] = phi i32 [ [[OR]], [[LOOP]] ], [ [[TMP12]], [[MIDDLE_BLOCK]] ]
+; CHECK-NEXT:    ret i32 [[OR_LCSSA]]
+;
+entry:
+  %conv = zext i1 %cmp to i32
+  br label %loop
+
+loop:
+  %iv = phi i32 [ 1, %entry ], [ %inc, %loop ]
+  %or13 = phi i32 [ 0, %entry ], [ %or, %loop ]
+  %and = and i32 %or13, 1
+  %or = or i32 %and, %conv
+  %inc = add i32 %iv, 1
+  %ec = icmp eq i32 %iv, 0
+  br i1 %ec, label %exit, label %loop
+
+exit:
+  ret i32 %or
+}
+
 declare void @llvm.assume(i1 noundef) #0
 
 attributes #0 = { "target-cpu"="penryn" }