[VPlan] Ignore incoming values with constant false mask. (#89384)

Ignore incoming values with constant false masks when trying to simplify
VPBlendRecipes.

As a follow-on optimization, we should also be able to drop all incoming
values with false masks by creating a new VPBlendRecipe with those
operands dropped.

PR: #89384

Author: fhahn

llvm/lib/Transforms/Vectorize/VPlanPatternMatch.h

@@ -51,8 +51,9 @@ template <typename Class> struct bind_ty {
 };
 
 /// Match a specified integer value or vector of all elements of that
-/// value.
-struct specific_intval {
+/// value. \p BitWidth optionally specifies the bitwidth the matched constant
+/// must have. If it is 0, the matched constant can have any bitwidth.
+template <unsigned BitWidth = 0> struct specific_intval {
   APInt Val;
 
   specific_intval(APInt V) : Val(std::move(V)) {}
@@ -66,15 +67,21 @@ struct specific_intval {
       if (const auto *C = dyn_cast<Constant>(V))
         CI = dyn_cast_or_null<ConstantInt>(
             C->getSplatValue(/*AllowPoison=*/false));
+    if (!CI)
+      return false;
 
-    return CI && APInt::isSameValue(CI->getValue(), Val);
+    assert((BitWidth == 0 || CI->getBitWidth() == BitWidth) &&
+           "Trying the match constant with unexpected bitwidth.");
+    return APInt::isSameValue(CI->getValue(), Val);
   }
 };
 
-inline specific_intval m_SpecificInt(uint64_t V) {
-  return specific_intval(APInt(64, V));
+inline specific_intval<0> m_SpecificInt(uint64_t V) {
+  return specific_intval<0>(APInt(64, V));
 }
 
+inline specific_intval<1> m_False() { return specific_intval<1>(APInt(64, 0)); }
+
 /// Matching combinators
 template <typename LTy, typename RTy> struct match_combine_or {
   LTy L;

llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp

@@ -884,18 +884,19 @@ void VPlanTransforms::clearReductionWrapFlags(VPlan &Plan) {
 
 /// Try to simplify recipe \p R.
 static void simplifyRecipe(VPRecipeBase &R, VPTypeAnalysis &TypeInfo) {
+  using namespace llvm::VPlanPatternMatch;
   // Try to remove redundant blend recipes.
   if (auto *Blend = dyn_cast<VPBlendRecipe>(&R)) {
     VPValue *Inc0 = Blend->getIncomingValue(0);
     for (unsigned I = 1; I != Blend->getNumIncomingValues(); ++I)
-      if (Inc0 != Blend->getIncomingValue(I))
+      if (Inc0 != Blend->getIncomingValue(I) &&
+          !match(Blend->getMask(I), m_False()))
         return;
     Blend->replaceAllUsesWith(Inc0);
     Blend->eraseFromParent();
     return;
   }
 
-  using namespace llvm::VPlanPatternMatch;
   VPValue *A;
   if (match(&R, m_Trunc(m_ZExtOrSExt(m_VPValue(A))))) {
     VPValue *Trunc = R.getVPSingleValue();

llvm/test/Transforms/LoopVectorize/unused-blend-mask-for-first-operand.ll

@@ -10,17 +10,13 @@ define void @test_not_first_lane_only_constant(ptr %A, ptr noalias %B)  {
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    br i1 false, label [[SCALAR_PH:%.*]], label [[VECTOR_PH:%.*]]
 ; CHECK:       vector.ph:
-; CHECK-NEXT:    [[BROADCAST_SPLATINSERT3:%.*]] = insertelement <4 x ptr> poison, ptr [[B]], i64 0
-; CHECK-NEXT:    [[BROADCAST_SPLAT4:%.*]] = shufflevector <4 x ptr> [[BROADCAST_SPLATINSERT3]], <4 x ptr> poison, <4 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:    [[OFFSET_IDX:%.*]] = trunc i32 [[INDEX]] to i16
 ; CHECK-NEXT:    [[TMP0:%.*]] = add i16 [[OFFSET_IDX]], 0
 ; CHECK-NEXT:    [[TMP1:%.*]] = getelementptr inbounds i16, ptr [[A]], i16 [[TMP0]]
-; CHECK-NEXT:    [[PREDPHI:%.*]] = select <4 x i1> zeroinitializer, <4 x ptr> poison, <4 x ptr> [[BROADCAST_SPLAT4]]
-; CHECK-NEXT:    [[TMP12:%.*]] = extractelement <4 x ptr> [[PREDPHI]], i32 0
-; CHECK-NEXT:    [[TMP13:%.*]] = load i16, ptr [[TMP12]], align 2
+; CHECK-NEXT:    [[TMP13:%.*]] = load i16, ptr %B, align 2
 ; CHECK-NEXT:    [[BROADCAST_SPLATINSERT5:%.*]] = insertelement <4 x i16> poison, i16 [[TMP13]], i64 0
 ; CHECK-NEXT:    [[BROADCAST_SPLAT6:%.*]] = shufflevector <4 x i16> [[BROADCAST_SPLATINSERT5]], <4 x i16> poison, <4 x i32> zeroinitializer
 ; CHECK-NEXT:    [[TMP2:%.*]] = getelementptr inbounds i16, ptr [[TMP1]], i32 0