Revert "[VPlan] Remove legacy costing inside VPBlendRecipe::computeCost (#171846)"

This reverts commit fd5f53a.

It's triggering legacy cost model assertions reported in
#171846 (comment)

Author: lukel97

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -7081,11 +7081,6 @@ static bool planContainsAdditionalSimplifications(VPlan &Plan,
           return true;
       }
 
-      // The legacy cost model costs non-header phis with a scalar VF as a phi,
-      // but scalar unrolled VPlans will have VPBlendRecipes which emit selects.
-      if (VF.isScalar() && isa<VPBlendRecipe>(&R))
-        return true;
-
       /// If a VPlan transform folded a recipe to one producing a single-scalar,
       /// but the original instruction wasn't uniform-after-vectorization in the
       /// legacy cost model, the legacy cost overestimates the actual cost.

llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp

@@ -2551,6 +2551,11 @@ void VPVectorPointerRecipe::printRecipe(raw_ostream &O, const Twine &Indent,
 
 InstructionCost VPBlendRecipe::computeCost(ElementCount VF,
                                            VPCostContext &Ctx) const {
+  // Handle cases where only the first lane is used the same way as the legacy
+  // cost model.
+  if (vputils::onlyFirstLaneUsed(this))
+    return Ctx.TTI.getCFInstrCost(Instruction::PHI, Ctx.CostKind);
+
   Type *ResultTy = toVectorTy(Ctx.Types.inferScalarType(this), VF);
   Type *CmpTy = toVectorTy(Type::getInt1Ty(Ctx.Types.getContext()), VF);
   return (getNumIncomingValues() - 1) *

llvm/test/Transforms/LoopVectorize/X86/replicate-uniform-call.ll

@@ -8,25 +8,48 @@ target triple = "x86_64-unknown-linux-gnu"
 define void @smax_call_uniform(ptr %dst, i64 %x) {
 ; CHECK-LABEL: define void @smax_call_uniform(
 ; CHECK-SAME: ptr [[DST:%.*]], i64 [[X:%.*]]) {
-; CHECK-NEXT:  [[ENTRY:.*]]:
+; CHECK-NEXT:  [[ENTRY:.*:]]
 ; CHECK-NEXT:    [[C:%.*]] = icmp ult i8 -68, -69
 ; CHECK-NEXT:    [[MUL:%.*]] = mul nuw nsw i64 [[X]], 0
-; CHECK-NEXT:    br label %[[LOOP_HEADER:.*]]
-; CHECK:       [[LOOP_HEADER]]:
-; CHECK-NEXT:    [[IV:%.*]] = phi i64 [ 0, %[[ENTRY]] ], [ [[INDEX_NEXT:%.*]], %[[LOOP_LATCH:.*]] ]
-; CHECK-NEXT:    br i1 [[C]], label %[[LOOP_LATCH]], label %[[ELSE:.*]]
-; CHECK:       [[ELSE]]:
-; CHECK-NEXT:    [[REM:%.*]] = urem i64 [[MUL]], [[X]]
-; CHECK-NEXT:    [[SMAX:%.*]] = tail call i64 @llvm.smax.i64(i64 [[REM]], i64 0)
-; CHECK-NEXT:    br label %[[LOOP_LATCH]]
-; CHECK:       [[LOOP_LATCH]]:
-; CHECK-NEXT:    [[PREDPHI7:%.*]] = phi i64 [ 1, %[[LOOP_HEADER]] ], [ [[SMAX]], %[[ELSE]] ]
+; CHECK-NEXT:    br label %[[VECTOR_PH:.*]]
+; CHECK:       [[VECTOR_PH]]:
+; CHECK-NEXT:    [[BROADCAST_SPLATINSERT:%.*]] = insertelement <2 x i1> poison, i1 [[C]], i64 0
+; CHECK-NEXT:    [[BROADCAST_SPLAT:%.*]] = shufflevector <2 x i1> [[BROADCAST_SPLATINSERT]], <2 x i1> poison, <2 x i32> zeroinitializer
+; CHECK-NEXT:    [[TMP1:%.*]] = xor <2 x i1> [[BROADCAST_SPLAT]], splat (i1 true)
+; CHECK-NEXT:    br label %[[VECTOR_BODY:.*]]
+; CHECK:       [[VECTOR_BODY]]:
+; CHECK-NEXT:    [[INDEX:%.*]] = phi i64 [ 0, %[[VECTOR_PH]] ], [ [[INDEX_NEXT:%.*]], %[[PRED_UREM_CONTINUE6:.*]] ]
+; CHECK-NEXT:    [[TMP2:%.*]] = extractelement <2 x i1> [[TMP1]], i32 0
+; CHECK-NEXT:    br i1 [[TMP2]], label %[[PRED_UREM_IF:.*]], label %[[PRED_UREM_CONTINUE:.*]]
+; CHECK:       [[PRED_UREM_IF]]:
+; CHECK-NEXT:    br label %[[PRED_UREM_CONTINUE]]
+; CHECK:       [[PRED_UREM_CONTINUE]]:
+; CHECK-NEXT:    [[TMP5:%.*]] = extractelement <2 x i1> [[TMP1]], i32 1
+; CHECK-NEXT:    br i1 [[TMP5]], label %[[PRED_UREM_IF1:.*]], label %[[PRED_UREM_CONTINUE2:.*]]
+; CHECK:       [[PRED_UREM_IF1]]:
+; CHECK-NEXT:    br label %[[PRED_UREM_CONTINUE2]]
+; CHECK:       [[PRED_UREM_CONTINUE2]]:
+; CHECK-NEXT:    [[TMP7:%.*]] = extractelement <2 x i1> [[TMP1]], i32 0
+; CHECK-NEXT:    br i1 [[TMP7]], label %[[PRED_UREM_IF3:.*]], label %[[PRED_UREM_CONTINUE4:.*]]
+; CHECK:       [[PRED_UREM_IF3]]:
+; CHECK-NEXT:    br label %[[PRED_UREM_CONTINUE4]]
+; CHECK:       [[PRED_UREM_CONTINUE4]]:
+; CHECK-NEXT:    [[TMP10:%.*]] = extractelement <2 x i1> [[TMP1]], i32 1
+; CHECK-NEXT:    br i1 [[TMP10]], label %[[PRED_UREM_IF5:.*]], label %[[PRED_UREM_CONTINUE6]]
+; CHECK:       [[PRED_UREM_IF5]]:
+; CHECK-NEXT:    br label %[[PRED_UREM_CONTINUE6]]
+; CHECK:       [[PRED_UREM_CONTINUE6]]:
+; CHECK-NEXT:    [[TMP13:%.*]] = tail call i64 @llvm.smax.i64(i64 0, i64 0)
+; CHECK-NEXT:    [[PREDPHI7:%.*]] = select i1 [[C]], i64 1, i64 [[TMP13]]
 ; CHECK-NEXT:    [[TMP17:%.*]] = add i64 [[PREDPHI7]], 1
 ; CHECK-NEXT:    [[TMP19:%.*]] = getelementptr i64, ptr [[DST]], i64 [[TMP17]]
 ; CHECK-NEXT:    store i64 0, ptr [[TMP19]], align 8
-; CHECK-NEXT:    [[INDEX_NEXT]] = add i64 [[IV]], 1
+; CHECK-NEXT:    store i64 0, ptr [[TMP19]], align 8
+; CHECK-NEXT:    [[INDEX_NEXT]] = add nuw i64 [[INDEX]], 4
 ; CHECK-NEXT:    [[TMP20:%.*]] = icmp eq i64 [[INDEX_NEXT]], 1024
-; CHECK-NEXT:    br i1 [[TMP20]], label %[[EXIT:.*]], label %[[LOOP_HEADER]]
+; CHECK-NEXT:    br i1 [[TMP20]], label %[[MIDDLE_BLOCK:.*]], label %[[VECTOR_BODY]], !llvm.loop [[LOOP0:![0-9]+]]
+; CHECK:       [[MIDDLE_BLOCK]]:
+; CHECK-NEXT:    br label %[[EXIT:.*]]
 ; CHECK:       [[EXIT]]:
 ; CHECK-NEXT:    ret void
 ;
@@ -58,3 +81,8 @@ exit:
 }
 
 declare i64 @llvm.smax.i64(i64, i64)
+;.
+; CHECK: [[LOOP0]] = distinct !{[[LOOP0]], [[META1:![0-9]+]], [[META2:![0-9]+]]}
+; CHECK: [[META1]] = !{!"llvm.loop.isvectorized", i32 1}
+; CHECK: [[META2]] = !{!"llvm.loop.unroll.runtime.disable"}
+;.