[VPlan] Simplify live-ins if they are SCEVConstant.

The legacy cost model in some parts checks if any of the operands are
constants via SCEV. Update VPlan construction to replace live-ins that
are constants via SCEV with such constants. This means VPlans (and
codegen) reflects what we computing the cost of and removes another case
where the legacy and VPlan cost model diverged.

Fixes llvm#105722.

llvm/lib/Transforms/Vectorize/VPRecipeBuilder.h

@@ -13,6 +13,7 @@
 #include "VPlan.h"
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/PointerUnion.h"
+#include "llvm/Analysis/ScalarEvolutionExpressions.h"
 #include "llvm/IR/IRBuilder.h"
 
 namespace llvm {
@@ -173,6 +174,11 @@ class VPRecipeBuilder {
       if (auto *R = Ingredient2Recipe.lookup(I))
         return R->getVPSingleValue();
     }
+    ScalarEvolution &SE = *PSE.getSE();
+    if (!isa<Constant>(V) && SE.isSCEVable(V->getType()))
+      if (auto *C = dyn_cast<SCEVConstant>(PSE.getSE()->getSCEV(V)))
+        return Plan.getOrAddLiveIn(C->getValue());
+
     return Plan.getOrAddLiveIn(V);
   }
 };

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

@@ -931,6 +931,71 @@ exit:
   ret void
 }
 
+; Test case for https://github.com/llvm/llvm-project/issues/105722.
+define i64 @live_in_known_1_via_scev() {
+; CHECK-LABEL: @live_in_known_1_via_scev(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[SEL:%.*]] = select i1 false, i32 3, i32 0
+; CHECK-NEXT:    br label [[PH:%.*]]
+; CHECK:       ph:
+; CHECK-NEXT:    [[P:%.*]] = phi i32 [ 1, [[ENTRY:%.*]] ]
+; CHECK-NEXT:    [[N:%.*]] = add nuw nsw i32 [[SEL]], 6
+; CHECK-NEXT:    [[P_EXT:%.*]] = zext nneg i32 [[P]] to i64
+; 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 i32 [ 0, [[VECTOR_PH]] ], [ [[INDEX_NEXT:%.*]], [[VECTOR_BODY]] ]
+; CHECK-NEXT:    [[VEC_PHI:%.*]] = phi <4 x i64> [ <i64 3, i64 1, i64 1, i64 1>, [[VECTOR_PH]] ], [ [[VEC_PHI]], [[VECTOR_BODY]] ]
+; CHECK-NEXT:    [[BROADCAST_SPLATINSERT:%.*]] = insertelement <4 x i32> poison, i32 [[INDEX]], i64 0
+; CHECK-NEXT:    [[BROADCAST_SPLAT:%.*]] = shufflevector <4 x i32> [[BROADCAST_SPLATINSERT]], <4 x i32> poison, <4 x i32> zeroinitializer
+; CHECK-NEXT:    [[VEC_IV:%.*]] = add <4 x i32> [[BROADCAST_SPLAT]], <i32 0, i32 1, i32 2, i32 3>
+; CHECK-NEXT:    [[TMP0:%.*]] = icmp ule <4 x i32> [[VEC_IV]], <i32 5, i32 5, i32 5, i32 5>
+; CHECK-NEXT:    [[TMP1:%.*]] = select <4 x i1> [[TMP0]], <4 x i64> [[VEC_PHI]], <4 x i64> [[VEC_PHI]]
+; CHECK-NEXT:    [[INDEX_NEXT]] = add i32 [[INDEX]], 4
+; CHECK-NEXT:    [[TMP2:%.*]] = icmp eq i32 [[INDEX_NEXT]], 8
+; CHECK-NEXT:    br i1 [[TMP2]], label [[MIDDLE_BLOCK:%.*]], label [[VECTOR_BODY]], !llvm.loop [[LOOP22:![0-9]+]]
+; CHECK:       middle.block:
+; CHECK-NEXT:    [[TMP3:%.*]] = call i64 @llvm.vector.reduce.mul.v4i64(<4 x i64> [[TMP1]])
+; CHECK-NEXT:    br i1 true, label [[EXIT:%.*]], label [[SCALAR_PH]]
+; CHECK:       scalar.ph:
+; CHECK-NEXT:    [[BC_RESUME_VAL:%.*]] = phi i32 [ 8, [[MIDDLE_BLOCK]] ], [ 0, [[PH]] ]
+; CHECK-NEXT:    [[BC_MERGE_RDX:%.*]] = phi i64 [ [[TMP3]], [[MIDDLE_BLOCK]] ], [ 3, [[PH]] ]
+; CHECK-NEXT:    br label [[LOOP:%.*]]
+; CHECK:       loop:
+; CHECK-NEXT:    [[IV:%.*]] = phi i32 [ [[BC_RESUME_VAL]], [[SCALAR_PH]] ], [ [[IV_NEXT:%.*]], [[LOOP]] ]
+; CHECK-NEXT:    [[RED:%.*]] = phi i64 [ [[BC_MERGE_RDX]], [[SCALAR_PH]] ], [ [[RED_MUL:%.*]], [[LOOP]] ]
+; CHECK-NEXT:    [[RED_MUL]] = mul nsw i64 [[RED]], [[P_EXT]]
+; CHECK-NEXT:    [[IV_NEXT]] = add nuw nsw i32 [[IV]], 1
+; CHECK-NEXT:    [[EC:%.*]] = icmp eq i32 [[IV_NEXT]], [[N]]
+; CHECK-NEXT:    br i1 [[EC]], label [[EXIT]], label [[LOOP]], !llvm.loop [[LOOP23:![0-9]+]]
+; CHECK:       exit:
+; CHECK-NEXT:    [[RES:%.*]] = phi i64 [ [[RED_MUL]], [[LOOP]] ], [ [[TMP3]], [[MIDDLE_BLOCK]] ]
+; CHECK-NEXT:    ret i64 [[RES]]
+;
+entry:
+  %sel = select i1 false, i32 3, i32 0
+  br label %ph
+
+ph:
+  %p = phi i32 [ 1, %entry ]
+  %N = add nuw nsw i32 %sel, 6
+  %p.ext = zext nneg i32 %p to i64
+  br label %loop
+
+loop:
+  %iv = phi i32 [ 0, %ph ], [ %iv.next, %loop ]
+  %red = phi i64 [ 3, %ph ], [ %red.mul,  %loop ]
+  %red.mul = mul nsw i64 %red, %p.ext
+  %iv.next = add nuw nsw i32 %iv, 1
+  %ec = icmp eq i32 %iv.next, %N
+  br i1 %ec, label %exit, label %loop
+
+exit:
+  %res = phi i64 [ %red.mul, %loop ]
+  ret i64 %res
+}
+
 declare void @llvm.assume(i1 noundef) #0
 
 attributes #0 = { "target-cpu"="penryn" }