[SLP] Remove -slp-optimize-identity-hor-reduction-ops option (#106238)

This code has been unchanged for two years; let's simplify the code
and remove configurability which makes the code harder to follow.

Author: preames

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

@@ -136,13 +136,6 @@ static cl::opt<bool> ShouldStartVectorizeHorAtStore(
     cl::desc(
         "Attempt to vectorize horizontal reductions feeding into a store"));
 
-// NOTE: If AllowHorRdxIdenityOptimization is true, the optimization will run
-// even if we match a reduction but do not vectorize in the end.
-static cl::opt<bool> AllowHorRdxIdenityOptimization(
-    "slp-optimize-identity-hor-reduction-ops", cl::init(true), cl::Hidden,
-    cl::desc("Allow optimization of original scalar identity operations on "
-             "matched horizontal reductions."));
-
 static cl::opt<int>
 MaxVectorRegSizeOption("slp-max-reg-size", cl::init(128), cl::Hidden,
     cl::desc("Attempt to vectorize for this register size in bits"));
@@ -17599,10 +17592,9 @@ class HorizontalReduction {
               return Num + Vals.size();
             });
         NumReducedVals < ReductionLimit &&
-        (!AllowHorRdxIdenityOptimization ||
-         all_of(ReducedVals, [](ArrayRef<Value *> RedV) {
+        all_of(ReducedVals, [](ArrayRef<Value *> RedV) {
            return RedV.size() < 2 || !allConstant(RedV) || !isSplat(RedV);
-         }))) {
+         })) {
       for (ReductionOpsType &RdxOps : ReductionOps)
         for (Value *RdxOp : RdxOps)
           V.analyzedReductionRoot(cast<Instruction>(RdxOp));
@@ -17732,8 +17724,7 @@ class HorizontalReduction {
       }
 
       // Emit code for constant values.
-      if (AllowHorRdxIdenityOptimization && Candidates.size() > 1 &&
-          allConstant(Candidates)) {
+      if (Candidates.size() > 1 && allConstant(Candidates)) {
         Value *Res = Candidates.front();
         ++VectorizedVals.try_emplace(Candidates.front(), 0).first->getSecond();
         for (Value *VC : ArrayRef(Candidates).drop_front()) {
@@ -17748,15 +17739,14 @@ class HorizontalReduction {
 
       unsigned NumReducedVals = Candidates.size();
       if (NumReducedVals < ReductionLimit &&
-          (NumReducedVals < 2 || !AllowHorRdxIdenityOptimization ||
-           !isSplat(Candidates)))
+          (NumReducedVals < 2 || !isSplat(Candidates)))
         continue;
 
       // Check if we support repeated scalar values processing (optimization of
       // original scalar identity operations on matched horizontal reductions).
-      IsSupportedHorRdxIdentityOp =
-          AllowHorRdxIdenityOptimization && RdxKind != RecurKind::Mul &&
-          RdxKind != RecurKind::FMul && RdxKind != RecurKind::FMulAdd;
+      IsSupportedHorRdxIdentityOp = RdxKind != RecurKind::Mul &&
+                                    RdxKind != RecurKind::FMul &&
+                                    RdxKind != RecurKind::FMulAdd;
       // Gather same values.
       MapVector<Value *, unsigned> SameValuesCounter;
       if (IsSupportedHorRdxIdentityOp)

llvm/test/Transforms/SLPVectorizer/AArch64/buildvector-reduce.ll

@@ -1,6 +1,5 @@
 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
 ; RUN: opt -S -passes=slp-vectorizer < %s -mtriple=arm64-apple-macosx | FileCheck %s
-; RUN: opt -S -passes=slp-vectorizer < %s -mtriple=arm64-apple-macosx -slp-optimize-identity-hor-reduction-ops=false | FileCheck %s --check-prefix=NO-IDENTITY
 
 define i8 @test() {
 ; CHECK-LABEL: @test(
@@ -12,17 +11,6 @@ define i8 @test() {
 ; CHECK-NEXT:    [[TMP0]] = mul i32 [[CALL278]], 8
 ; CHECK-NEXT:    br label [[FOR_BODY]]
 ;
-; NO-IDENTITY-LABEL: @test(
-; NO-IDENTITY-NEXT:  entry:
-; NO-IDENTITY-NEXT:    br label [[FOR_BODY:%.*]]
-; NO-IDENTITY:       for.body:
-; NO-IDENTITY-NEXT:    [[SUM:%.*]] = phi i32 [ [[TMP2:%.*]], [[FOR_BODY]] ], [ 0, [[ENTRY:%.*]] ]
-; NO-IDENTITY-NEXT:    [[CALL278:%.*]] = call i32 @fn(i32 [[SUM]])
-; NO-IDENTITY-NEXT:    [[TMP0:%.*]] = insertelement <8 x i32> poison, i32 [[CALL278]], i32 0
-; NO-IDENTITY-NEXT:    [[TMP1:%.*]] = shufflevector <8 x i32> [[TMP0]], <8 x i32> poison, <8 x i32> zeroinitializer
-; NO-IDENTITY-NEXT:    [[TMP2]] = call i32 @llvm.vector.reduce.add.v8i32(<8 x i32> [[TMP1]])
-; NO-IDENTITY-NEXT:    br label [[FOR_BODY]]
-;
 entry:
   br label %for.body