[SLP]Do not vectorize small (<=2) buildvector/buildvalue sequences with MaxVF==true.

[alexey-bataev]

If MaxVFOnly for buildvector/buildvalue vectorization is set to true and the
total number of elements to vectorize is <= 2, better to try to
vectorize reductions at first, which may produce larger tree (reductions
have a limit of at least 4 elements to vectorize). Smaller
buildvector/buildvalue sequence will be attempted to vectorize later,
with MaxVFOnly set to false.

[llvmbot]

@llvm/pr-subscribers-llvm-transforms

Author: Alexey Bataev (alexey-bataev)

Changes

If MaxVFOnly for buildvector/buildvalue vectorization is set to true and the
total number of elements to vectorize is <= 2, better to try to
vectorize reductions at first, which may produce larger tree (reductions
have a limit of at least 4 elements to vectorize). Smaller
buildvector/buildvalue sequence will be attempted to vectorize later,
with MaxVFOnly set to false.

---

Full diff: https://github.com/llvm/llvm-project/pull/98957.diff

3 Files Affected:

• (modified) llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp (+16)
• (modified) llvm/test/Transforms/SLPVectorizer/AArch64/scalarization-overhead.ll (+18-19)
• (modified) llvm/test/Transforms/SLPVectorizer/X86/reused-extractelements.ll (+1-2)

diff --git a/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp b/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
index 74a16d3fbcad6..629ddeea1534a 100644
--- a/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
+++ b/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
@@ -18254,6 +18254,14 @@ bool SLPVectorizerPass::vectorizeInsertValueInst(InsertValueInst *IVI,
   if (!findBuildAggregate(IVI, TTI, BuildVectorOpds, BuildVectorInsts))
     return false;
 
+  if (MaxVFOnly && BuildVectorOpds.size() == 2) {
+    R.getORE()->emit([&]() {
+      return OptimizationRemarkMissed(SV_NAME, "NotPossible", IVI)
+             << "Cannot SLP vectorize list: only 2 elements of buildvalue, "
+                "trying reduction first.";
+    });
+    return false;
+  }
   LLVM_DEBUG(dbgs() << "SLP: array mappable to vector: " << *IVI << "\n");
   // Aggregate value is unlikely to be processed in vector register.
   return tryToVectorizeList(BuildVectorOpds, R, MaxVFOnly);
@@ -18270,6 +18278,14 @@ bool SLPVectorizerPass::vectorizeInsertElementInst(InsertElementInst *IEI,
        isFixedVectorShuffle(BuildVectorOpds, Mask)))
     return false;
 
+  if (MaxVFOnly && BuildVectorInsts.size() == 2) {
+    R.getORE()->emit([&]() {
+      return OptimizationRemarkMissed(SV_NAME, "NotPossible", IEI)
+             << "Cannot SLP vectorize list: only 2 elements of buildvector, "
+                "trying reduction first.";
+    });
+    return false;
+  }
   LLVM_DEBUG(dbgs() << "SLP: array mappable to vector: " << *IEI << "\n");
   return tryToVectorizeList(BuildVectorInsts, R, MaxVFOnly);
 }
diff --git a/llvm/test/Transforms/SLPVectorizer/AArch64/scalarization-overhead.ll b/llvm/test/Transforms/SLPVectorizer/AArch64/scalarization-overhead.ll
index a24cb81541d7c..c6209fd71063a 100644
--- a/llvm/test/Transforms/SLPVectorizer/AArch64/scalarization-overhead.ll
+++ b/llvm/test/Transforms/SLPVectorizer/AArch64/scalarization-overhead.ll
@@ -10,33 +10,32 @@ define fastcc i64 @zot(float %arg, float %arg1, float %arg2, float %arg3, float
 ; CHECK-NEXT:    [[TMP1:%.*]] = insertelement <4 x float> [[TMP0]], float [[ARG3:%.*]], i32 2
 ; CHECK-NEXT:    [[TMP2:%.*]] = shufflevector <4 x float> [[TMP1]], <4 x float> poison, <4 x i32> <i32 0, i32 1, i32 2, i32 2>
 ; CHECK-NEXT:    [[TMP3:%.*]] = fmul fast <4 x float> <float 0.000000e+00, float 0.000000e+00, float 1.000000e+00, float 1.000000e+00>, [[TMP2]]
-; CHECK-NEXT:    [[TMP4:%.*]] = insertelement <2 x float> <float poison, float 0.000000e+00>, float [[ARG3]], i32 0
-; CHECK-NEXT:    [[TMP5:%.*]] = fadd fast <2 x float> [[TMP4]], <float 1.000000e+00, float 0.000000e+00>
-; CHECK-NEXT:    [[TMP6:%.*]] = shufflevector <2 x float> [[TMP5]], <2 x float> poison, <4 x i32> <i32 0, i32 1, i32 poison, i32 poison>
-; CHECK-NEXT:    [[TMP7:%.*]] = shufflevector <4 x float> [[TMP2]], <4 x float> [[TMP6]], <4 x i32> <i32 4, i32 5, i32 2, i32 3>
-; CHECK-NEXT:    [[TMP8:%.*]] = fadd fast <4 x float> [[TMP7]], <float 2.000000e+00, float 1.000000e+00, float 1.000000e+00, float 1.000000e+00>
+; CHECK-NEXT:    [[VAL12:%.*]] = fadd fast float [[ARG3]], 1.000000e+00
+; CHECK-NEXT:    [[TMP4:%.*]] = insertelement <4 x float> [[TMP2]], float [[VAL12]], i32 0
+; CHECK-NEXT:    [[TMP5:%.*]] = insertelement <4 x float> [[TMP4]], float 0.000000e+00, i32 1
+; CHECK-NEXT:    [[TMP6:%.*]] = fadd fast <4 x float> [[TMP5]], <float 2.000000e+00, float 1.000000e+00, float 1.000000e+00, float 1.000000e+00>
 ; CHECK-NEXT:    br i1 [[ARG6:%.*]], label [[BB18:%.*]], label [[BB57:%.*]]
 ; CHECK:       bb18:
-; CHECK-NEXT:    [[TMP9:%.*]] = phi <4 x float> [ [[TMP8]], [[BB:%.*]] ]
-; CHECK-NEXT:    [[TMP10:%.*]] = extractelement <4 x float> [[TMP8]], i32 2
-; CHECK-NEXT:    [[VAL23:%.*]] = fmul fast float [[TMP10]], 2.000000e+00
-; CHECK-NEXT:    [[TMP11:%.*]] = extractelement <4 x float> [[TMP8]], i32 3
-; CHECK-NEXT:    [[VAL24:%.*]] = fmul fast float [[TMP11]], 3.000000e+00
+; CHECK-NEXT:    [[TMP7:%.*]] = phi <4 x float> [ [[TMP6]], [[BB:%.*]] ]
+; CHECK-NEXT:    [[TMP8:%.*]] = extractelement <4 x float> [[TMP6]], i32 2
+; CHECK-NEXT:    [[VAL23:%.*]] = fmul fast float [[TMP8]], 2.000000e+00
+; CHECK-NEXT:    [[TMP9:%.*]] = extractelement <4 x float> [[TMP6]], i32 3
+; CHECK-NEXT:    [[VAL24:%.*]] = fmul fast float [[TMP9]], 3.000000e+00
 ; CHECK-NEXT:    br i1 [[ARG7:%.*]], label [[BB25:%.*]], label [[BB57]]
 ; CHECK:       bb25:
-; CHECK-NEXT:    [[TMP12:%.*]] = phi <4 x float> [ [[TMP9]], [[BB18]] ]
-; CHECK-NEXT:    [[TMP13:%.*]] = extractelement <4 x float> [[TMP3]], i32 1
+; CHECK-NEXT:    [[TMP10:%.*]] = phi <4 x float> [ [[TMP7]], [[BB18]] ]
+; CHECK-NEXT:    [[TMP11:%.*]] = extractelement <4 x float> [[TMP3]], i32 1
 ; CHECK-NEXT:    br label [[BB30:%.*]]
 ; CHECK:       bb30:
 ; CHECK-NEXT:    [[VAL31:%.*]] = phi float [ [[VAL55:%.*]], [[BB30]] ], [ 0.000000e+00, [[BB25]] ]
-; CHECK-NEXT:    [[VAL32:%.*]] = phi float [ [[TMP13]], [[BB30]] ], [ 0.000000e+00, [[BB25]] ]
-; CHECK-NEXT:    [[TMP14:%.*]] = load <4 x i8>, ptr [[ARG5:%.*]], align 1
-; CHECK-NEXT:    [[TMP15:%.*]] = uitofp <4 x i8> [[TMP14]] to <4 x float>
-; CHECK-NEXT:    [[TMP16:%.*]] = fsub fast <4 x float> [[TMP15]], [[TMP3]]
-; CHECK-NEXT:    [[TMP17:%.*]] = fmul fast <4 x float> [[TMP16]], [[TMP12]]
-; CHECK-NEXT:    [[TMP18:%.*]] = call fast float @llvm.vector.reduce.fadd.v4f32(float -0.000000e+00, <4 x float> [[TMP17]])
+; CHECK-NEXT:    [[VAL32:%.*]] = phi float [ [[TMP11]], [[BB30]] ], [ 0.000000e+00, [[BB25]] ]
+; CHECK-NEXT:    [[TMP12:%.*]] = load <4 x i8>, ptr [[ARG5:%.*]], align 1
+; CHECK-NEXT:    [[TMP13:%.*]] = uitofp <4 x i8> [[TMP12]] to <4 x float>
+; CHECK-NEXT:    [[TMP14:%.*]] = fsub fast <4 x float> [[TMP13]], [[TMP3]]
+; CHECK-NEXT:    [[TMP15:%.*]] = fmul fast <4 x float> [[TMP14]], [[TMP10]]
+; CHECK-NEXT:    [[TMP16:%.*]] = call fast float @llvm.vector.reduce.fadd.v4f32(float -0.000000e+00, <4 x float> [[TMP15]])
 ; CHECK-NEXT:    [[VAL55]] = tail call fast float @llvm.minnum.f32(float [[VAL31]], float [[ARG1:%.*]])
-; CHECK-NEXT:    [[VAL56:%.*]] = tail call fast float @llvm.maxnum.f32(float [[ARG2:%.*]], float [[TMP18]])
+; CHECK-NEXT:    [[VAL56:%.*]] = tail call fast float @llvm.maxnum.f32(float [[ARG2:%.*]], float [[TMP16]])
 ; CHECK-NEXT:    call void @ham(float [[VAL55]], float [[VAL56]])
 ; CHECK-NEXT:    br i1 [[ARG8:%.*]], label [[BB30]], label [[BB57]]
 ; CHECK:       bb57:
diff --git a/llvm/test/Transforms/SLPVectorizer/X86/reused-extractelements.ll b/llvm/test/Transforms/SLPVectorizer/X86/reused-extractelements.ll
index bf4903fd19b09..2f5936357d0f5 100644
--- a/llvm/test/Transforms/SLPVectorizer/X86/reused-extractelements.ll
+++ b/llvm/test/Transforms/SLPVectorizer/X86/reused-extractelements.ll
@@ -7,8 +7,7 @@
 ; YAML-NEXT: Name:            NotPossible
 ; YAML-NEXT: Function:        g
 ; YAML-NEXT: Args:
-; YAML-NEXT:   - String:          'Cannot SLP vectorize list: vectorization was impossible'
-; YAML-NEXT:   - String:          ' with available vectorization factors'
+; YAML-NEXT:   - String:          'Cannot SLP vectorize list: only 2 elements of buildvector, trying reduction first.'
 
 define <2 x i32> @g(<2 x i32> %x, i32 %a, i32 %b) {
 ; CHECK-LABEL: @g(

[RKSimon]

LGTM