[llvm] 219d451 - [Analysis][AArch64] Make fixed-width ordered reductions slightly more expensive

[David Sherwood via llvm-commits]

Author: David Sherwood
Date: 2021-08-18T17:01:56+01:00
New Revision: 219d4518fce9aafcb5eba9b92fb778837f0a4827

URL: https://github.com/llvm/llvm-project/commit/219d4518fce9aafcb5eba9b92fb778837f0a4827
DIFF: https://github.com/llvm/llvm-project/commit/219d4518fce9aafcb5eba9b92fb778837f0a4827.diff

LOG: [Analysis][AArch64] Make fixed-width ordered reductions slightly more expensive

For tight loops like this:

  float r = 0;
  for (int i = 0; i < n; i++) {
    r += a[i];
  }

it's better not to vectorise at -O3 using fixed-width ordered reductions
on AArch64 targets. Although the resulting number of instructions in the
generated code ends up being comparable to not vectorising at all, there
may be additional costs on some CPUs, for example perhaps the scheduling
is worse. It makes sense to deter vectorisation in tight loops.

Differential Revision: https://reviews.llvm.org/D108292

Added: 
    

Modified: 
    llvm/lib/Target/AArch64/AArch64TargetTransformInfo.cpp
    llvm/test/Analysis/CostModel/AArch64/reduce-fadd.ll
    llvm/test/Transforms/LoopVectorize/AArch64/strict-fadd-cost.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Target/AArch64/AArch64TargetTransformInfo.cpp b/llvm/lib/Target/AArch64/AArch64TargetTransformInfo.cpp
index 1c20dddfbf4b9..869c2c121177c 100644
--- a/llvm/lib/Target/AArch64/AArch64TargetTransformInfo.cpp
+++ b/llvm/lib/Target/AArch64/AArch64TargetTransformInfo.cpp
@@ -1999,8 +1999,13 @@ AArch64TTIImpl::getArithmeticReductionCost(unsigned Opcode, VectorType *ValTy,
                                            Optional<FastMathFlags> FMF,
                                            TTI::TargetCostKind CostKind) {
   if (TTI::requiresOrderedReduction(FMF)) {
-    if (!isa<ScalableVectorType>(ValTy))
-      return BaseT::getArithmeticReductionCost(Opcode, ValTy, FMF, CostKind);
+    if (auto *FixedVTy = dyn_cast<FixedVectorType>(ValTy)) {
+      InstructionCost BaseCost =
+          BaseT::getArithmeticReductionCost(Opcode, ValTy, FMF, CostKind);
+      // Add on extra cost to reflect the extra overhead on some CPUs. We still
+      // end up vectorizing for more computationally intensive loops.
+      return BaseCost + FixedVTy->getNumElements();
+    }
 
     if (Opcode != Instruction::FAdd)
       return InstructionCost::getInvalid();

diff  --git a/llvm/test/Analysis/CostModel/AArch64/reduce-fadd.ll b/llvm/test/Analysis/CostModel/AArch64/reduce-fadd.ll
index 935f245fcdd2c..b1f0c1d714d35 100644
--- a/llvm/test/Analysis/CostModel/AArch64/reduce-fadd.ll
+++ b/llvm/test/Analysis/CostModel/AArch64/reduce-fadd.ll
@@ -2,10 +2,10 @@
 
 define void @strict_fp_reductions() {
 ; CHECK-LABEL: strict_fp_reductions
-; CHECK-NEXT: Cost Model: Found an estimated cost of 17 for instruction: %fadd_v4f32 = call float @llvm.vector.reduce.fadd.v4f32(float 0.000000e+00, <4 x float> undef)
-; CHECK-NEXT: Cost Model: Found an estimated cost of 34 for instruction: %fadd_v8f32 = call float @llvm.vector.reduce.fadd.v8f32(float 0.000000e+00, <8 x float> undef)
-; CHECK-NEXT: Cost Model: Found an estimated cost of 7 for instruction: %fadd_v2f64 = call double @llvm.vector.reduce.fadd.v2f64(double 0.000000e+00, <2 x double> undef)
-; CHECK-NEXT: Cost Model: Found an estimated cost of 14 for instruction: %fadd_v4f64 = call double @llvm.vector.reduce.fadd.v4f64(double 0.000000e+00, <4 x double> undef)
+; CHECK-NEXT: Cost Model: Found an estimated cost of 21 for instruction: %fadd_v4f32 = call float @llvm.vector.reduce.fadd.v4f32(float 0.000000e+00, <4 x float> undef)
+; CHECK-NEXT: Cost Model: Found an estimated cost of 42 for instruction: %fadd_v8f32 = call float @llvm.vector.reduce.fadd.v8f32(float 0.000000e+00, <8 x float> undef)
+; CHECK-NEXT: Cost Model: Found an estimated cost of 9 for instruction: %fadd_v2f64 = call double @llvm.vector.reduce.fadd.v2f64(double 0.000000e+00, <2 x double> undef)
+; CHECK-NEXT: Cost Model: Found an estimated cost of 18 for instruction: %fadd_v4f64 = call double @llvm.vector.reduce.fadd.v4f64(double 0.000000e+00, <4 x double> undef)
   %fadd_v4f32 = call float @llvm.vector.reduce.fadd.v4f32(float 0.0, <4 x float> undef)
   %fadd_v8f32 = call float @llvm.vector.reduce.fadd.v8f32(float 0.0, <8 x float> undef)
   %fadd_v2f64 = call double @llvm.vector.reduce.fadd.v2f64(double 0.0, <2 x double> undef)

diff  --git a/llvm/test/Transforms/LoopVectorize/AArch64/strict-fadd-cost.ll b/llvm/test/Transforms/LoopVectorize/AArch64/strict-fadd-cost.ll
index 1f309ff3cac14..20ed971e409f3 100644
--- a/llvm/test/Transforms/LoopVectorize/AArch64/strict-fadd-cost.ll
+++ b/llvm/test/Transforms/LoopVectorize/AArch64/strict-fadd-cost.ll
@@ -6,8 +6,8 @@
 
 target triple="aarch64-unknown-linux-gnu"
 
-; CHECK-VF4: Found an estimated cost of 17 for VF 4 For instruction:   %add = fadd float %0, %sum.07
-; CHECK-VF8: Found an estimated cost of 34 for VF 8 For instruction:   %add = fadd float %0, %sum.07
+; CHECK-VF4: Found an estimated cost of 21 for VF 4 For instruction:   %add = fadd float %0, %sum.07
+; CHECK-VF8: Found an estimated cost of 42 for VF 8 For instruction:   %add = fadd float %0, %sum.07
 
 define float @fadd_strict32(float* noalias nocapture readonly %a, i64 %n) {
 entry:
@@ -28,8 +28,8 @@ for.end:
 }
 
 
-; CHECK-VF4: Found an estimated cost of 14 for VF 4 For instruction:   %add = fadd double %0, %sum.07
-; CHECK-VF8: Found an estimated cost of 28 for VF 8 For instruction:   %add = fadd double %0, %sum.07
+; CHECK-VF4: Found an estimated cost of 18 for VF 4 For instruction:   %add = fadd double %0, %sum.07
+; CHECK-VF8: Found an estimated cost of 36 for VF 8 For instruction:   %add = fadd double %0, %sum.07
 
 define double @fadd_strict64(double* noalias nocapture readonly %a, i64 %n) {
 entry: