[TTI][AArch64] Add preferFixedIfEqualToScalable hook

This adds a new hook to prefer fixed width loop vectorization over
scalable. This will be used in the loop vectoriser to generate more NEON
code instead of SVE if cost-model assigns equal costs to fixed and
scalable versions of the vectorised loop.

This is used in #95819.

Author: sjoerdmeijer

llvm/include/llvm/Analysis/TargetTransformInfo.h

@@ -1674,6 +1674,11 @@ class TargetTransformInfo {
         false; ///< If op is an fp min/max, whether NaNs may be present.
   };
 
+  /// \returns True if the targets prefers fixed width vectorization if the
+  /// loop vectorizer's cost-model assigns an equal cost to the fixed and
+  /// scalable version of the vectorized loop.
+  bool preferFixedIfEqualToScalable() const;
+
   /// \returns True if the target prefers reductions in loop.
   bool preferInLoopReduction(unsigned Opcode, Type *Ty,
                              ReductionFlags Flags) const;
@@ -2143,6 +2148,7 @@ class TargetTransformInfo::Concept {
   virtual unsigned getStoreVectorFactor(unsigned VF, unsigned StoreSize,
                                         unsigned ChainSizeInBytes,
                                         VectorType *VecTy) const = 0;
+  virtual bool preferFixedIfEqualToScalable() const = 0;
   virtual bool preferInLoopReduction(unsigned Opcode, Type *Ty,
                                      ReductionFlags) const = 0;
   virtual bool preferPredicatedReductionSelect(unsigned Opcode, Type *Ty,
@@ -2873,6 +2879,9 @@ class TargetTransformInfo::Model final : public TargetTransformInfo::Concept {
                                 VectorType *VecTy) const override {
     return Impl.getStoreVectorFactor(VF, StoreSize, ChainSizeInBytes, VecTy);
   }
+  bool preferFixedIfEqualToScalable() const override {
+    return Impl.preferFixedIfEqualToScalable();
+  }
   bool preferInLoopReduction(unsigned Opcode, Type *Ty,
                              ReductionFlags Flags) const override {
     return Impl.preferInLoopReduction(Opcode, Ty, Flags);

llvm/include/llvm/Analysis/TargetTransformInfoImpl.h

@@ -913,6 +913,8 @@ class TargetTransformInfoImplBase {
     return VF;
   }
 
+  bool preferFixedIfEqualToScalable() const { return false; }
+
   bool preferInLoopReduction(unsigned Opcode, Type *Ty,
                              TTI::ReductionFlags Flags) const {
     return false;

llvm/lib/Analysis/TargetTransformInfo.cpp

@@ -1282,6 +1282,10 @@ unsigned TargetTransformInfo::getStoreVectorFactor(unsigned VF,
   return TTIImpl->getStoreVectorFactor(VF, StoreSize, ChainSizeInBytes, VecTy);
 }
 
+bool TargetTransformInfo::preferFixedIfEqualToScalable() const {
+  return TTIImpl->preferFixedIfEqualToScalable();
+}
+
 bool TargetTransformInfo::preferInLoopReduction(unsigned Opcode, Type *Ty,
                                                 ReductionFlags Flags) const {
   return TTIImpl->preferInLoopReduction(Opcode, Ty, Flags);

llvm/lib/Target/AArch64/AArch64Features.td

@@ -244,6 +244,9 @@ def FeatureExperimentalZeroingPseudos
 def FeatureUseScalarIncVL : SubtargetFeature<"use-scalar-inc-vl",
   "UseScalarIncVL", "true", "Prefer inc/dec over add+cnt">;
 
+def FeatureUseFixedIfEqualToScalable : SubtargetFeature<"use-fixed-if-equal-to-scalable",
+  "UseFixedIfEqualToScalable", "true", "Prefer fixed width loop vectorization over scalable if cost-model assigns equal costs">;
+
 def FeatureBF16 : Extension<"bf16", "BF16",
     "Enable BFloat16 Extension (FEAT_BF16)", [],
     "FEAT_BF16", "+bf16", 280>;

llvm/lib/Target/AArch64/AArch64Processors.td

@@ -489,6 +489,7 @@ def TuneNeoverseV2 : SubtargetFeature<"neoversev2", "ARMProcFamily", "NeoverseV2
                                       FeatureALULSLFast,
                                       FeaturePostRAScheduler,
                                       FeatureEnableSelectOptimize,
+                                      FeatureUseFixedIfEqualToScalable,
                                       FeaturePredictableSelectIsExpensive]>;
 
 def TuneNeoverseV3 : SubtargetFeature<"neoversev3", "ARMProcFamily", "NeoverseV3",

llvm/lib/Target/AArch64/AArch64TargetTransformInfo.h

@@ -371,6 +371,10 @@ class AArch64TTIImpl : public BasicTTIImplBase<AArch64TTIImpl> {
     return TailFoldingStyle::DataWithoutLaneMask;
   }
 
+  bool preferFixedIfEqualToScalable() const {
+    return ST->useFixedIfEqualToScalable();
+  }
+
   bool preferPredicateOverEpilogue(TailFoldingInfo *TFI);
 
   bool supportsScalableVectors() const { return ST->hasSVE(); }