[llvm] [TTI][AArch64] Add preferFixedIfEqualToScalable hook (PR #95818)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-llvm-analysis

@llvm/pr-subscribers-backend-aarch64

Author: Sjoerd Meijer (sjoerdmeijer)

<details>
<summary>Changes</summary>

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.

---
Full diff: https://github.com/llvm/llvm-project/pull/95818.diff


6 Files Affected:

- (modified) llvm/include/llvm/Analysis/TargetTransformInfo.h (+6) 
- (modified) llvm/include/llvm/Analysis/TargetTransformInfoImpl.h (+4) 
- (modified) llvm/lib/Analysis/TargetTransformInfo.cpp (+4) 
- (modified) llvm/lib/Target/AArch64/AArch64Features.td (+3) 
- (modified) llvm/lib/Target/AArch64/AArch64Processors.td (+1) 
- (modified) llvm/lib/Target/AArch64/AArch64TargetTransformInfo.h (+2) 


``````````diff
diff --git a/llvm/include/llvm/Analysis/TargetTransformInfo.h b/llvm/include/llvm/Analysis/TargetTransformInfo.h
index f55f21c94a85a..84b811818beee 100644
--- a/llvm/include/llvm/Analysis/TargetTransformInfo.h
+++ b/llvm/include/llvm/Analysis/TargetTransformInfo.h
@@ -1674,6 +1674,8 @@ class TargetTransformInfo {
         false; ///< If op is an fp min/max, whether NaNs may be present.
   };
 
+  bool preferFixedIfEqualToScalable() const;
+
   /// \returns True if the target prefers reductions in loop.
   bool preferInLoopReduction(unsigned Opcode, Type *Ty,
                              ReductionFlags Flags) const;
@@ -2143,6 +2145,7 @@ 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 +2876,9 @@ 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);
diff --git a/llvm/include/llvm/Analysis/TargetTransformInfoImpl.h b/llvm/include/llvm/Analysis/TargetTransformInfoImpl.h
index 7828bdc1f1f43..9679c9ec6bc4e 100644
--- a/llvm/include/llvm/Analysis/TargetTransformInfoImpl.h
+++ b/llvm/include/llvm/Analysis/TargetTransformInfoImpl.h
@@ -913,6 +913,10 @@ class TargetTransformInfoImplBase {
     return VF;
   }
 
+  bool preferFixedIfEqualToScalable() const {
+    return false;
+  }
+
   bool preferInLoopReduction(unsigned Opcode, Type *Ty,
                              TTI::ReductionFlags Flags) const {
     return false;
diff --git a/llvm/lib/Analysis/TargetTransformInfo.cpp b/llvm/lib/Analysis/TargetTransformInfo.cpp
index 7e721cbc87f3f..27a7f5b32d3cf 100644
--- a/llvm/lib/Analysis/TargetTransformInfo.cpp
+++ b/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);
diff --git a/llvm/lib/Target/AArch64/AArch64Features.td b/llvm/lib/Target/AArch64/AArch64Features.td
index ffb899a301459..988630769afdb 100644
--- a/llvm/lib/Target/AArch64/AArch64Features.td
+++ b/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>;
diff --git a/llvm/lib/Target/AArch64/AArch64Processors.td b/llvm/lib/Target/AArch64/AArch64Processors.td
index cc33765307fb4..a0263f0164f4f 100644
--- a/llvm/lib/Target/AArch64/AArch64Processors.td
+++ b/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",
diff --git a/llvm/lib/Target/AArch64/AArch64TargetTransformInfo.h b/llvm/lib/Target/AArch64/AArch64TargetTransformInfo.h
index feec1a4289c3a..13e66f9ea1913 100644
--- a/llvm/lib/Target/AArch64/AArch64TargetTransformInfo.h
+++ b/llvm/lib/Target/AArch64/AArch64TargetTransformInfo.h
@@ -371,6 +371,8 @@ class AArch64TTIImpl : public BasicTTIImplBase<AArch64TTIImpl> {
     return TailFoldingStyle::DataWithoutLaneMask;
   }
 
+  bool preferFixedIfEqualToScalable() const { return ST->useFixedIfEqualToScalable(); }
+
   bool preferPredicateOverEpilogue(TailFoldingInfo *TFI);
 
   bool supportsScalableVectors() const { return ST->hasSVE(); }

``````````

</details>


https://github.com/llvm/llvm-project/pull/95818