[llvm] LAA: rearrange in preparation to scale strides (NFC) (PR #122113)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-analysis

Author: Ramkumar Ramachandra (artagnon)

<details>
<summary>Changes</summary>

Rearrange the DepDistanceAndSizeInfo struct in preparation to scale strides. getDependenceDistanceStrideAndSize now returns the data of CommonStride, MaxStride, and clarifies when to rety with runtime checks, in place of (unscaled) strides.

---
Full diff: https://github.com/llvm/llvm-project/pull/122113.diff


2 Files Affected:

- (modified) llvm/include/llvm/Analysis/LoopAccessAnalysis.h (+10-6) 
- (modified) llvm/lib/Analysis/LoopAccessAnalysis.cpp (+25-12) 


``````````diff
diff --git a/llvm/include/llvm/Analysis/LoopAccessAnalysis.h b/llvm/include/llvm/Analysis/LoopAccessAnalysis.h
index a35bc7402d1a89..7bc8c4deae1a36 100644
--- a/llvm/include/llvm/Analysis/LoopAccessAnalysis.h
+++ b/llvm/include/llvm/Analysis/LoopAccessAnalysis.h
@@ -366,16 +366,20 @@ class MemoryDepChecker {
 
   struct DepDistanceStrideAndSizeInfo {
     const SCEV *Dist;
-    uint64_t StrideA;
-    uint64_t StrideB;
+    uint64_t MaxStride;
+    std::optional<uint64_t> CommonStride;
+    bool ShouldRetryWithRuntimeCheck;
     uint64_t TypeByteSize;
     bool AIsWrite;
     bool BIsWrite;
 
-    DepDistanceStrideAndSizeInfo(const SCEV *Dist, uint64_t StrideA,
-                                 uint64_t StrideB, uint64_t TypeByteSize,
-                                 bool AIsWrite, bool BIsWrite)
-        : Dist(Dist), StrideA(StrideA), StrideB(StrideB),
+    DepDistanceStrideAndSizeInfo(const SCEV *Dist, uint64_t MaxStride,
+                                 std::optional<uint64_t> CommonStride,
+                                 bool ShouldRetryWithRuntimeCheck,
+                                 uint64_t TypeByteSize, bool AIsWrite,
+                                 bool BIsWrite)
+        : Dist(Dist), MaxStride(MaxStride), CommonStride(CommonStride),
+          ShouldRetryWithRuntimeCheck(ShouldRetryWithRuntimeCheck),
           TypeByteSize(TypeByteSize), AIsWrite(AIsWrite), BIsWrite(BIsWrite) {}
   };
 
diff --git a/llvm/lib/Analysis/LoopAccessAnalysis.cpp b/llvm/lib/Analysis/LoopAccessAnalysis.cpp
index 2c75d5625cb66d..38e9145826c08e 100644
--- a/llvm/lib/Analysis/LoopAccessAnalysis.cpp
+++ b/llvm/lib/Analysis/LoopAccessAnalysis.cpp
@@ -1994,8 +1994,23 @@ MemoryDepChecker::getDependenceDistanceStrideAndSize(
       DL.getTypeStoreSizeInBits(ATy) == DL.getTypeStoreSizeInBits(BTy);
   if (!HasSameSize)
     TypeByteSize = 0;
-  return DepDistanceStrideAndSizeInfo(Dist, std::abs(StrideAPtrInt),
-                                      std::abs(StrideBPtrInt), TypeByteSize,
+
+  StrideAPtrInt = std::abs(StrideAPtrInt);
+  StrideBPtrInt = std::abs(StrideBPtrInt);
+
+  uint64_t MaxStride = std::max(StrideAPtrInt, StrideBPtrInt);
+
+  std::optional<uint64_t> CommonStride;
+  if (StrideAPtrInt == StrideBPtrInt)
+    CommonStride = StrideAPtrInt;
+
+  // TODO: Historically, we don't retry with runtime checks unless the
+  // (unscaled) strides are the same. Fix this once the condition for runtime
+  // checks in isDependent is fixed.
+  bool ShouldRetryWithRuntimeCheck = CommonStride.has_value();
+
+  return DepDistanceStrideAndSizeInfo(Dist, MaxStride, CommonStride,
+                                      ShouldRetryWithRuntimeCheck, TypeByteSize,
                                       AIsWrite, BIsWrite);
 }
 
@@ -2011,23 +2026,21 @@ MemoryDepChecker::isDependent(const MemAccessInfo &A, unsigned AIdx,
   if (std::holds_alternative<Dependence::DepType>(Res))
     return std::get<Dependence::DepType>(Res);
 
-  auto &[Dist, StrideA, StrideB, TypeByteSize, AIsWrite, BIsWrite] =
+  auto &[Dist, MaxStride, CommonStride, ShouldRetryWithRuntimeCheck,
+         TypeByteSize, AIsWrite, BIsWrite] =
       std::get<DepDistanceStrideAndSizeInfo>(Res);
   bool HasSameSize = TypeByteSize > 0;
 
-  std::optional<uint64_t> CommonStride =
-      StrideA == StrideB ? std::make_optional(StrideA) : std::nullopt;
   if (isa<SCEVCouldNotCompute>(Dist)) {
-    // TODO: Relax requirement that there is a common stride to retry with
-    // non-constant distance dependencies.
-    FoundNonConstantDistanceDependence |= CommonStride.has_value();
+    // TODO: Relax requirement that there is a common unscaled stride to retry
+    // with non-constant distance dependencies.
+    FoundNonConstantDistanceDependence |= ShouldRetryWithRuntimeCheck;
     LLVM_DEBUG(dbgs() << "LAA: Dependence because of uncomputable distance.\n");
     return Dependence::Unknown;
   }
 
   ScalarEvolution &SE = *PSE.getSE();
   auto &DL = InnermostLoop->getHeader()->getDataLayout();
-  uint64_t MaxStride = std::max(StrideA, StrideB);
 
   // If the distance between the acecsses is larger than their maximum absolute
   // stride multiplied by the symbolic maximum backedge taken count (which is an
@@ -2086,7 +2099,7 @@ MemoryDepChecker::isDependent(const MemAccessInfo &A, unsigned AIdx,
         // condition to consider retrying with runtime checks. Historically, we
         // did not set it when strides were different but there is no inherent
         // reason to.
-        FoundNonConstantDistanceDependence |= CommonStride.has_value();
+        FoundNonConstantDistanceDependence |= ShouldRetryWithRuntimeCheck;
         return Dependence::Unknown;
       }
       if (!HasSameSize ||
@@ -2105,7 +2118,7 @@ MemoryDepChecker::isDependent(const MemAccessInfo &A, unsigned AIdx,
   int64_t MinDistance = SE.getSignedRangeMin(Dist).getSExtValue();
   // Below we only handle strictly positive distances.
   if (MinDistance <= 0) {
-    FoundNonConstantDistanceDependence |= CommonStride.has_value();
+    FoundNonConstantDistanceDependence |= ShouldRetryWithRuntimeCheck;
     return Dependence::Unknown;
   }
 
@@ -2118,7 +2131,7 @@ MemoryDepChecker::isDependent(const MemAccessInfo &A, unsigned AIdx,
     // condition to consider retrying with runtime checks. Historically, we
     // did not set it when strides were different but there is no inherent
     // reason to.
-    FoundNonConstantDistanceDependence |= CommonStride.has_value();
+    FoundNonConstantDistanceDependence |= ShouldRetryWithRuntimeCheck;
   }
 
   if (!HasSameSize) {

``````````

</details>


https://github.com/llvm/llvm-project/pull/122113