[llvm] 17912f3 - LAA: refactor dependence class to prep for scaled strides (NFC) (#122113)

[via llvm-commits]

Author: Ramkumar Ramachandra
Date: 2025-01-09T16:05:17Z
New Revision: 17912f336bd3d3db09b367c155fb44148ea4de24

URL: https://github.com/llvm/llvm-project/commit/17912f336bd3d3db09b367c155fb44148ea4de24
DIFF: https://github.com/llvm/llvm-project/commit/17912f336bd3d3db09b367c155fb44148ea4de24.diff

LOG: LAA: refactor dependence class to prep for scaled strides (NFC) (#122113)

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

Added: 
    

Modified: 
    llvm/include/llvm/Analysis/LoopAccessAnalysis.h
    llvm/lib/Analysis/LoopAccessAnalysis.cpp

Removed: 
    


################################################################################
diff  --git a/llvm/include/llvm/Analysis/LoopAccessAnalysis.h b/llvm/include/llvm/Analysis/LoopAccessAnalysis.h
index a35bc7402d1a89..31374a128856c7 100644
--- a/llvm/include/llvm/Analysis/LoopAccessAnalysis.h
+++ b/llvm/include/llvm/Analysis/LoopAccessAnalysis.h
@@ -366,16 +366,26 @@ class MemoryDepChecker {
 
   struct DepDistanceStrideAndSizeInfo {
     const SCEV *Dist;
-    uint64_t StrideA;
-    uint64_t StrideB;
+
+    /// Strides could either be scaled (in bytes, taking the size of the
+    /// underlying type into account), or unscaled (in indexing units; unscaled
+    /// stride = scaled stride / size of underlying type). Here, strides are
+    /// unscaled.
+    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 
diff erent 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 
diff erent but there is no inherent
     // reason to.
-    FoundNonConstantDistanceDependence |= CommonStride.has_value();
+    FoundNonConstantDistanceDependence |= ShouldRetryWithRuntimeCheck;
   }
 
   if (!HasSameSize) {