[llvm] [LAA] Support different strides & non constant dep distances using SCEV. (PR #88039)

[Nikita Popov via llvm-commits]

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@@ -2011,68 +2012,109 @@ MemoryDepChecker::Dependence::DepType MemoryDepChecker::isDependent(
   if (std::holds_alternative<Dependence::DepType>(Res))
     return std::get<Dependence::DepType>(Res);
 
-  const auto &[Dist, Stride, TypeByteSize, AIsWrite, BIsWrite] =
-      std::get<std::tuple<const SCEV *, uint64_t, uint64_t, bool, bool>>(Res);
+  const auto &[Dist, StrideA, StrideB, TypeByteSize, AIsWrite, BIsWrite] =
+      std::get<
+          std::tuple<const SCEV *, uint64_t, uint64_t, uint64_t, bool, bool>>(
+          Res);
   bool HasSameSize = TypeByteSize > 0;
 
+  uint64_t CommonStride = StrideA == StrideB ? StrideA : 0;
+  if (isa<SCEVCouldNotCompute>(Dist)) {
+    FoundNonConstantDistanceDependence = true;
+    LLVM_DEBUG(dbgs() << "LAA: Dependence because of uncomputable distance.\n");
+    return Dependence::Unknown;
+  }
+
   ScalarEvolution &SE = *PSE.getSE();
   auto &DL = InnermostLoop->getHeader()->getModule()->getDataLayout();
-  if (!isa<SCEVCouldNotCompute>(Dist) && HasSameSize &&
+  if (HasSameSize && CommonStride &&
       isSafeDependenceDistance(DL, SE, *(PSE.getBackedgeTakenCount()), *Dist,
-                               Stride, TypeByteSize))
+                               CommonStride, TypeByteSize))
     return Dependence::NoDep;
 
   const SCEVConstant *C = dyn_cast<SCEVConstant>(Dist);
-  if (!C) {
-    LLVM_DEBUG(dbgs() << "LAA: Dependence because of non-constant distance\n");
-    FoundNonConstantDistanceDependence = true;
-    return Dependence::Unknown;
-  }
-
-  const APInt &Val = C->getAPInt();
-  int64_t Distance = Val.getSExtValue();
 
   // Attempt to prove strided accesses independent.
-  if (std::abs(Distance) > 0 && Stride > 1 && HasSameSize &&
-      areStridedAccessesIndependent(std::abs(Distance), Stride, TypeByteSize)) {
-    LLVM_DEBUG(dbgs() << "LAA: Strided accesses are independent\n");
-    return Dependence::NoDep;
+  if (C) {
+    const APInt &Val = C->getAPInt();
+    int64_t Distance = Val.getSExtValue();
+
+    if (std::abs(Distance) > 0 && CommonStride > 1 && HasSameSize &&
+        areStridedAccessesIndependent(std::abs(Distance), CommonStride,
+                                      TypeByteSize)) {
+      LLVM_DEBUG(dbgs() << "LAA: Strided accesses are independent\n");
+      return Dependence::NoDep;
+    }
+  }
+
+  // Write to the same location with the same size.
+  if (SE.isKnownPredicate(CmpInst::ICMP_EQ, Dist,
+                          SE.getZero(Dist->getType()))) {
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nikic wrote:

Don't use isKnownPredicate with ICMP_EQ -- directly check whether Dist is zero. (As such, should be part of the `if (C)` branch).

https://github.com/llvm/llvm-project/pull/88039