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

[Florian Hahn via llvm-commits]

================
@@ -2028,68 +2029,105 @@ 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] =
+  const auto &[Dist, StrideA, StrideB, TypeByteSize, AIsWrite, BIsWrite] =
       std::get<DepDistanceStrideAndSizeInfo>(Res);
   bool HasSameSize = TypeByteSize > 0;
 
+  uint64_t CommonStride = StrideA == StrideB ? StrideA : 0;
+  if (isa<SCEVCouldNotCompute>(Dist)) {
+    FoundNonConstantDistanceDependence |= CommonStride;
+    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))
----------------
fhahn wrote:

At the moment the comment in the function implies that it assumes equal strides which is why I left it as-is for now, to limit the scope of the initial patch. 

It tries to prove `|Dist| > BackedgeTakenCount * Stride`, so using the maximum should conservatively be correct. 

A similar reasoning can be applied at the end of the function too, which also is currently guarded by having a common stride. I am also planning on addressing that separately.

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