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

[Michael Kruse via llvm-branch-commits]

================
@@ -2028,41 +2029,61 @@ 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;
 
+  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;
+    LLVM_DEBUG(dbgs() << "LAA: Dependence because of uncomputable distance.\n");
+    return Dependence::Unknown;
+  }
+
   ScalarEvolution &SE = *PSE.getSE();
   auto &DL = InnermostLoop->getHeader()->getModule()->getDataLayout();
   // If the distance between the acecsses is larger than their absolute stride
   // multiplied by the backedge taken count, the accesses are independet, i.e.
   // they are far enough appart that accesses won't access the same location
   // across all loop ierations.
-  if (!isa<SCEVCouldNotCompute>(Dist) && HasSameSize &&
+  if (HasSameSize && CommonStride &&
----------------
Meinersbur wrote:

`isSafeDependenceDistance` could be easily modified as well to support different sizes, but it's unrelated to this patch.

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