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

[Michael Kruse via llvm-branch-commits]

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@@ -2071,34 +2092,51 @@ MemoryDepChecker::Dependence::DepType MemoryDepChecker::isDependent(
     // NOTE: There is no need to update MaxSafeVectorWidthInBits after call to
     // couldPreventStoreLoadForward, even if it changed MinDepDistBytes, since a
     // forward dependency will allow vectorization using any width.
-    if (IsTrueDataDependence && EnableForwardingConflictDetection &&
-        (!HasSameSize || couldPreventStoreLoadForward(Val.abs().getZExtValue(),
-                                                      TypeByteSize))) {
-      LLVM_DEBUG(dbgs() << "LAA: Forward but may prevent st->ld forwarding\n");
-      return Dependence::ForwardButPreventsForwarding;
+    if (IsTrueDataDependence && EnableForwardingConflictDetection) {
+      if (!C) {
+        // TODO: Relax requirement that there is a common stride to retry with
+        // non-constant distance dependencies.
+        FoundNonConstantDistanceDependence |= !!CommonStride;
+        return Dependence::Unknown;
+      }
+      if (!HasSameSize ||
+          couldPreventStoreLoadForward(C->getAPInt().abs().getZExtValue(),
+                                       TypeByteSize)) {
+        LLVM_DEBUG(
+            dbgs() << "LAA: Forward but may prevent st->ld forwarding\n");
+        return Dependence::ForwardButPreventsForwarding;
+      }
     }
 
     LLVM_DEBUG(dbgs() << "LAA: Dependence is negative\n");
     return Dependence::Forward;
   }
 
-  // Write to the same location with the same size.
-  if (Val == 0) {
-    if (HasSameSize)
-      return Dependence::Forward;
-    LLVM_DEBUG(
-        dbgs() << "LAA: Zero dependence difference but different type sizes\n");
+  if (!C) {
+    // 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 non-constant distance\n");
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Meinersbur wrote:

This is the bail-out moved here from line 2047. Is this done so return Dependence::Forward with negative distance can be returned also on non-constant distances?

`FoundNonConstantDistanceDependence` is not set if there is no common stride, but what does it have to do with the distance between the pointers? Before this patch it would set that flag independent of any stride.

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