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

[Michael Kruse via llvm-commits]

================
@@ -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");
----------------
Meinersbur wrote:

Not that I like the arbitrary-looking `!!CommonStride` scattered around the code, but at least I understand now. `FoundNonConstantDistanceDependence` sounded like something that's safer to set to true when in doubt. Should really be called `ReconsiderWithRuntimeChecks`. If I may suggest an alternative TODO:
```
// TODO: FoundNonConstantDistanceDependence is used as a necessary condition to consider retrying
// with runtime checks. Historically, we did not set it when strides were different but there is no inherent
// reason to.
```


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