[llvm] LAA: be more precise on different store sizes (PR #122318)

[Ramkumar Ramachandra via llvm-commits]

================
@@ -1989,11 +1990,24 @@ MemoryDepChecker::getDependenceDistanceStrideAndSize(
     return MemoryDepChecker::Dependence::Unknown;
   }
 
-  uint64_t TypeByteSize = DL.getTypeAllocSize(ATy);
-  bool HasSameSize =
-      DL.getTypeStoreSizeInBits(ATy) == DL.getTypeStoreSizeInBits(BTy);
-  if (!HasSameSize)
-    TypeByteSize = 0;
+  // When the distance is zero, we're reading/writing the same memory location:
+  // check that the store sizes are equal. Otherwise, fail with an unknown
+  // dependence for which we should not generate runtime checks.
+  TypeSize AStoreSz = DL.getTypeStoreSize(ATy),
+           BStoreSz = DL.getTypeStoreSize(BTy);
+  if (Dist->isZero() && AStoreSz != BStoreSz) {
----------------
artagnon wrote:

I think these hunks in the patch are correct, and that we were being overly conservative earlier. See also: positive LV test change.

```diff
@@ -2102,8 +2110,7 @@ MemoryDepChecker::isDependent(const MemAccessInfo &A, unsigned AIdx,
         FoundNonConstantDistanceDependence |= ShouldRetryWithRuntimeCheck;
         return Dependence::Unknown;
       }
-      if (!HasSameSize ||
-          couldPreventStoreLoadForward(
+      if (couldPreventStoreLoadForward(
               ConstDist->getAPInt().abs().getZExtValue(), TypeByteSize)) {
         LLVM_DEBUG(
             dbgs() << "LAA: Forward but may prevent st->ld forwarding\n");
@@ -2134,12 +2141,6 @@ MemoryDepChecker::isDependent(const MemAccessInfo &A, unsigned AIdx,
     FoundNonConstantDistanceDependence |= ShouldRetryWithRuntimeCheck;
   }

-  if (!HasSameSize) {
-    LLVM_DEBUG(dbgs() << "LAA: ReadWrite-Write positive dependency with "
-                         "different type sizes\n");
-    return Dependence::Unknown;
-  }
-
```

https://github.com/llvm/llvm-project/pull/122318