[MLIR] Define memory effects for memref.prefetch operation (llvm#151261)

Currently `memref.prefetch` has no memory side effects, which are
necessary for some optimizations.

This PR adds the needed side effect, as recommended in
https://discourse.llvm.org/t/modeling-volatility-with-memory-effects/67946

This PR was created after a discussion on this specific topic here
https://discourse.llvm.org/t/memref-prefetch-op-has-no-memory-side-effects-decoration-in-the-def-td-file/87482

---------

Signed-off-by: Alexandre Eichenberger <[email protected]>

Author: AlexandreEichenberger

mlir/include/mlir/Dialect/MemRef/IR/MemRefOps.td

@@ -1414,7 +1414,14 @@ def MemRef_PrefetchOp : MemRef_Op<"prefetch"> {
     instruction cache.
   }];
 
-  let arguments = (ins AnyMemRef:$memref, Variadic<Index>:$indices,
+  // The memref argument is labeled with a side effect to enforce a
+  // relative ordering of the prefetch and other memory operations targeting
+  // that memory stream.
+  // We need it to be a write otherwise the operation would be trivially removed
+  // since it does not produce a value.
+
+  let arguments = (ins Arg<AnyMemRef, "prefetch address", [MemWrite]> :$memref,
+                       Variadic<Index>:$indices,
                        BoolAttr:$isWrite,
                        ConfinedAttr<I32Attr, [IntMinValue<0>,
                                           IntMaxValue<3>]>:$localityHint,

mlir/test/Dialect/Bufferization/Transforms/OwnershipBasedBufferDeallocation/misc-other.mlir

@@ -17,4 +17,12 @@ func.func @func_with_assert(%arg0: index, %arg1: index) {
 func.func @func_with_assume_alignment(%arg0: memref<128xi8>) {
   %0 = memref.assume_alignment %arg0, 64 : memref<128xi8>
   return
-}
+}
+
+// CHECK-LABEL: func @func_with_prefetch(
+//       CHECK: memref.prefetch %arg0[%c0, %c0], read, locality<1>, data : memref<4x8xf32>
+func.func @func_with_prefetch(%arg0: memref<4x8xf32>) {
+  %c0 = arith.constant 0 : index
+  memref.prefetch %arg0[%c0, %c0], read, locality<1>, data : memref<4x8xf32>
+  return
+}