[clang] [CIR] Add alias analysis skeleton (PR #214357)

[Erich Keane via cfe-commits]

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@@ -0,0 +1,94 @@
+//===- CIRBasicAliasAnalysis.cpp - Basic CIR Alias Analysis ---------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "clang/CIR/Dialect/Analysis/CIRBasicAliasAnalysis.h"
+#include "clang/CIR/Dialect/IR/CIRDialect.h"
+#include "mlir/Interfaces/SideEffectInterfaces.h"
+
+using namespace llvm;
+using namespace cir;
+
+//===----------------------------------------------------------------------===//
+// Helpers
+//===----------------------------------------------------------------------===//
+
+mlir::Value CIRBasicAliasAnalysis::getUnderlyingObject(mlir::Value val) {
+  // TODO: Walk through cir.ptr_stride, cir.cast, cir.get_member, etc.
+  // to find the root allocation (cir.alloca, cir.global_addr, function args).
+  return val;
+}
+
+bool CIRBasicAliasAnalysis::areDistinctObjects(mlir::Value lhs, mlir::Value rhs) {
+  // Two values are distinct allocations if they originate from different
+  // cir.alloca operations (or other allocation ops) in the same function.
+  // TODO: Extend to cover global addresses, function arguments with noalias,
+  // and heap allocations.
+  mlir::Value lhsObj = getUnderlyingObject(lhs);
+  mlir::Value rhsObj = getUnderlyingObject(rhs);
+
+  if (lhsObj == rhsObj)
+    return false;
+
+  // Different cir.alloca ops in the same function cannot alias.
+  if (mlir::isa_and_nonnull<cir::AllocaOp>(lhsObj.getDefiningOp()) &&
+      mlir::isa_and_nonnull<cir::AllocaOp>(rhsObj.getDefiningOp()))
+    return true;
+
+  return false;
+}
+
+//===----------------------------------------------------------------------===//
+// CIRBasicAliasAnalysis
+//===----------------------------------------------------------------------===//
+
+mlir::AliasResult CIRBasicAliasAnalysis::alias(mlir::Value lhs, mlir::Value rhs) {
+  if (lhs == rhs)
+    return mlir::AliasResult::MustAlias;
+
+  if (areDistinctObjects(lhs, rhs))
+    return mlir::AliasResult::NoAlias;
+
+  // Conservative fallback — the aggregate will try other implementations.
+  return mlir::AliasResult::MayAlias;
+}
+
+mlir::ModRefResult CIRBasicAliasAnalysis::getModRef(mlir::Operation *op,
+                                                    mlir::Value location) {
+  // Pure operations (no side effects) neither modify nor reference memory.
+  if (auto effects = mlir::dyn_cast<mlir::MemoryEffectOpInterface>(op)) {
+    if (effects.hasNoEffect())
+      return mlir::ModRefResult::getNoModRef();
+
+    SmallVector<mlir::MemoryEffects::EffectInstance> effectList;
+    effects.getEffects(effectList);
+
+    bool mod = false;
+    bool ref = false;
+    for (auto &effect : effectList) {
+      // Only count effects on the queried location (or unknown location).
+      mlir::Value effectVal = effect.getValue();
+      if (effectVal && effectVal != location)
+        continue;
+      if (mlir::isa<mlir::MemoryEffects::Write>(effect.getEffect()))
+        mod = true;
+      else if (mlir::isa<mlir::MemoryEffects::Read>(effect.getEffect()))
+        ref = true;
+    }
+
+    if (!mod && !ref)
----------------
erichkeane wrote:

Perhaps a dumb suggestion.. what if you made `mod` and `ref` `mlir::ModRefResult`, and set them to the '`getMod` and `getRef` in the loop above, then used `merge` (or `intersect`? Stop making me think about linear algebra) to combine them instead of the below?

WDYT?

https://github.com/llvm/llvm-project/pull/214357