[flang] approximate alias analysis support for hlfir.designate

Add a rough alias analysis rule for hlfir.designate which just follows
the memref argument. This could be extended in the future to take into
account the indices or derived type fields accessed to spot for provably
non-overlapping cases. In the meantime, we need a flag to ensure we
never say "MustAlias" when following a value through a hlfir.designate
because the designate analysis is only approximate.

Differential Revision: https://reviews.llvm.org/D157718

Author: tblah

flang/include/flang/Optimizer/Analysis/AliasAnalysis.h

@@ -55,6 +55,9 @@ class AliasAnalysis {
     mlir::Type valueType;
     /// Attributes of the memory source object, e.g. Target.
     Attributes attributes;
+    /// Have we lost precision following the source such that
+    /// even an exact match cannot be MustAlias?
+    bool approximateSource;
 
     /// Print information about the memory source to `os`.
     void print(llvm::raw_ostream &os) const;

flang/lib/Optimizer/Analysis/AliasAnalysis.cpp

@@ -71,6 +71,7 @@ bool AliasAnalysis::Source::isRecordWithPointerComponent() const {
 AliasResult AliasAnalysis::alias(Value lhs, Value rhs) {
   auto lhsSrc = getSource(lhs);
   auto rhsSrc = getSource(rhs);
+  bool approximateSource = lhsSrc.approximateSource || rhsSrc.approximateSource;
   LLVM_DEBUG(llvm::dbgs() << "AliasAnalysis::alias\n";
              llvm::dbgs() << "  lhs: " << lhs << "\n";
              llvm::dbgs() << "  lhsSrc: " << lhsSrc << "\n";
@@ -86,8 +87,11 @@ AliasResult AliasAnalysis::alias(Value lhs, Value rhs) {
     return AliasResult::MayAlias;
 
   if (lhsSrc.kind == rhsSrc.kind) {
-    if (lhsSrc.u == rhsSrc.u)
+    if (lhsSrc.u == rhsSrc.u) {
+      if (approximateSource)
+        return AliasResult::MayAlias;
       return AliasResult::MustAlias;
+    }
 
     // Allocate and global memory address cannot physically alias
     if (lhsSrc.kind == SourceKind::Allocate ||
@@ -195,6 +199,7 @@ AliasAnalysis::Source AliasAnalysis::getSource(mlir::Value v) {
   SourceKind type{SourceKind::Unknown};
   mlir::Type ty;
   bool breakFromLoop{false};
+  bool approximateSource{false};
   mlir::SymbolRefAttr global;
   Source::Attributes attributes;
   while (defOp && !breakFromLoop) {
@@ -234,6 +239,19 @@ AliasAnalysis::Source AliasAnalysis::getSource(mlir::Value v) {
           v = op.getMemref();
           defOp = v.getDefiningOp();
         })
+        .Case<hlfir::DesignateOp>([&](auto op) {
+          // Track further through the memory indexed into
+          // => if the source arrays/structures don't alias then nor do the
+          //    results of hlfir.designate
+          v = op.getMemref();
+          defOp = v.getDefiningOp();
+          // TODO: there will be some cases which provably don't alias if one
+          // takes into account the component or indices, which are currently
+          // ignored here - leading to false positives
+          // because of this limitation, we need to make sure we never return
+          // MustAlias after going through a designate operation
+          approximateSource = true;
+        })
         .Default([&](auto op) {
           defOp = nullptr;
           breakFromLoop = true;
@@ -252,9 +270,9 @@ AliasAnalysis::Source AliasAnalysis::getSource(mlir::Value v) {
     }
 
   if (type == SourceKind::Global)
-    return {global, type, ty, attributes};
+    return {global, type, ty, attributes, approximateSource};
 
-  return {v, type, ty, attributes};
+  return {v, type, ty, attributes, approximateSource};
 }
 
 } // namespace fir

flang/test/Analysis/AliasAnalysis/alias-analysis-4.fir

@@ -0,0 +1,73 @@
+// check that hlfir.designate can be followed by alias analysis
+
+// use --mlir-disable-threading so that the AA queries are serialised
+// as well as its diagnostic output.
+// RUN: fir-opt %s --test-fir-alias-analysis -split-input-file --mlir-disable-threading 2>&1 | FileCheck %s
+
+// designate for a derived type component:
+// module m
+//   type t
+//     real :: array(42)
+//   end type t
+//   type (t) :: glbl
+// contains
+//   subroutine test(arg)
+//     real :: arg(42)
+//     glbl%array = arg
+//   end subroutine test
+// end module m
+
+// A global can't alias with a dummy argument
+// CHECK: arg#0 <-> glbl%array#0: NoAlias
+
+module attributes {fir.defaultkind = "a1c4d8i4l4r4", fir.kindmap = "", llvm.target_triple = "aarch64-unknown-linux-gnu"} {
+  fir.global @_QMmEglbl : !fir.type<_QMmTt{array:!fir.array<42xf32>}> {
+    %0 = fir.undefined !fir.type<_QMmTt{array:!fir.array<42xf32>}>
+    fir.has_value %0 : !fir.type<_QMmTt{array:!fir.array<42xf32>}>
+  }
+  func.func @_QMmPtest(%arg0: !fir.ref<!fir.array<42xf32>> {fir.bindc_name = "arg"}) {
+    %c42 = arith.constant 42 : index
+    %0 = fir.address_of(@_QMmEglbl) : !fir.ref<!fir.type<_QMmTt{array:!fir.array<42xf32>}>>
+    %1:2 = hlfir.declare %0 {uniq_name = "_QMmEglbl"} : (!fir.ref<!fir.type<_QMmTt{array:!fir.array<42xf32>}>>) -> (!fir.ref<!fir.type<_QMmTt{array:!fir.array<42xf32>}>>, !fir.ref<!fir.type<_QMmTt{array:!fir.array<42xf32>}>>)
+    %2 = fir.shape %c42 : (index) -> !fir.shape<1>
+    %3:2 = hlfir.declare %arg0(%2) {uniq_name = "_QMmFtestEarg", test.ptr = "arg"} : (!fir.ref<!fir.array<42xf32>>, !fir.shape<1>) -> (!fir.ref<!fir.array<42xf32>>, !fir.ref<!fir.array<42xf32>>)
+    %4 = hlfir.designate %1#0{"array"} shape %2 {test.ptr = "glbl%array"} : (!fir.ref<!fir.type<_QMmTt{array:!fir.array<42xf32>}>>, !fir.shape<1>) -> !fir.ref<!fir.array<42xf32>>
+    hlfir.assign %3#0 to %4 : !fir.ref<!fir.array<42xf32>>, !fir.ref<!fir.array<42xf32>>
+    return
+  }
+}
+
+// -----
+
+// designate for an array element
+
+// two dummy arguments don't alias
+// CHECK: array0#0 <-> array1#0: NoAlias
+
+func.func @array_element(%arg0: !fir.ref<!fir.array<2x42xi32>>, %arg1: !fir.ref<!fir.array<2x42xi32>>) {
+    %c0 = arith.constant 0 : index
+    %c1 = arith.constant 1 : index
+    %c41 = arith.constant 41 : index
+    %c42 = arith.constant 42 : index
+    %shape = fir.shape %c42 : (index) -> !fir.shape<1>
+    %array0 = hlfir.designate %arg0 (%c0, %c0:%c41:%c1) shape %shape {test.ptr = "array0"} : (!fir.ref<!fir.array<2x42xi32>>, index, index, index, index, !fir.shape<1>) -> !fir.box<!fir.array<?xi32>>
+    %array1 = hlfir.designate %arg1 (%c1, %c0:%c41:%c1) shape %shape {test.ptr = "array1"} : (!fir.ref<!fir.array<2x42xi32>>, index, index, index, index, !fir.shape<1>) -> !fir.box<!fir.array<?xi32>>
+    return
+}
+
+// -----
+
+// FIXME: designate doesn't understand non-overlappning array indices
+// make sure that we say MayAlias and not MustAlias until array indexes are understood
+// CHECK: array2#0 <-> array3#0: MayAlias
+
+func.func @array_element_same_source(%arg0: !fir.ref<!fir.array<2x42xi32>>) {
+    %c0 = arith.constant 0 : index
+    %c1 = arith.constant 1 : index
+    %c41 = arith.constant 41 : index
+    %c42 = arith.constant 42 : index
+    %shape = fir.shape %c42 : (index) -> !fir.shape<1>
+    %array2 = hlfir.designate %arg0 (%c0, %c0:%c41:%c1) shape %shape {test.ptr = "array2"} : (!fir.ref<!fir.array<2x42xi32>>, index, index, index, index, !fir.shape<1>) -> !fir.box<!fir.array<?xi32>>
+    %array3 = hlfir.designate %arg0 (%c1, %c0:%c41:%c1) shape %shape {test.ptr = "array3"} : (!fir.ref<!fir.array<2x42xi32>>, index, index, index, index, !fir.shape<1>) -> !fir.box<!fir.array<?xi32>>
+    return
+}