[CodeGen] Add TBAA struct path info for array members

This enables the LLVM optimizer to view accesses to distinct struct
members as independent, also for array members. For example, the
following two stores no longer alias:

    struct S { int a[10]; int b; };
    void test(S *p, int i) {
      p->a[i] = ...;
      p->b = ...;
    }

Array members were already added to TBAA struct type nodes in commit
57493e2. Here, we extend a path tag for an array subscript expression.

Author: brunodf-snps

clang/lib/CodeGen/CGExpr.cpp

@@ -4503,7 +4503,32 @@ LValue CodeGenFunction::EmitArraySubscriptExpr(const ArraySubscriptExpr *E,
         E->getType(), !getLangOpts().PointerOverflowDefined, SignedIndices,
         E->getExprLoc(), &arrayType, E->getBase());
     EltBaseInfo = ArrayLV.getBaseInfo();
-    EltTBAAInfo = CGM.getTBAAInfoForSubobject(ArrayLV, E->getType());
+    if (!CGM.getCodeGenOpts().NewStructPathTBAA) {
+      // Since CodeGenTBAA::getTypeInfoHelper only handles array types for
+      // new struct path TBAA, we must a use a plain access.
+      EltTBAAInfo = CGM.getTBAAInfoForSubobject(ArrayLV, E->getType());
+    } else if (ArrayLV.getTBAAInfo().isMayAlias()) {
+      EltTBAAInfo = TBAAAccessInfo::getMayAliasInfo();
+    } else if (ArrayLV.getTBAAInfo().isIncomplete()) {
+      EltTBAAInfo = CGM.getTBAAAccessInfo(E->getType());
+    } else {
+      // Extend struct path from base lvalue, similar to EmitLValueForField.
+      // If no base type has been assigned for the array access, then try to
+      // generate one.
+      EltTBAAInfo = ArrayLV.getTBAAInfo();
+      if (!EltTBAAInfo.BaseType) {
+        EltTBAAInfo.BaseType = CGM.getTBAABaseTypeInfo(ArrayLV.getType());
+        assert(!EltTBAAInfo.Offset &&
+               "Nonzero offset for an access with no base type!");
+      }
+      // The index into the array is a runtime value. We use the same struct
+      // path for all array elements (that of the element at index 0). So we
+      // set the access type and size, but do not have to adjust
+      // EltTBAAInfo.Offset.
+      EltTBAAInfo.AccessType = CGM.getTBAATypeInfo(E->getType());
+      EltTBAAInfo.Size =
+          getContext().getTypeSizeInChars(E->getType()).getQuantity();
+    }
   } else {
     // The base must be a pointer; emit it with an estimate of its alignment.
     Addr = EmitPointerWithAlignment(E->getBase(), &EltBaseInfo, &EltTBAAInfo);

clang/test/CodeGen/tbaa-array.cpp

@@ -10,6 +10,8 @@
 struct A { int i; };
 struct B { A a[1]; };
 struct C { int i; int x[3]; };
+struct D { int n; int arr[]; }; // flexible array member
+extern int AA[];                // incomplete array type
 
 int foo(B *b) {
 // CHECK-LABEL: _Z3fooP1B
@@ -28,16 +30,30 @@ int bar(C *c) {
 
 int bar2(C *c) {
 // CHECK-NEW-LABEL: _Z4bar2P1C
-// CHECK-NEW: load i32, {{.*}}, !tbaa [[TAG_int:!.*]]
+// CHECK-NEW: load i32, {{.*}}, !tbaa [[TAG_C_x:!.*]]
   return c->x[2];
 }
 
 int bar3(C *c, int j) {
 // CHECK-NEW-LABEL: _Z4bar3P1Ci
-// CHECK-NEW: load i32, {{.*}}, !tbaa [[TAG_int:!.*]]
+// CHECK-NEW: load i32, {{.*}}, !tbaa [[TAG_C_x]]
   return c->x[j];
 }
 
+int bar4(D *d) {
+// CHECK-NEW-LABEL: _Z4bar4P1D
+// CHECK-NEW: load i32, {{.*}}, !tbaa [[TAG_int:!.*]]
+// CHECK-NEW: load i32, {{.*}}, !tbaa [[TAG_int:!.*]]
+    return d->arr[d->n];
+}
+
+int bar5(int j) {
+// CHECK-NEW-LABEL: _Z4bar5i
+// CHECK-NEW: load i32, {{.*}}, !tbaa [[TAG_int:!.*]]
+// CHECK-NEW: load i32, {{.*}}, !tbaa [[TAG_int:!.*]]
+    return AA[2] + AA[j];
+}
+
 // CHECK-DAG: [[TAG_A_i]] = !{[[TYPE_A:!.*]], [[TYPE_int:!.*]], i64 0}
 // CHECK-DAG: [[TYPE_A]] = !{!"_ZTS1A", !{{.*}}, i64 0}
 // CHECK-DAG: [[TYPE_int]] = !{!"int", !{{.*}}, i64 0}
@@ -49,3 +65,4 @@ int bar3(C *c, int j) {
 // CHECK-NEW-DAG: [[TAG_A_i]] = !{[[TYPE_A]], [[TYPE_int]], i64 0, i64 4}
 // CHECK-NEW-DAG: [[TYPE_C:!.*]] = !{[[TYPE_char]], i64 16, !"_ZTS1C", [[TYPE_int]], i64 0, i64 4, [[TYPE_int]], i64 4, i64 12}
 // CHECK-NEW-DAG: [[TAG_C_i]] = !{[[TYPE_C:!.*]], [[TYPE_int:!.*]], i64 0, i64 4}
+// CHECK-NEW-DAG: [[TAG_C_x]] = !{[[TYPE_C:!.*]], [[TYPE_int:!.*]], i64 4, i64 4}