c++: Fix up cp_build_array_ref COND_EXPR handling [PR120471]

The following testcase is miscompiled since the introduction of UBSan,
cp_build_array_ref COND_EXPR handling replaces
(cond ? a : b)[idx] with cond ? a[idx] : b[idx], but if there are
SAVE_EXPRs inside of idx, they will be evaluated just in one of the
branches and the other uses uninitialized temporaries.

Fixed by keeping doing what it did if idx doesn't have side effects
and is invariant.  Otherwise if op1/op2 are ARRAY_TYPE arrays with
invariant addresses or pointers with invariant values, use
SAVE_EXPR <op0>, SAVE_EXPR <idx>, SAVE_EXPR <op0> as a new condition
and SAVE_EXPR <idx> instead of idx for the recursive calls.
Otherwise punt, but if op1/op2 are ARRAY_TYPE, furthermore call
cp_default_conversion on array, so that COND_EXPR with ARRAY_TYPE doesn't
survive in the IL until expansion.

2025-07-01  Jakub Jelinek  <jakub@redhat.com>

	PR c++/120471
gcc/cp/
	* typeck.cc (cp_build_array_ref) <case COND_EXPR>: If idx is not
	INTEGER_CST, don't optimize the case (but cp_default_conversion on
	array early if it has ARRAY_TYPE) or use
	SAVE_EXPR <op0>, SAVE_EXPR <idx>, SAVE_EXPR <op0> as new op0 depending
	on flag_strong_eval_order and whether op1 and op2 are arrays with
	invariant address or tree invariant pointers.  Formatting fixes.
gcc/testsuite/
	* g++.dg/ubsan/pr120471.C: New test.
	* g++.dg/parse/pr120471.C: New test.

(cherry picked from commit 988e87b66882875b14a6cab11c17516863c74a63)

diff --git a/gcc/cp/typeck.cc b/gcc/cp/typeck.cc
index 89ff595ec5132700f960a63385ca61bd06ebca9e..19dfaf18928ff6299a34e6f61a5c71de650b8b7e 100644 (file)
--- a/gcc/cp/typeck.cc
+++ b/gcc/cp/typeck.cc
@@ -3811,13 +3811,129 @@ cp_build_array_ref (location_t loc, tree array, tree idx,
       }
 
     case COND_EXPR:
-      ret = build_conditional_expr
-              (loc, TREE_OPERAND (array, 0),
-              cp_build_array_ref (loc, TREE_OPERAND (array, 1), idx,
-                                  complain),
-              cp_build_array_ref (loc, TREE_OPERAND (array, 2), idx,
-                                  complain),
-              complain);
+      tree op0, op1, op2;
+      op0 = TREE_OPERAND (array, 0);
+      op1 = TREE_OPERAND (array, 1);
+      op2 = TREE_OPERAND (array, 1);
+      if (TREE_SIDE_EFFECTS (idx) || !tree_invariant_p (idx))
+       {
+         /* If idx could possibly have some SAVE_EXPRs, turning
+            (op0 ? op1 : op2)[idx] into
+            op0 ? op1[idx] : op2[idx] can lead into temporaries
+            initialized in one conditional path and uninitialized
+            uses of them in the other path.
+            And if idx is a really large expression, evaluating it
+            twice is also not optimal.
+            On the other side, op0 must be sequenced before evaluation
+            of op1 and op2 and for C++17 op0, op1 and op2 must be
+            sequenced before idx.
+            If idx is INTEGER_CST, we can just do the optimization
+            without any SAVE_EXPRs, if op1 and op2 are both ARRAY_TYPE
+            VAR_DECLs or COMPONENT_REFs thereof (so their address
+            is constant or relative to frame), optimize into
+            (SAVE_EXPR <op0>, SAVE_EXPR <idx>, SAVE_EXPR <op0>)
+            ? op1[SAVE_EXPR <idx>] : op2[SAVE_EXPR <idx>]
+            Otherwise avoid this optimization.  */
+         if (flag_strong_eval_order == 2)
+           {
+             if (TREE_CODE (TREE_TYPE (array)) == ARRAY_TYPE)
+               {
+                 tree xop1 = op1;
+                 tree xop2 = op2;
+                 while (xop1 && handled_component_p (xop1))
+                   {
+                     switch (TREE_CODE (xop1))
+                       {
+                       case ARRAY_REF:
+                       case ARRAY_RANGE_REF:
+                         if (!tree_invariant_p (TREE_OPERAND (xop1, 1))
+                             || TREE_OPERAND (xop1, 2) != NULL_TREE
+                             || TREE_OPERAND (xop1, 3) != NULL_TREE)
+                           {
+                             xop1 = NULL_TREE;
+                             continue;
+                           }
+                         break;
+
+                       case COMPONENT_REF:
+                         if (TREE_OPERAND (xop1, 2) != NULL_TREE)
+                           {
+                             xop1 = NULL_TREE;
+                             continue;
+                           }
+                         break;
+
+                       default:
+                         break;
+                       }
+                     xop1 = TREE_OPERAND (xop1, 0);
+                   }
+                 if (xop1)
+                   STRIP_ANY_LOCATION_WRAPPER (xop1);
+                 while (xop2 && handled_component_p (xop2))
+                   {
+                     switch (TREE_CODE (xop2))
+                       {
+                       case ARRAY_REF:
+                       case ARRAY_RANGE_REF:
+                         if (!tree_invariant_p (TREE_OPERAND (xop2, 1))
+                             || TREE_OPERAND (xop2, 2) != NULL_TREE
+                             || TREE_OPERAND (xop2, 3) != NULL_TREE)
+                           {
+                             xop2 = NULL_TREE;
+                             continue;
+                           }
+                         break;
+
+                       case COMPONENT_REF:
+                         if (TREE_OPERAND (xop2, 2) != NULL_TREE)
+                           {
+                             xop2 = NULL_TREE;
+                             continue;
+                           }
+                         break;
+
+                       default:
+                         break;
+                       }
+                     xop2 = TREE_OPERAND (xop2, 0);
+                   }
+                 if (xop2)
+                   STRIP_ANY_LOCATION_WRAPPER (xop2);
+
+                 if (!xop1
+                     || !xop2
+                     || !(CONSTANT_CLASS_P (xop1)
+                          || decl_address_invariant_p (xop1))
+                     || !(CONSTANT_CLASS_P (xop2)
+                          || decl_address_invariant_p (xop2)))
+                   {
+                     /* Force default conversion on array if
+                        we can't optimize this and array is ARRAY_TYPE
+                        COND_EXPR, we can't leave COND_EXPRs with
+                        ARRAY_TYPE in the IL.  */
+                     array = cp_default_conversion (array, complain);
+                     if (error_operand_p (array))
+                       return error_mark_node;
+                     break;
+                   }
+               }
+             else if (!POINTER_TYPE_P (TREE_TYPE (array))
+                      || !tree_invariant_p (op1)
+                      || !tree_invariant_p (op2))
+               break;
+           }
+         if (TREE_SIDE_EFFECTS (idx))
+           {
+             idx = save_expr (idx);
+             op0 = save_expr (op0);
+             tree tem = build_compound_expr (loc, op0, idx);
+             op0 = build_compound_expr (loc, tem, op0);
+           }
+       }
+      op1 = cp_build_array_ref (loc, op1, idx, complain);
+      op2 = cp_build_array_ref (loc, op2, idx, complain);
+      ret = build_conditional_expr (loc, op0, op1, op2, complain);
       protected_set_expr_location (ret, loc);
       return ret;
 

diff --git a/gcc/testsuite/g++.dg/parse/pr120471.C b/gcc/testsuite/g++.dg/parse/pr120471.C
new file mode 100644 (file)
index 0000000..ad47e38
--- /dev/null
+++ b/gcc/testsuite/g++.dg/parse/pr120471.C
@@ -0,0 +1,42 @@
+// PR c++/120471
+// { dg-do compile }
+
+extern int a1[], a2[], a3[], a4[];
+
+int corge (int);
+
+int
+foo (int p)
+{
+  return (p ? a1 : a2)[1];
+}
+
+int
+bar (int p, int q)
+{
+  return (p ? a1 : a2)[q];
+}
+
+int
+garply (int p, int q)
+{
+  return (p ? a1 : a2)[corge (q)];
+}
+
+int
+baz (int p, int q)
+{
+  return (p ? q ? a1 : a2 : q ? a3 : a4)[1];
+}
+
+int
+qux (int p, int q, int r)
+{
+  return (p ? q ? a1 : a2 : q ? a3 : a4)[r];
+}
+
+int
+fred (int p, int q, int r)
+{
+  return (p ? q ? a1 : a2 : q ? a3 : a4)[corge (r)];
+}

diff --git a/gcc/testsuite/g++.dg/ubsan/pr120471.C b/gcc/testsuite/g++.dg/ubsan/pr120471.C
new file mode 100644 (file)
index 0000000..31b781f
--- /dev/null
+++ b/gcc/testsuite/g++.dg/ubsan/pr120471.C
@@ -0,0 +1,21 @@
+// PR c++/120471
+// { dg-do run }
+// { dg-options "-fsanitize=undefined" }
+
+volatile int b[1], a[1];
+
+void
+foo (int x)
+{
+  volatile int c = 21;
+  volatile int v = (x % 2 ? b : a)[c % 3];
+  if (v != 0)
+    __builtin_abort ();
+}
+
+int
+main ()
+{
+  foo (1);
+  foo (2);
+}