On 6/30/25 4:24 PM, Jakub Jelinek wrote:
On Mon, Jun 30, 2025 at 03:55:46PM -0400, Jason Merrill wrote:
Might go with my earlier !TREE_SIDE_EFFECTS && tree_invariant_p suggestion?

If you mean the following, it passes the 2 testcases and I can surely
bootstrap/regtest it tonight.

Yes, that's what I meant, but did you also see my other two comments in the body of the patch?

2025-06-30  Jakub Jelinek  <ja...@redhat.com>

        PR c++/120471
        * 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.

        * g++.dg/ubsan/pr120471.C: New test.
        * g++.dg/parse/pr120471.C: New test.

--- gcc/cp/typeck.cc.jj 2025-06-30 12:19:16.398242029 +0200
+++ gcc/cp/typeck.cc    2025-06-30 22:12:55.313206118 +0200
@@ -4001,13 +4001,91 @@ cp_build_array_ref (location_t loc, tree
        }
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 (op1)) == ARRAY_TYPE)
+               {
+                 tree xop1 = op1;
+                 while (TREE_CODE (xop1) == COMPONENT_REF)
+                   xop1 = TREE_OPERAND (xop1, 0);
+                 STRIP_ANY_LOCATION_WRAPPER (xop1);
+                 if (!decl_address_invariant_p (xop1))
+                   {
+                     /* Force default conversion on array if
+                        we can't optimize this and array has ARRAY_TYPE
+                        COND_EXPR, we can't leave COND_EXPRs with
+                        ARRAY_TYPE in the IL.  */
+                     if (TREE_CODE (TREE_TYPE (array)) == ARRAY_TYPE)
+                       {
+                         array = cp_default_conversion (array, complain);
+                         if (error_operand_p (array))
+                           return error_mark_node;
+                       }
+                     break;
+                   }
+               }
+             else if (!POINTER_TYPE_P (TREE_TYPE (op1))
+                      || !tree_invariant_p (op1))
+               break;
+             if (TREE_CODE (TREE_TYPE (op2)) == ARRAY_TYPE)
+               {
+                 tree xop2 = op2;
+                 while (TREE_CODE (xop2) == COMPONENT_REF)
+                   xop2 = TREE_OPERAND (xop2, 0);
+                 STRIP_ANY_LOCATION_WRAPPER (xop2);
+                 if (!decl_address_invariant_p (xop2))
+                   {
+                     /* Force default conversion on array if
+                        we can't optimize this and array has ARRAY_TYPE
+                        COND_EXPR, we can't leave COND_EXPRs with
+                        ARRAY_TYPE in the IL.  */
+                     if (TREE_CODE (TREE_TYPE (array)) == ARRAY_TYPE)
+                       {
+                         array = cp_default_conversion (array, complain);
+                         if (error_operand_p (array))
+                           return error_mark_node;
+                       }
+                     break;
+                   }
+               }
+             else if (!POINTER_TYPE_P (TREE_TYPE (op2))
+                      || !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;
--- gcc/testsuite/g++.dg/ubsan/pr120471.C.jj 2025-06-30 18:19:57.290356305 +0200
+++ gcc/testsuite/g++.dg/ubsan/pr120471.C       2025-06-30 18:19:57.290356305 
+0200
@@ -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);
+}
--- gcc/testsuite/g++.dg/parse/pr120471.C.jj    2025-06-30 19:01:55.875225849 
+0200
+++ gcc/testsuite/g++.dg/parse/pr120471.C       2025-06-30 22:14:27.381023195 
+0200
@@ -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)];
+}

        Jakub


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