c++: Fix up constexpr handling of char/signed char/short pre/post inc/decrement ...

signed char, char or short int pre/post inc/decrement are represented by
normal {PRE,POST}_{INC,DEC}REMENT_EXPRs in the FE and only gimplification
ensures that the {PLUS,MINUS}_EXPR is done in unsigned version of those
types:
    case PREINCREMENT_EXPR:
    case PREDECREMENT_EXPR:
    case POSTINCREMENT_EXPR:
    case POSTDECREMENT_EXPR:
      {
        tree type = TREE_TYPE (TREE_OPERAND (*expr_p, 0));
        if (INTEGRAL_TYPE_P (type) && c_promoting_integer_type_p (type))
          {
            if (!TYPE_OVERFLOW_WRAPS (type))
              type = unsigned_type_for (type);
            return gimplify_self_mod_expr (expr_p, pre_p, post_p, 1, type);
          }
        break;
      }
This means during constant evaluation we need to do it similarly (either
using unsigned_type_for or using widening to integer_type_node).
The following patch does the latter.

2022-10-24  Jakub Jelinek  <jakub@redhat.com>

	PR c++/105774
	* constexpr.c (cxx_eval_increment_expression): For signed types
	that promote to int, evaluate PLUS_EXPR or MINUS_EXPR in int type.

	* g++.dg/cpp1y/constexpr-105774.C: New test.

(cherry picked from commit da8c362c4c18cff2f2dfd5c4706bdda7576899a4)

diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index 369920eda90987633577f858de0ea8848ae43f49..62cb2c5e06a8b51efcaa0c5945e2ddc2b017968d 100644 (file)
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -5248,6 +5248,18 @@ cxx_eval_increment_expression (const constexpr_ctx *ctx, tree t,
        offset = fold_build1 (NEGATE_EXPR, TREE_TYPE (offset), offset);
       mod = fold_build2 (POINTER_PLUS_EXPR, type, val, offset);
     }
+  else if (c_promoting_integer_type_p (type)
+          && !TYPE_UNSIGNED (type)
+          && TYPE_PRECISION (type) < TYPE_PRECISION (integer_type_node))
+    {
+      offset = fold_convert (integer_type_node, offset);
+      mod = fold_convert (integer_type_node, val);
+      tree t = fold_build2 (inc ? PLUS_EXPR : MINUS_EXPR, integer_type_node,
+                           mod, offset);
+      mod = fold_convert (type, t);
+      if (TREE_OVERFLOW_P (mod) && !TREE_OVERFLOW_P (t))
+       TREE_OVERFLOW (mod) = false;
+    }
   else
     mod = fold_build2 (inc ? PLUS_EXPR : MINUS_EXPR, type, val, offset);
   if (!ptr)

diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-105774.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-105774.C
new file mode 100644 (file)
index 0000000..8ca6fce
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-105774.C
@@ -0,0 +1,15 @@
+// PR c++/105774
+// { dg-do compile { target c++14 } }
+
+constexpr signed char
+foo ()
+{
+#if __SCHAR_MAX__ < __INT_MAX__
+  signed char x = __SCHAR_MAX__;
+#else
+  signed char x = 0;
+#endif
+  return ++x;
+}
+
+constexpr auto a = foo ();