[PATCH] c++: Fix up constexpr handling of char/signed char/short pre/post inc/decrement [PR105774]

[PATCH] c++: Fix up constexpr handling of char/signed char/short pre/post inc/decrement [PR105774]

[Jakub Jelinek]

Hi!

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.

Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

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

	PR c++/105774
	* constexpr.cc (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.

--- gcc/cp/constexpr.cc.jj	2022-10-21 18:04:47.869797312 +0200
+++ gcc/cp/constexpr.cc	2022-10-23 18:43:27.003390282 +0200
@@ -6234,6 +6234,18 @@ cxx_eval_increment_expression (const con
 	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)
--- gcc/testsuite/g++.dg/cpp1y/constexpr-105774.C.jj	2022-10-23 18:44:15.587729613 +0200
+++ gcc/testsuite/g++.dg/cpp1y/constexpr-105774.C	2022-10-23 18:33:54.754170726 +0200
@@ -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 ();

	Jakub

Re: [PATCH] c++: Fix up constexpr handling of char/signed char/short pre/post inc/decrement [PR105774]

[Jason Merrill]

On 10/24/22 03:27, Jakub Jelinek wrote:
> Hi!
> 
> 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.
> 
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

OK.

> 2022-10-24  Jakub Jelinek  <jakub@redhat.com>
> 
> 	PR c++/105774
> 	* constexpr.cc (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.
> 
> --- gcc/cp/constexpr.cc.jj	2022-10-21 18:04:47.869797312 +0200
> +++ gcc/cp/constexpr.cc	2022-10-23 18:43:27.003390282 +0200
> @@ -6234,6 +6234,18 @@ cxx_eval_increment_expression (const con
>   	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)
> --- gcc/testsuite/g++.dg/cpp1y/constexpr-105774.C.jj	2022-10-23 18:44:15.587729613 +0200
> +++ gcc/testsuite/g++.dg/cpp1y/constexpr-105774.C	2022-10-23 18:33:54.754170726 +0200
> @@ -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 ();
> 
> 	Jakub
>