[gcc r13-2563] stddef.h: Add C2x unreachable macro

[Joseph Myers <jsm28@gcc.gnu.org>]

https://gcc.gnu.org/g:a1a53dc7d87969d230e9ca51fcab59f3a72e5f6e

commit r13-2563-ga1a53dc7d87969d230e9ca51fcab59f3a72e5f6e
Author: Joseph Myers <joseph@codesourcery.com>
Date:   Fri Sep 9 14:11:21 2022 +0000

    stddef.h: Add C2x unreachable macro
    
    C2x adds a macro unreachable to stddef.h, with the same semantics as
    __builtin_unreachable.  Define this macro accordingly.
    
    Bootstrapped with no regressions for x86_64-pc-linux-gnu.
    
    gcc/
            * ginclude/stddef.h [__STDC_VERSION__ > 201710L] (unreachable):
            New macro.
    
    gcc/testsuite/
            * gcc.dg/c11-unreachable-1.c, gcc.dg/c2x-unreachable-1.c: New
            tests.

Diff:
---
 gcc/ginclude/stddef.h                    |  4 ++++
 gcc/testsuite/gcc.dg/c11-unreachable-1.c |  9 +++++++++
 gcc/testsuite/gcc.dg/c2x-unreachable-1.c | 29 +++++++++++++++++++++++++++++
 3 files changed, 42 insertions(+)

diff --git a/gcc/ginclude/stddef.h b/gcc/ginclude/stddef.h
index 315ff786694..3d29213e8f1 100644
--- a/gcc/ginclude/stddef.h
+++ b/gcc/ginclude/stddef.h
@@ -451,6 +451,10 @@ typedef struct {
 #endif
 #endif /* C23.  */
 
+#if defined __STDC_VERSION__ && __STDC_VERSION__ > 201710L
+#define unreachable() (__builtin_unreachable ())
+#endif
+
 #endif /* _STDDEF_H was defined this time */
 
 #endif /* !_STDDEF_H && !_STDDEF_H_ && !_ANSI_STDDEF_H && !__STDDEF_H__
diff --git a/gcc/testsuite/gcc.dg/c11-unreachable-1.c b/gcc/testsuite/gcc.dg/c11-unreachable-1.c
new file mode 100644
index 00000000000..28e48392ed1
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/c11-unreachable-1.c
@@ -0,0 +1,9 @@
+/* Test unreachable not defined in <stddef.h> for C11.  */
+/* { dg-do preprocess } */
+/* { dg-options "-std=c11 -pedantic-errors" } */
+
+#include <stddef.h>
+
+#ifdef unreachable
+#error "unreachable defined"
+#endif
diff --git a/gcc/testsuite/gcc.dg/c2x-unreachable-1.c b/gcc/testsuite/gcc.dg/c2x-unreachable-1.c
new file mode 100644
index 00000000000..468f1f87ebb
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/c2x-unreachable-1.c
@@ -0,0 +1,29 @@
+/* Test unreachable in <stddef.h> for C2x.  */
+/* { dg-do run } */
+/* { dg-options "-std=c2x -pedantic-errors -O2" } */
+
+#include <stddef.h>
+
+#ifndef unreachable
+#error "unreachable not defined"
+#endif
+
+extern void *p;
+extern __typeof__ (unreachable ()) *p;
+
+volatile int x = 1;
+
+extern void not_defined (void);
+
+extern void exit (int);
+
+int
+main ()
+{
+  if (x == 2)
+    {
+      unreachable ();
+      not_defined ();
+    }
+  exit (0);
+}