[patch] Fix PR C++/82872

[Eric Botcazou <ebotcazou@adacore.com>]

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Hi,

this is an ICE in the C++ FE on an offsetof construct with __PTRDIFF_MAX__ and 
it happens only with this specific index:

In file included from pr82872.c:4:
pr82872.c: In function 'size_t foo()':
pr82872.c:10:10: internal compiler error: in ignore_overflows, at cp/cvt.c:583
   return offsetof (struct S, a[__PTRDIFF_MAX__]);
          ^~~~~~~~
0x65ef7f ignore_overflows
        /home/eric/svn/gcc/gcc/cp/cvt.c:583
0x661bcc ocp_convert(tree_node*, tree_node*, int, int, int)
        /home/eric/svn/gcc/gcc/cp/cvt.c:817
0x6636e0 convert(tree_node*, tree_node*)
        /home/eric/svn/gcc/gcc/cp/cvt.c:1580
0x882b62 fold_offsetof(tree_node*)
        /home/eric/svn/gcc/gcc/c-family/c-common.c:6282
0x801c0c finish_offsetof(tree_node*, tree_node*, unsigned int)
        /home/eric/svn/gcc/gcc/cp/semantics.c:4041
0x74596b cp_parser_builtin_offsetof
        /home/eric/svn/gcc/gcc/cp/parser.c:9828

The C++ FE is quite picky about constant integer overflow during conversion 
operations and checks that they can only _introduce_ overflows and never make 
them disappear, the latter case occuring here in convert_to_integer_1.

Tested on x86_64-suse-linux, OK for the mainline?


2017-12-21  Eric Botcazou  <ebotcazou@adacore.com>

	PR C++/82872
	* convert.c (convert_to_integer_1) <POINTER_TYPE>: Do not return
	the shared zero if the input has overflowed.


2017-12-21  Eric Botcazou  <ebotcazou@adacore.com>

	* c-c++-common/pr82872.c: New test.

-- 
Eric Botcazou

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/* PR c++/82872 */
/* { dg-do compile } */

#include <stddef.h>

struct S { int i, a[1]; };

size_t foo (void)
{
  return offsetof (struct S, a[__PTRDIFF_MAX__]);
}

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Index: convert.c
===================================================================
--- convert.c	(revision 255850)
+++ convert.c	(working copy)
@@ -671,7 +671,7 @@ convert_to_integer_1 (tree type, tree ex
     {
     case POINTER_TYPE:
     case REFERENCE_TYPE:
-      if (integer_zerop (expr))
+      if (integer_zerop (expr) && !TREE_OVERFLOW (expr))
 	return build_int_cst (type, 0);
 
       /* Convert to an unsigned integer of the correct width first, and from