[Bug other/56881] Miscompilation (optimisation failure?) causing NULL dereference and segfault at runtime

["mikpe at it dot uu.se" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56881

Mikael Pettersson <mikpe at it dot uu.se> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|mikpe at it dot uu.se       |

--- Comment #8 from Mikael Pettersson <mikpe at it dot uu.se> 2013-04-14 08:56:47 UTC ---
The error is in the test case.  It overrides the libc memmove() with its own
implementation, but that implementation fails to follow the specification.  In
particular, it returns NULL rather than memmove()'s first parameter.

GCC now optimizes based on this aspect of the specification, so things go wrong
at runtime.

Correcting the test case as follows allows it to work with gcc 4.8 and 4.9:

--- unix.c.~1~  2013-03-06 23:17:26.000000000 +0100
+++ unix.c      2013-04-14 10:45:24.651407693 +0200
@@ -110,7 +110,7 @@ memmove(void *dp, const void *sp, size_t
        unsigned char *cdp, *csp;

        if (n<=0)
-               return 0;
+               return dp;
        cdp = dp;
        csp = (unsigned char *)sp;
        if (cdp < csp) {
@@ -124,6 +124,6 @@ memmove(void *dp, const void *sp, size_t
                        *--cdp = *--csp;
                } while (--n);
        }
-       return 0;
+       return dp;
 }
 #endif

Not a bug in GCC.  Please close as INVALID.