Description of problem:
I don't know if this is a bug in gcc or in the XEmacs code.  Any help you can give me in figuring that out is much appreciated.

XEmacs fails to build on x86_64 with GCC 4.7.0, although it builds successfully on i686.  I have tracked the build failure down to this code in src/lread.c, lines 2989-92:

  if (!NILP (s->tail))
    XCDR (s->tail) = read0 (readcharfun);
  else
    s->head = read0 (readcharfun);

I'm not very good with Intel assembly, but a look at the generated code for this function seems to show that this is being compiled down to just the "else" branch; i.e., I don't see any code corresponding to the conditional or to the "then" branch.  When this code is executed, either s->head is non-nil and s->tail is nil, or the two pointers are aliases.  XEmacs is compiled with -fno-strict-aliasing.

Version-Release number of selected component (if applicable):
gcc-4.7.0-0.6.fc17.x86_64

How reproducible:
Always.

Steps to Reproduce:
1. Attempt to rpmbuild the xemacs package.  The build will fail.
2. Create a file, say /tmp/test.el, with these contents: "(princ '(1 . 2))".
3. cd ~/rpmbuild/BUILD/xemacs-21.5.31/src
4. gdb ./temacs
5. At the gdb prompt, type "break lread.c:2989".
6. run -nd -no-packages -batch -l /tmp/test.el
7. Observe that s->head and s->tail are equal (use "pobj s->tail" to see the Lisp interpretation of the object), and non-nil.
8. Step forward, and watch the "else" branch be taken.

Actual results:
The wrong branch of the conditional is taken.  Indeed, the correct branch doesn't seem to exist in the object code.

Expected results:
The correct branch should not be compiled away, and should be taken.

Additional info: