[Bug rtl-optimization/58759] [4.9 Regression] wrong code (segfaults) at -O2 on x86_64-linux-gnu in 32-bit mode

["law at redhat dot com" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58759

Jeffrey A. Law <law at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |law at redhat dot com

--- Comment #4 from Jeffrey A. Law <law at redhat dot com> ---
I don't offhand see how this can be a jump threading problem.

./xgcc -B./ -O2 -fdbg-cnt=registered_jump_thread:0 j.c -m32 -g
-fdump-tree-all-blocks-vops-details -dap -S
[law@lugnut gcc]$ ./xgcc -B./ -O2 -fdbg-cnt=registered_jump_thread:0 j.c -m32 
dbg_cnt 'registered_jump_thread' set to 0
[law@lugnut gcc]$ ./a.out
Segmentation fault (core dumped)


Note carefully the -fdbg-cnt flag.  That turns off jump threading.

Using gdb and the raw assembly code we have the following faulting instruction
(shortly after the call to foo():

        movl    %edx, i+12

Hmm, that's strange since "i" is in readonly memory:

        .section        .rodata
        .align 4
        .type   i, @object
        .size   i, 16
i:

If we look at the faulting instruction in the .reload dump we have:

(insn 100 105 47 6 (set (mem/u/c:SI (const:SI (plus:SI (symbol_ref:SI ("i")
[flags 0x2]  <var_decl 0x7f5ee20427b8 i>)
                    (const_int 12 [0xc]))) [4 i+12 S4 A32])
        (reg:SI 1 dx [127])) j.c:30 86 {*movsi_internal}
     (nil))

Yup, that's a write to readonly memory.