[Bug c++/52558] write introduction incorrect wrt the C++11 memory model

[amacleod at redhat dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52558

--- Comment #8 from Andrew Macleod <amacleod at redhat dot com> 2012-03-12 15:50:13 UTC ---
We can still perform store motion out of a loop, we just can't put the store on
a path which is executed if the loop isn't executed.

In this case, we actually made the code *slower*.  Before LIM, there was a load
of g1, a compare and return.
        movl    g_1(%rip), %edx
        xorl    %eax, %eax
        testl   %edx, %edx
        jne     .L1
.L4:
        addl    $1, %eax
        movl    $0, g_2(%rip)
        cmpl    $4, %eax
        jne     .L4
.L1:
        rep
        ret


  LIM makes it have a load of g_1, a load of g_2 and a store to g_2 before
returning.

       .cfi_startproc
        movl    g_1(%rip), %edx
        movl    g_2(%rip), %eax
        testl   %edx, %edx
        jne     .L2
        movl    $0, g_2(%rip)
        ret
.L2:
        movl    %eax, g_2(%rip)
        xorl    %eax, %eax
        ret



-O3 corrects this mistake and returns it to the more optimal results.


I would argue this testcase shows LIM actually making the code worse in this
case as well.