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

[rguenther at suse dot de]

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

--- Comment #9 from rguenther at suse dot de <rguenther at suse dot de> 2012-03-12 15:55:27 UTC ---
On Mon, 12 Mar 2012, amacleod at redhat dot com wrote:

> 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.

Usually loops are executed ;)  At least that is what we assume if
we don't know better.  But yes, splitting the exit block(s)
is a solution, so, if you fix this please go this way.

Richard.