On Fri, Apr 01, 2016 at 03:35:19PM +0200, Bernd Schmidt wrote:
> On 04/01/2016 03:14 PM, Jakub Jelinek wrote:
> >As the testcase below shows, we can end up with lots of useless
> >instructions from multi-word arithmetics.
> >simplify-rtx.c can optimize x {&,|,^}= {0,-1}, but while
> >x &= 0 or x {|,^}= -1 are optimized into constants and CSE can handle those
> >fine, we keep x &= -1 and x {|,^}= 0 in the IL until expansion if x
> >is a MEM. There are two issues, one is that cse_insn has for a few years
> >code that wants to prevent partially overlapping MEM->MEM moves,
> >but actually doesn't realize that fully overlapping MEM->MEM noop moves
> >are fine. And the second one is that on most backends, there are no
> >MEM->MEM move instructions, so we need to delete the useless insns instead,
> >because it can't match.
> >
> >Fixed thusly, bootstrapped/regtested on x86_64-linux and i686-linux.
> >Is this something we want for 6.x or defer for stage1?
>
> It seems like a stage1 thing to me unless it's a regression. But you're in a
> better position to make that call.
I guess it can wait for stage1.
> >+ /* Similarly, lots of targets don't allow no-op
> >+ (set (mem x) (mem x)) moves. */
> >+ else if (n_sets == 1
> >+ && MEM_P (trial)
> >+ && MEM_P (dest)
> >+ && rtx_equal_p (trial, dest)
> >+ && !side_effects_p (dest)
> >+ && (cfun->can_delete_dead_exceptions
> >+ || insn_nothrow_p (insn)))
>
> Looks like this block of code is practically duplicated - I'd prefer a
> helper function set_of_equal_mems_removable_p or something. Ok with that
> change.
Perhaps instead just set a bool in the second hunk and just test that at the
third hunk's condition?
Jakub
