Re: [RFC] new post-reload compare elimination pass

[Jeff Law <law at redhat dot com>]

On 11/29/10 16:39, Richard Henderson wrote:
> Since NickC is away for the next month or so, I spent some of the
> long weekend trying to help him out with the mn10300 comparison
> problem for which he's already sent two revisions.  The patch is
> along the lines I proposed here:
>
>    http://gcc.gnu.org/ml/gcc-patches/2010-11/msg01076.html
>
> This adds a new post-reload pass, which is only enabled by backends
> that require it, e.g.
>
> > +  /* Enable comparison elimination, unless explicitly disabled.  */
> > +  if (optimize&&  !global_options_set.x_flag_compare_elim_after_reload)
> > +    flag_compare_elim_after_reload = 1;
> as will be seen in the mn10300 backend patch that uses this.
>
> I tried to use the df.h interfaces more, but I couldn't seem to find
> any easy way to walk a def-def chain.  I'd need that to easily notice
> the LUID of the clobber of the flags register preceding a compare.  In
> the end it seemed easiest to simply walk the insn chain and keep track.
>
> Comments wrt this specific patch?
At first glance this reminded me a lot of the old comparison elimination 
code in final, as it should since the code we want to generate is the 
same -- the major difference is this code twiddles the RTL to match 
reality where the old code waited until it was late enough on the 
pipeline that it could just change the assembly code without downstream 
consequences.

With that in mind, I looked over the old code in final.c to see if there 
were any high level gotchas -- the only ones that stuck out where checks 
to ensure the comparison didn't have a volatile operand or an operand 
with an auto-increment addressing mode.    I don't see similar checks in 
your new pass.

I don't think it's necessary right now, but handling conditional moves, 
conditional traps & setcc insns might be useful one day as well.

Overall, the pass looks pretty simple.  I presume you verified it 
actually eliminated some comparisons on the mn103 port?  Did NickC give 
you a testcase or two to work with?

Jeff