Re: [RFC 0/6] Flags outputs for asms

[Segher Boessenkool <segher at kernel dot crashing dot org>]

Hi Richard,

On Fri, May 08, 2015 at 01:15:25PM -0700, Richard Henderson wrote:
> But it *does* try to match an intermediate pattern,
> 
> (set (reg:CCGC 17 flags)
>     (compare:CCGC (reg:CCGC 17 flags)
>         (const_int 0 [0])))
> 
> which can be considered a no-op move.

Maybe we should teach combine this is a no-op, then?  Then everything
should work as-is?  Combine knows about no-op moves (they don't cost,
and it deletes them itself).

> Jeff or Segher, is it worth complicating the can_combine_p test near line 1958
> 
>       /* Make sure that the value that is to be substituted for the register
>          does not use any registers whose values alter in between.  However,
>          If the insns are adjacent, a use can't cross a set even though we
>          think it might (this can happen for a sequence of insns each setting
>          the same destination; last_set of that register might point to
>          a NOTE).  If INSN has a REG_EQUIV note, the register is always
>          equivalent to the memory so the substitution is valid even if there
>          are intervening stores.  Also, don't move a volatile asm or
>          UNSPEC_VOLATILE across any other insns.  */
>       || (! all_adjacent
>           && (((!MEM_P (src)
>                 || ! find_reg_note (insn, REG_EQUIV, src))
>                && use_crosses_set_p (src, DF_INSN_LUID (insn)))
>               || (GET_CODE (src) == ASM_OPERANDS && MEM_VOLATILE_P (src))
>               || GET_CODE (src) == UNSPEC_VOLATILE))
> 
> to notice that the set is one of SUCC or SUCC2, and is thus included in the
> insns being combined?  That does seem cleaner and more general than the hacky
> i386 nop_cmp pattern, but would certainly require tons more testing...

"Cleaner"?  In this code?  Heh.

use_crosses_set_p often estimates pessimistically.  Is that what is
happening here?  Using real dataflow in combine would fix that (and many
other problems).  Not that that helps you right now ;-)

I'll build with your patches tomorrow and investigate.


Segher