emit_no_conflict_block breaks some conditional moves

[Greg McGary]

James E Wilson <wilson@specifixinc.com> writes:

> Greg McGary wrote:
> > I found that
> > emit_no_conflict_block() reordered insns gen'd by
> > expand_doubleword_shift() in a way that violated dependency between
> > compares and associated conditional-move insns that had the target
> > register as destination.
> 
> You didn't say precisely what went wrong, but I'd guess you have
>      cmpsi ...
>      movsicc target, ...
>      cmpsi ...
>      movsicc target, ...
> which got reordered to
>      cmpsi ...
>      cmpsi ...
>      movsicc target, ...
>      movsicc target, ...
> which obviously does not work if your condition code register is a
> hard register.

Correct.  FYI, the two "cmpsi" insns are identical and redundant, so
don't conflict, however, all bit-logic and shift insns on this CPU
clobber condition codes, and the CC-producing cmpsi insns are
separated from their consumers by CC-clobbering logic & shift insns.

> Perhaps a check like
>      && GET_MODE_CLASS (GET_MODE (SET_DEST (set))) != MODE_CC
> or maybe check for any hard register
>      && (SET_DEST (set) != REG
>          || REGNO (set) >= FIRST_PSEUDO_REGISTER)
> Safer is probably to do both checks, so that we only reject CCmode
> hard regs here, e.g.
>      && (GET_MODE_CLASS (GET_MODE (SET_DEST (set))) != MODE_CC
>          || SET_DEST (set) != REG
>          || REGNO (set) >= FIRST_PSEUDO_REGISTER))
> which should handle the specific case you ran into.

That will do fine for ports that have conditional move, but without
movsicc, you'll have this case:

    cmpsi ...
    bcc 1f
    movsi target, ...
  1:
    cmpsi ...
    bcc 2f
    movsi target, ...
  2:
    <cc-clobbering insns...>

which without the above fix will be reordered:

    cmpsi ...
    bcc 1f
  1:
    cmpsi ...
    bcc 2f
  2:
    <cc-clobbering insns...>
    movsi target, ...
    movsi target, ...

while with the above fix, will be reordered:

    bcc 1f
  1:  
    bcc 2f
  2:
    <cc-clobbering insns...>
    cmpsi ...
    movsi target, ...
    cmpsi ...
    movsi target, ...

Here, the branches and labels need to also travel with the cmpsi and movsi.

Greg