Re: [PATCH 40/50] rtlanal.c:for_each_inc_dec

[Jeff Law <law at redhat dot com>]

On 08/26/14 13:28, Richard Sandiford wrote:
> [Jeff, sorry for the duplicate, sent the original from an account that
> adds disclaimers.]
No worries.  Given the 3000+ messages that were waiting for me when I 
got back from PTO, what's another duplicate here and there :-)

[ Big snip. ]

> > invalid and cope with it if we ever find one.  Perhaps a bit of
> > ENABLE_CHECKING to detect if we ever create such a note?
>
> I suppose an assert means that it'd be up to each piece of code that
> creates a note to check whether the equivalent value has autoinc addresses.
> How about just dropping those REG_EQUAL and REG_EQUIV notes instead,
> like we already do for ASM_OPERANDS?
I can live with that.

> Here I've extended it to all notes with side effects.  The additional
> cases are:
>
>      case CLOBBER:
>        /* Reject CLOBBER with a non-VOID mode.  These are made by combine.c
> 	 when some combination can't be done.  If we see one, don't think
> 	 that we can simplify the expression.  */
>        return (GET_MODE (x) != VOIDmode);
>
>      [...snip autoincs...]
>      case CALL:
>      case UNSPEC_VOLATILE:
>        return 1;
>
>      case MEM:
>      case ASM_INPUT:
>      case ASM_OPERANDS:
>        if (MEM_VOLATILE_P (x))
> 	return 1;
>
> The combine clobbers shouldn't make their way into a note anyway,
> since it represents a failed optimisation.  ASM_INPUT is a top-level
> rtx rather than a SET_SRC, so isn't important.  Checking for volatile
> ASM_OPERANDS is just a subset of the current:
>
>        /* Don't add ASM_OPERAND REG_EQUAL/REG_EQUIV notes.
> 	 It serves no useful purpose and breaks eliminate_regs.  */
>        if (GET_CODE (datum) == ASM_OPERANDS)
> 	return NULL_RTX;
>
> So the remaining cases are CALL, UNSPEC_VOLATILE and volatile MEMs.
> I think UNSPEC_VOLATILE and volatile MEMs really do fall into the same
> category as auto-inc/dec.
Agreed.

 Const CALLs should be OK in practice,
> but I'm not sure why they'd ever need to be treated as having
> side effects.
They probably don't, but I think that's independent of your changes. 
There may be subtle things that depend on the const calls having "side 
effects" -- for example, they might be considered as clobbering argument 
saveback aeras on the stack and other such nonsense.



  Other CALLs are more dangerous.  In practice the only
> interesting notes for calls are (a) that the call address is equal
> to some other rtx, which is recorded in the insn that sets the
> address register rather than on the call itself and (b) that the result
> of the call is equivalent to some non-CALL rtx (e.g. after a libcall).
Clearly dropping these is safe as well.  And in the case of a libcall, 
the note shouldn't look like a call, the note should be the "obvious" 
RTL form that the libcall is implementing.  ie instead of a call to 
mulsi the note looks like (mult (a) (b))




> Tested on x86_64-linux-gnu.  OK to install?
>
> Thanks,
> Richard
>
>
> gcc/
> 	* emit-rtl.c (set_unique_reg_note): Discard notes with side effects.
OK.

Jeff