Re: PATCH: fine-tuning for can_store_by_pieces

[Sandra Loosemore <sandra@codesourcery.com>]

Richard Sandiford wrote:

> Hmm, I'm not sure I follow.  You seem to be implying that 1-byte stores
> are always done "by pieces" when optimize_size, but I don't think that's
> true.  I was referring the 1-instruction bound in code like this:
> 
>       if (host_integerp (len, 1)
> --->	  && !(optimize_size && tree_low_cst (len, 1) > 1)
> 	  && can_store_by_pieces (tree_low_cst (len, 1),
> 				  builtin_memset_read_str, &c, dest_align))
> 	{
> 	  val_rtx = force_reg (TYPE_MODE (unsigned_char_type_node),
> 			       val_rtx);
> 	  store_by_pieces (dest_mem, tree_low_cst (len, 1),
> 			   builtin_memset_gen_str, val_rtx, dest_align, 0);
> 	}
>       else if (!set_storage_via_setmem (dest_mem, len_rtx, val_rtx,
> 					dest_align, expected_align,
> 					expected_size))
> 	goto do_libcall;
>        
> This code still uses can_store_by_pieces for single-byte stores when
> optimize_size (and can still fall back to setmem or libcalls for that
> case if can_store_by_pieces returns false, although I agree that's an
> odd thing to do for single-byte stores).  What I was objecting to was
> that the target doesn't get any chance to say that _2-byte stores_ (or
> bigger) are better implemented "by pieces" than via a setmem or libcall
> pattern.
> 
> You referred to this limit yourself when I queried the MIPS
> optimize_size value of SET_RATIO.  You said that the value only really
> matters for 1-byte stores, and looking at the patch, I thought I could
> see why.  All calls to can_store_by_pieces with a "true" argument seemed
> to be guarded by a check like the above.  So the suggestion to move the
> check was really following on from that.  As far as I could tell,
> CLEAR_RATIO and CLEAR_BY_PIECES_P have no single-byte limit for
> optimize_size, so I was thinking it would be better if SET_RATIO and
> SET_BY_PIECES_P didn't either.

Oh, OK.  I misunderstood what you were unhappy about there.  I tried 
just removing the offending clause of the if statement, and got a small 
decrease in size across the board for CSiBE tests (just -Os, -Os 
-mips16, and -Os -mabicalls) on mips32r2.

I'm not sure it's necessary to move the optimize_size test to the 
default definition of SET_RATIO.  We currently have:

#ifndef MOVE_RATIO
#if defined (HAVE_movmemqi) || defined (HAVE_movmemhi) || defined 
(HAVE_movmemsi) || defined (HAVE_movmemdi) || defined (HAVE_movmemti)
#define MOVE_RATIO 2
#else
/* If we are optimizing for space (-Os), cut down the default move 
ratio.  */
#define MOVE_RATIO (optimize_size ? 3 : 15)
#endif

#endif
/* If a memory set (to value other than zero) operation would take
    SET_RATIO or more simple move-instruction sequences, we will do a movmem
    or libcall instead.  */
#ifndef SET_RATIO
#define SET_RATIO MOVE_RATIO
#endif

...so SET_RATIO is already guaranteed to default to a small value if 
optimize_size is true.  Only problem is if folks are concerned that this 
change might be a Bad Thing for other back ends; all my other changes 
have tried not to change the behavior on anything other than MIPS.

-Sandra