Re: [PATCH] MIPS: don't expand large block move

["Maciej W. Rozycki" <macro@orcam.me.uk>]

On Fri, 19 May 2023, Jeff Law wrote:

> > diff --git a/gcc/config/mips/mips.cc b/gcc/config/mips/mips.cc
> > index ca491b981a3..00f26d5e923 100644
> > --- a/gcc/config/mips/mips.cc
> > +++ b/gcc/config/mips/mips.cc
> > @@ -8313,6 +8313,12 @@ mips_expand_block_move (rtx dest, rtx src, rtx
> > length)
> >   	}
> >         else if (optimize)
> >   	{
> > +	  /* When the length is big enough, the lib call has better performace
> > +	     than load/store insns.
> > +	     In most platform, the value is about 64-128.
> > +	     And in fact lib call may be optimized with SIMD */
> > +	  if (INTVAL(length) >= 64)
> > +	    return false;
> Just a formatting nit.  Space between INTVAL and the open paren for its
> argument list.

 This is oddly wrapped too.  I'd move "performace" (typo there!) to the 
second line, to align better with the rest of the text.

 Plus s/platform/platforms/ and there's a full stop missing along with two 
spaces at the end.  Also there's inconsistent style around <= and >=; the 
GNU Coding Standards ask for spaces around binary operators.  And "don't" 
in the change heading ought to be capitalised.

 In fact, I'd justify the whole paragraph as each sentence doesn't have to 
start on a new line, and the commit description could benefit from some 
reformatting too, as it's now odd to read.

> OK with that change.

 I think the conditional would be better readable if it was flattened 
though:

      if (INTVAL (length) <= MIPS_MAX_MOVE_BYTES_STRAIGHT)
	...
      else if (INTVAL (length) >= 64)
	...
      else if (optimize)
	...

or even:

      if (INTVAL (length) <= MIPS_MAX_MOVE_BYTES_STRAIGHT)
	...
      else if (INTVAL (length) < 64 && optimize)
	...

One just wouldn't write it as proposed if creating the whole piece from 
scratch rather than retrofitting this extra conditional.

 Ultimately it may have to be tunable as LWL/LWR, etc. may be subject to 
fusion and may be faster after all.

  Maciej