Re: [RS6000] Fix PR61098, Poor code setting count register

[Alan Modra <amodra@gmail.com>]

On Thu, May 08, 2014 at 09:48:35AM -0400, David Edelsohn wrote:
> The history is 32 bit HWI.

Right.

> The ChangeLog does not mention the changes to rs6000.md nor rs6000-protos.h.

Oops, added.

	* config/rs6000/rs6000.md (movsi_internal1_single+1): Update
	call to rs6000_emit_set_const in splitter.
	(movdi_internal64+2, +3): Likewise.
	* config/rs6000/rs6000-protos.h (rs6000_emit_set_const): Update
	prototype.

> Please do not remove all of the comments from the two functions. The
> comments should provide some documentation about the different
> purposes of the two functions other than setting DEST to a CONST.

I believe my updated comment covers the complete purpose of the
function nowadays.  The comments I removed are out-dated, and should
have been removed a long time ago..  rs6000_emit_set_const does not
even look at N, it always returns a non-zero result, and the return is
only tested for non-zero.  I removed MODE too, because that is always
the same as GET_MODE (dest).

> Why did you remove the test for NULL dest?
> 
> -      if (dest == NULL)
> -       dest = gen_reg_rtx (mode);
> 
> That could occur, at least it used to occur.

I'm sure we can't get a NULL dest nowadays.  All (three) uses of
rs6000_emit_set_const occur in splitters.  They all must have passed a
gpc_reg_operand constraint on operands[0] before calling
rs6000_emit_set_const, so if NULL were possible we'd segfault in
gpc_reg_operand.

> I think that the way you rearranged the invocations of copy_rtx() in
> rs6000_emit_set_long_const() is okay, but it would be good for someone
> else to double check.

Yeah, that function is a bit messy.  I took the approach of always use
a bare "dest" once in the last instruction emitted, with every other
use getting hit with copy_rtx.  The previous approach was similar,
but used the bare "dest" on the first instruction emitted.  Obviously
you don't need copy_rtx anywhere with the new code when
can_create_pseudo_p is true, but I felt it wasn't worth optimising
that for the added source complication.

-- 
Alan Modra
Australia Development Lab, IBM