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RE: (R5900) Implementing Vector Support


Woon yung Liu <ysai187@yahoo.com> writes:
> On Wednesday, June 1, 2016 5:45 AM, Richard Henderson <rth@redhat.com>
> wrote:
> > This is almost always incorrect, and certainly before reload.
> > You need to use gen_lowpart.  There are examples in the code
> 
> > fragments that I sent the other week.
> 
> The problem is that gen_lowpart() doesn't seem to support casting to
> other modes of the same size.
> When I use it, the assert within gen_lowpart_general() will fail due to
> gen_lowpart_common() rejecting the operation (new mode size is not
> smaller than the old).

The conclusion we came to when developing MSA is that simplify_gen_subreg
is the way to go for converting between vector modes:

simplify_gen_subreg (new_mode, rtx, old_mode, 0)

I'm not sure there is much need to change modes after reload so do it
upfront at expand time or when splitting and you should be OK.

See trunk mips.c for a number of uses of this when converting vector modes.
 
> > You need to read the gcc internals documentation.  They are all three
> different
> 
> > uses, though there is some overlap between define_insn and
> define_expand.
> 
> I actually read the GCC internals documentation before I even begun any
> attempt at this, but there was still a lot that I did not successfully
> grasp.
> 
> I'll go with define_expand then.

define_expand only provides a way to generate an instruction or sequence
of instructions to achieve the overall goal. You must also have
define_insn definitions for any pattern you emit or the generated code
will fail to match.

A define_insn_and_split is just shorthand for a define_insn where one
or more output patterns are '#' (split) and you want to define the
split alongside the instruction rather than as a separate define_split.
As far as I understand the difference is syntactic sugar.

> But I am already doubting that I will complete this port as I can no
> longer see a favourable conclusion.

It may take time but I'm sure we can help talk through the problems. As
a new GCC developer you are a welcome addition to the community.

Thanks,
Matthew


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