Re: MIPS opcodes patch: change move from andu to or

[Andrew Haley <aph at pasanda dot cygnus dot co dot uk>]

> Date: 25 Oct 1999 13:31:12 -0400
> From: Ian Lance Taylor <ian@zembu.com>
> 
>    Date: Mon, 25 Oct 1999 11:19:28 -0400 (EDT)
>    From: Gavin Romig-Koch <gavin@cygnus.com>
> 
>    OK to commit?
> 
>    For opcodes:
> 	   * mips-opc.c (move): Use "or" rather than "addu" or "daddu".
> 
> We deliberately chose ``addu'' and ``daddu'' rather than ``or''
> because several MIPS chips have two pipelined addition units but only
> one pipelined logical unit.  Those chips can move two pairs of
> registers around at once if we use ``addu'', but they can only move
> one pair if we use ``or''.

Aha!  That's a *very* good reason.  I'll add a comment, if that's OK
with you.

> Is there any reason we should use ``or''?  Your comment mentions
> something about ``gcc -mips3 -gp32''; can't we fix gcc?

Yeah.  The problem is that -mgp32 is supposed to generate 32-bit code,
but any move insns get converted into "dadd".  I don't really think
that this is important, since the insns will have the same effect, but
it looks rather odd.

I suppose that we could change gcc to ouput "or" insns for 32-bit
moves, but that would degrade performance in the case that you
describe.

Andrew.