From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <gcc-return-43348-listarch-gcc=gcc.gnu.org@gcc.gnu.org>
Received: (qmail 13622 invoked by alias); 26 Dec 2001 02:06:01 -0000
Mailing-List: contact gcc-help@gcc.gnu.org; run by ezmlm
Precedence: bulk
List-Archive: <http://gcc.gnu.org/ml/gcc/>
List-Post: <mailto:gcc@gcc.gnu.org>
List-Help: <http://gcc.gnu.org/ml/>
Sender: gcc-owner@gcc.gnu.org
Received: (qmail 13593 invoked from network); 26 Dec 2001 02:05:58 -0000
Received: from unknown (HELO igw3.watson.ibm.com) (198.81.209.18)
  by sources.redhat.com with SMTP; 26 Dec 2001 02:05:58 -0000
Received: from sp1n293en1.watson.ibm.com (sp1n293en1.watson.ibm.com [9.2.112.57])
	by igw3.watson.ibm.com (8.11.4/8.11.4) with ESMTP id fBQ23in18888;
	Tue, 25 Dec 2001 21:04:04 -0500
Received: from makai.watson.ibm.com (makai.watson.ibm.com [9.2.216.144])
	by sp1n293en1.watson.ibm.com (8.11.4/8.11.4) with ESMTP id fBQ23An30750;
	Tue, 25 Dec 2001 21:03:15 -0500
Received: from watson.ibm.com (localhost [127.0.0.1])
	by makai.watson.ibm.com (AIX4.3/8.9.3/8.9.3/01-10-2000) with ESMTP id VAA29248;
	Tue, 25 Dec 2001 21:02:42 -0500
Message-Id: <200112260202.VAA29248@makai.watson.ibm.com>
To: Bernd Schmidt <bernds@redhat.co.uk>, Jan Hubicka <jh@suse.cz>,
   Richard Henderson <rth@redhat.com>
cc: gcc@gcc.gnu.org, degger@fhm.edu
Subject: cant_combine_insn_p hard_reg->reg moves
In-Reply-To: Message from degger@fhm.edu 
   of "Tue, 18 Dec 2001 12:49:49 +0100." <20011218125633.C322E6E18@Nicole.fhm.edu> 
Date: Tue, 25 Dec 2001 23:13:00 -0000
From: David Edelsohn <dje@watson.ibm.com>
X-SW-Source: 2001-12/txt/msg01284.txt.bz2

	At Daniel's prompting, I finally tracked down the cause of the
code pessimization with respect to function arguments and return values.
Daniel tracked it down to somewhere in combine.

	The problem is cant_combine_insn_p disallowing hard registers.
Arguments and return values are specified as hard regs.  So this change
from December 2000 now often inserts a move between the hard reg allocated
to the pseudo of a computation and the hard regs of arguments and return
values.  This is rather pessimistic for some architecture, especially for
ABIs with register-based calling conventions.

	How can this improved?  Could cant_combine_insn_p test for hard
regs with either REG_EQUIV involving computations of ARG_POINTER_REGNUM or
FUNCTION_VALUE_REGNO_P?

Thanks, David