From mboxrd@z Thu Jan  1 00:00:00 1970
From: Michael Hayes <m.hayes@elec.canterbury.ac.nz>
To: law@cygnus.com
Cc: Richard Henderson <rth@cygnus.com>, Jamie Lokier <egcs@tantalophile.demon.co.uk>, Marc Lehmann <pcg@goof.com>, egcs@cygnus.com
Subject: Re: GCC 2.7.2.3 good, EGCS 1.0.3 bad for x86 subtract then test
Date: Fri, 25 Dec 1998 18:17:00 -0000
Message-id: <13956.18174.396993.213410@ongaonga.elec.canterbury.ac.nz>
References: <no.id@egcs.cygnus.com> <19981218003619.B28066@cerebro.laendle> <19981220010520.A4999@tantalophile.demon.co.uk> <19981220223834.D16580@cerebro.laendle> <19981221105213.A5825@tantalophile.demon.co.uk> <19981223141815.C12309@dot.cygnus.com>
X-SW-Source: 1998-12/msg00910.html

Jeff Law writes:

 > Basically we had something like this before combine:
 > 
 > (set (reg1) (mem (blah))
 > (set (reg2) (mem (oof))
 > (set (reg3) (minus (reg1) (reg2))
 > 
 > Combine turned that into:
 > 
 > (set (reg3) (minus (mem (blah)) (reg2))
 > 
 > Which is allowed by the predicates for the subsi3 insn.
 > 
 > However, it loses because the first input does not match the output
 > operand.  This causes spills & reloads and bad code.

Why not use the extra condition to prevent the combination if the
first input does not match the output operand?

Michael.