From mboxrd@z Thu Jan  1 00:00:00 1970
From: Jeffrey A Law <law@cygnus.com>
To: Bill Currie <bcurrie@tssc.co.nz>
Cc: ak@muc.de, egcs@cygnus.com
Subject: Re: Feature request: ability to describe x86 register halves as contraints. 
Date: Tue, 30 Jun 1998 15:26:00 -0000
Message-id: <18651.899245271@hurl.cygnus.com>
References: <359962C1.6A80@tssc.co.nz>
X-SW-Source: 1998-07/msg00000.html

  In message <359962C1.6A80@tssc.co.nz>you write:
  > Jeffrey A Law wrote:
  > > First there'd have to be a way to describe hi/lo halves of registers
  > > in RTL.  I'm not aware of any such mechanism.
  > 
  > Couldn't (subreg:QI (reg:HI 0) 1) or (subreg:QI (reg:SI 0) 1) be used to
  > represent AH?  Trouble is, someone will have to teach egcs to use such
  > constructs (I'm not well versed on the operation of trees yet,
  > especially tree->rtl conversion)
Nope -- the constant at the end refers to a word sized object -- on
the x86 words are 32bits.

SUBREGs already have a well defined meaning.  I don't suggest overloading
another meaning on them.

  > Also, how difficult would it be to add eiter a new machine mode or a new
  > flag/interpretation of existing flags to represent halves of registers?
It's not a question of modes or flags, but of how to represent to the
compiler via the RTL how to access parts of a register.

jeff