From mboxrd@z Thu Jan  1 00:00:00 1970
From: Joern Rennecke <amylaar@cygnus.co.uk>
To: egcs@cygnus.com
Subject: Re: Reload patch to improve 386 code
Date: Thu, 21 Aug 1997 16:51:55 -0000
Message-id: <199708211632.RAA01567@phal.cygnus.co.uk>
In-reply-to: Pine.SOL.3.90.970821173955.733C-100000@maigret.informatik.rwth-aachen.de
X-SW-Source: 1997-08/0250.html

> > The problem with this is that you might have additional spills if you
> > remove reload inherits - when you inherit a reload that needs a secondary
> > reload, you extend the lifetime of only one reload register, instead of needing
> > two (or more, when a tertiary reload would be needed too.)
> 
> Are you sure? The inheritance code in choose_reload_regs is run after all
> spilling has been done, so I don't think removing it could cause additional
> spills.


You are right, I mixed up something here.  What I actually saw was when
an inherit was missed, the new reload - or its secondary reloads -
would clobber other reload registers, which could otherwise have been used
for subsequent inherits.
reload_cse coudn't clean up the mess - it just changed the reload for the
missed inherit from a memory-register to a register-register move.
It isn't clever enough to reassign reload registers and then reclaim the
freed register for further cse.

> The reason I'm not happy with the inheritance code is that find_equiv_reg
> is overly conservative; e.g. it misses cases like this:
> 
> movl %eax,16(%esp)
> movl %eax,some_symbol
> addl 16(%esp),%ebx
> 
> where the addl could inherit the value in %eax. My idea is to teach

I think my inheritance patches would fix that too.