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Date: Thu, 3 Jul 2003 22:53:36 +0200
From: Corinna Vinschen <corinna-cygwin AT cygwin DOT com>
To: cygwin AT cygwin DOT com
Subject: Re: mmap() and gcc precompiled headers
Message-ID: <20030703205336.GA9643@cygbert.vinschen.de>
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References: <3F0476DE DOT 9010103 AT agilent DOT com>
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On Thu, Jul 03, 2003 at 11:33:02AM -0700, Earl Chew wrote:
> Corinna Vinschen wrote:
> >On Thu, Jul 03, 2003 at 10:19:42AM -0400, Christopher Faylor wrote:
> >>On Thu, Jul 03, 2003 at 11:47:28AM +0200, Corinna Vinschen wrote:
> >>>Do you mean something like this:
> >>>
> >>>  If addr is given, check if it's 64K aligned.  If not, align and
> >>>  raise the memory requirement accordingly.  Call MapViewOfFileEx
> >>>  with the aligned address.  If it works, return the addr given as
> >>>  parameter, otherwise return MapViewOfFileEx(NULL).
> 
> Patching gcc to use MAP_FIXED is possible, but I'm not sure that
> it's really necessary, and there's another complication.

IMO it's not a question of necessity but of correctness.

>    preferred_base = mmap (NULL, size,
>                           PROT_READ | PROT_WRITE, MAP_PRIVATE,
>                           f, offset);
> 
>    munmap(preferred_base, size);
> 
>    newbase = mmap (preferred_base, size,
>                    PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_FIXED,
>                    f, offset);
> 
> There's a good chance that this fails because preferred_base
> is the fudged value from MapViewOfFileEx() and is no longer 64k
> aligned (only 4k aligned), and MAP_FIXED contains a check
> to ensure that the address is 64k aligned :-(

It depends on offset being 0 or not.  Is gcc using offset !=0?

> So a patch to gcc to use MAP_FIXED would also require somehow
> making gcc aware of the 64k alignment.

No chance since it can't actually control the return value from mmap.
So I guess we should actually try it with your suggested approach.

> If addr is given, perform the 64k fixup. Essentially this means
> undoing the mmap64() fudge that might have been done previously:
> 
>   caddr_t ret = rec->get_address () + (off - gran_off);
> 
> (Adjust the size appropriately too.)

Adjusting the size?!?  We're just potentially moving the mapping addr,
this shouldn't have any influence to len resp. gran_len.

> In other words, addr -= (off - gran_off). This address should
> now be 64k aligned. Error if it isn't.
> 
> Use this address when calling MapViewOfFileEx(). If the call fails
> and MAP_FIXED is not set, then try again with addr = 0.

Yes, that's what I mentioned in my first reply.  I think it's worth
a try.  I've checked in a patch.  I've tested it and it looks pretty
good.


Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Developer                                mailto:cygwin AT cygwin DOT com
Red Hat, Inc.

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