From mboxrd@z Thu Jan  1 00:00:00 1970
From: Jeffrey A Law <law@cygnus.com>
To: Dave Love <d.love@dl.ac.uk>
Cc: egcs@cygnus.com
Subject: Re: egcs-1.1 release schedule 
Date: Sun, 21 Jun 1998 23:07:00 -0000
Message-id: <4986.898489891@hurl.cygnus.com>
References: <rzqaf79nru7.fsf@djlvig.dl.ac.uk>
X-SW-Source: 1998-06/msg00746.html

  In message < rzqaf79nru7.fsf@djlvig.dl.ac.uk >you write:
  > _Please_ include some means of allowing Fortran (at least) to get
  > stack-allocated doubles double-aligned on x86 (modulo libc).  (I hope
  > I haven't missed this going in at some stage!)  The one-line patch for
  > STACK_BOUNDARY used by g77 0.5.22 is good enough.
I'm still waiting on some kind of solution that doesn't totally break
the ABI.

To do this "right" you have to:

	* Make sure gcc always allocates stack in multiples of 8 bytes,
	  adding dummy outgoing args as necessary to keep the stack
	  properly aligned at call points.

	  You can't do this with STACK_BOUNDARY since that says we
	  will 100% always have a properly aligned stack, which can
	  never be true since we might be linking in code from
	  another compiler which didn't keep the stack suitably
	  aligned.

	  If the stack gets mis-aligned relative to STACK_BOUNDARY
	  combine could end up removing a seemingly useless
	  stack operation/address calculation.

	  The idea is to make sure the stack is 8 byte aligned in the
	  common cases, but not absolutely rely on it for correct code
	  generation.


	* Second, assuming that gcc always keeps the pointer aligned
	  for itself, then arrange for doubles to end up 8 byte
	  aligned relative to the stack pointer.

	  If the stack gets mis-aligned due to an old module, then
	  our doubles won't be aligned correctly, but the vast majority
	  of the time they will be suitably aligned.

	  I don't think there's any mechanism to do this when the
	  desired alignment is less than STACK_BOUNDARY.  I fact
	  I know that to be the case since I worked on a similar
	  problem recently.

	* The ABI is still going to mandate that some doubles in
	  argument lists are going to be mis-aligned.  We'd have
	  to arrange to copy them from the arglist into a suitable
	  stack slot.  This may be more trouble than its worth.



Note that some non-ABI breaking changes to align doubles and other
values have gone into the x86 compiler.  In particular we should be
properly aligning all data in the static store.

jeff