Where is it written?

Where is it written?

[George Anzinger]

I thought this would be simple, but...

Could someone point me at the info on calling conventions to be used
with
x86 processors.  I need this to write asm code correctly and I suspect
that it is a bit more formal than the various comments I have found in
the sources.  Is it, perhaps an Intel doc?  Or a gcc thing?
 
George
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Re: Where is it written?

[Michael Meissner]

On Fri, Nov 10, 2000 at 08:37:56AM -0800, George Anzinger wrote:
> I thought this would be simple, but...
> 
> Could someone point me at the info on calling conventions to be used
> with
> x86 processors.  I need this to write asm code correctly and I suspect
> that it is a bit more formal than the various comments I have found in
> the sources.  Is it, perhaps an Intel doc?  Or a gcc thing?

It may be out of print by now, but the original reference for the x86 ABI, is
the:

	System V Application Binary Interface
	Intel386 (tm) Processor Supplement

When Cygnus purchased the manual I have, many moons ago, it was published by
AT&T, with a copyright date of 1991, published by Prentice Hall, with an ISBN
number of 0-13-877689-X.  It most recently was published by SCO (possibly even
Caldera, which just bought SCO).  You can get an online version from:

	http://www.sco.com/developer/devspecs/abi386-4.pdf

-- 
Michael Meissner, Red Hat, Inc.
PMB 198, 174 Littleton Road #3, Westford, Massachusetts 01886, USA
Work:	  meissner@redhat.com		phone: +1 978-486-9304
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Re: Where is it written?

[Albert D. Cahalan]

Michael Meissner writes:

> It may be out of print by now, but the original reference
> for the x86 ABI, is the:
>
> 	System V Application Binary Interface
> 	Intel386 (tm) Processor Supplement
>
> When Cygnus purchased the manual I have, many moons ago,
> it was published by AT&T, with a copyright date of 1991,

Gee that looks old. Might there be better calling conventions
for the Pentium 4 or Athlon? Memory latency, vector registers,
and more direct access to floating-point registers may mean
we ought to change the calling conventions. One would start
with the kernel of course, because it stands alone.

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Re: Where is it written?

[Michael Meissner]

On Fri, Nov 10, 2000 at 07:11:37PM -0500, Albert D. Cahalan wrote:
> Michael Meissner writes:
> 
> > It may be out of print by now, but the original reference
> > for the x86 ABI, is the:
> >
> > 	System V Application Binary Interface
> > 	Intel386 (tm) Processor Supplement
> >
> > When Cygnus purchased the manual I have, many moons ago,
> > it was published by AT&T, with a copyright date of 1991,
> 
> Gee that looks old. Might there be better calling conventions
> for the Pentium 4 or Athlon? Memory latency, vector registers,
> and more direct access to floating-point registers may mean
> we ought to change the calling conventions. One would start
> with the kernel of course, because it stands alone.

Generally with ABIs you don't want to mess with it (otherwise you can't be
guaranteed that a library built by somebody else will be compatible with your
code, without all sorts of bits in the e_flags field).  It allows multiple
compilers to be provided that all interoperate (as long as they follow the same
spec).

Don't get me wrong -- in my 25 years of compiler hacking, I've never seen an
ABI that I was completely happy with, including ABI's that I designed myself.
ABIs by their nature are a compromise.  That particular ABI was short sighted
in that it wants only 32-bit alignment for doubles, instead of 64-bit alignment
for instance, and also doesn't align the stack to higher alignment boundaries.

-- 
Michael Meissner, Red Hat, Inc.
PMB 198, 174 Littleton Road #3, Westford, Massachusetts 01886, USA
Work:	  meissner@redhat.com		phone: +1 978-486-9304
Non-work: meissner@spectacle-pond.org	fax:   +1 978-692-4482
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Re: Where is it written?

[H. Peter Anvin]

Followup to:  <20001110192751.A2766@munchkin.spectacle-pond.org>
By author:    Michael Meissner <meissner@spectacle-pond.org>
In newsgroup: linux.dev.kernel
> 
> Generally with ABIs you don't want to mess with it (otherwise you can't be
> guaranteed that a library built by somebody else will be compatible with your
> code, without all sorts of bits in the e_flags field).  It allows multiple
> compilers to be provided that all interoperate (as long as they follow the same
> spec).
> 
> Don't get me wrong -- in my 25 years of compiler hacking, I've never seen an
> ABI that I was completely happy with, including ABI's that I designed myself.
> ABIs by their nature are a compromise.  That particular ABI was short sighted
> in that it wants only 32-bit alignment for doubles, instead of 64-bit alignment
> for instance, and also doesn't align the stack to higher alignment boundaries.
> 

We can mess with the ABI, but it requires a wholescale rev of the
entire system.  We have had such revs before -- each major rev of libc
is one -- but they are incredibly painful.  However, if we find
ourselves in a situation where there are enough reasons to introduce
libc.so.7 then perhaps looking at some revs to the ABI might be in
order -- passing arguments in registers and aligning the stack to 64
bits probably would be the main items.

	-hpa
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Re: Where is it written?

[Keith Owens]

On 10 Nov 2000 17:10:00 -0800, 
"H. Peter Anvin" <hpa@zytor.com> wrote:
>We can mess with the ABI, but it requires a wholescale rev of the
>entire system.

AFAICT, there is nothing stopping us from redoing the kernel ABI to
pass the first few parameters between kernel functions in registers.
As long as the syscall interface is unchanged, that ABI change will
only break binary modules (care_factor == 0).  The ABI type would need
to be added to the symbol version prefix, trivial.

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Re: Where is it written?

[H. Peter Anvin]

Followup to:  <11198.973906134@ocs3.ocs-net>
By author:    Keith Owens <kaos@ocs.com.au>
In newsgroup: linux.dev.kernel
>
> On 10 Nov 2000 17:10:00 -0800, 
> "H. Peter Anvin" <hpa@zytor.com> wrote:
> >We can mess with the ABI, but it requires a wholescale rev of the
> >entire system.
> 
> AFAICT, there is nothing stopping us from redoing the kernel ABI to
> pass the first few parameters between kernel functions in registers.
> As long as the syscall interface is unchanged, that ABI change will
> only break binary modules (care_factor == 0).  The ABI type would need
> to be added to the symbol version prefix, trivial.
> 

Yes, the kernel is very different; however, the big win for an ABI
change is in user space.

AFAIK, I think Linus tried this once, but ran into bugs in gcc.  We
might very well try again in 2.5.

	-hpa
-- 
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Re: Where is it written?

[Richard Henderson]

On Fri, Nov 10, 2000 at 05:33:34PM -0800, H. Peter Anvin wrote:
> AFAIK, I think Linus tried this once, but ran into bugs in gcc.
> We might very well try again in 2.5.

You'll definitely have to use a compiler later than gcc 2.95, since
there were in fact major bugs in this area.  I'd be interested in
hearing bug reports from someone trying with current cvs sources.


r~
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Re: Where is it written?

[Michael Meissner]

On Sat, Nov 11, 2000 at 12:28:54PM +1100, Keith Owens wrote:
> On 10 Nov 2000 17:10:00 -0800, 
> "H. Peter Anvin" <hpa@zytor.com> wrote:
> >We can mess with the ABI, but it requires a wholescale rev of the
> >entire system.
> 
> AFAICT, there is nothing stopping us from redoing the kernel ABI to
> pass the first few parameters between kernel functions in registers.
> As long as the syscall interface is unchanged, that ABI change will
> only break binary modules (care_factor == 0).  The ABI type would need
> to be added to the symbol version prefix, trivial.

Other than the minor little fact that -mregparm=n exposes several latent
compiler bugs, that since it is not part of the ABI, it isn't on anybody's
radar screen as needing to be fixed.  This is of course the downside of free
software, that volunteers tend to have their own ideas of what to work on.

Also note, that for -mregpar=n, it is important that variable argument
functions be declared properly in all callers, since they need to use the
standard calling sequence.

-- 
Michael Meissner, Red Hat, Inc.
PMB 198, 174 Littleton Road #3, Westford, Massachusetts 01886, USA
Work:	  meissner@redhat.com		phone: +1 978-486-9304
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Re: Where is it written?

[Andrea Arcangeli]

On Sat, Nov 11, 2000 at 12:17:04AM -0500, Michael Meissner wrote:
> Other than the minor little fact that -mregparm=n exposes several latent
> compiler bugs, that since it is not part of the ABI, it isn't on anybody's
> radar screen as needing to be fixed.  This is of course the downside of free

mregparm bug is supposed to been fixed in 2.95 by Bernd Schmit.

Andrea
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Re: Where is it written?

[Andrea Arcangeli]

On Fri, Nov 10, 2000 at 07:27:51PM -0500, Michael Meissner wrote:
> Don't get me wrong -- in my 25 years of compiler hacking, I've never seen an
> ABI that I was completely happy with, [..]

Ok, but I've seen only one that I'm completly unhappy with.  Can you think at
one case where it's better to push the parameter on the stack instead of
passing them through the callee clobbered ebx/eax/edx? Saving the pushes makes
a relevant performance difference (that's why we have FASTCALL in kernel to use
the sane calling convention even with the <2.95 gcc in fast paths).

Andrea
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Re: Where is it written?

[Peter Samuelson]

[Andrea Arcangeli]
> Can you think at one case where it's better to push the parameter on
> the stack instead of passing them through the callee clobbered
> ebx/eax/edx?

Well it's safer if you are lazy about prototyping varargs functions.
But of course by doing that you're treading on thin ice anyway, in
terms of type promotion and portability.  So I guess it's much better
to say "varargs functions MUST be prototyped" and use the registers.

I'd say go for it -- set up a mailing list and flesh out a better x86
ABI.  (Yes, this is the ubiquitous "someone besides me should..."
suggestion, I'm afraid I would look pretty foolish trying to help
design such.)  One issue: ideally you want to use 64-bit regs on AMD
Hammer for long longs, but then you leave out all legacy x68s. :(

AIUI gcc can cope OK with multiple ABIs to be chosen at runtime, am I
right?  IRIX, HP-UX and AIX all have both 32-bit and 64-bit ABIs.

Peter
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Re: Where is it written?

[H. Peter Anvin]

Followup to:  <20001111171749.A32100@wire.cadcamlab.org>
By author:    Peter Samuelson <peter@cadcamlab.org>
In newsgroup: linux.dev.kernel
> 
> [Andrea Arcangeli]
> > Can you think at one case where it's better to push the parameter on
> > the stack instead of passing them through the callee clobbered
> > ebx/eax/edx?
> 
> Well it's safer if you are lazy about prototyping varargs functions.
> But of course by doing that you're treading on thin ice anyway, in
> terms of type promotion and portability.  So I guess it's much better
> to say "varargs functions MUST be prototyped" and use the registers.
> 

It definitely is now.  At the time the original x86 ABI was created, a
lot of C code was still K&R, and thus prototypes didn't exist...

> 
> AIUI gcc can cope OK with multiple ABIs to be chosen at runtime, am I
> right?  IRIX, HP-UX and AIX all have both 32-bit and 64-bit ABIs.
> 

I don't think we want to introduce a new ABI in user space at this
time.  If we ever have to major-rev the ABI (libc.so.7), then we
should consider this.

	-hpa
-- 
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Re: Where is it written?

[Peter Samuelson]

[Peter Anvin]
> At the time the original x86 ABI was created, a lot of C code was
> still K&R, and thus prototypes didn't exist...

True enough.  That does explain a lot.  But what about the Alpha?  From
reading gcc source awhile back I seem to remember that most if not all
parameters are passed in registers.  How does *that* work with varargs
and no prototypes?  Or does it?

> I don't think we want to introduce a new ABI in user space at this
> time.  If we ever have to major-rev the ABI (libc.so.7), then we
> should consider this.

Ah, but kernel-side?  My point was that if gcc (and binutils) is
flexible enough to let you pick an ABI at runtime, perhaps a RISCoid
ABI for x86 could coexist with the SysV one, to be used initially for
self-contained code like the kernel.  (And later, a possible transition
in userspace.)

Peter
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Re: Where is it written?

[H. Peter Anvin]

Peter Samuelson wrote:
> 
> [Peter Anvin]
> > At the time the original x86 ABI was created, a lot of C code was
> > still K&R, and thus prototypes didn't exist...
> 
> True enough.  That does explain a lot.  But what about the Alpha?  From
> reading gcc source awhile back I seem to remember that most if not all
> parameters are passed in registers.  How does *that* work with varargs
> and no prototypes?  Or does it?
> 

It doesn't, but the Alpha is a *much* more recent ABI... the x86 ABI
really dates back to the 16-bit 8086-series CPUs.

> > I don't think we want to introduce a new ABI in user space at this
> > time.  If we ever have to major-rev the ABI (libc.so.7), then we
> > should consider this.
> 
> Ah, but kernel-side?  My point was that if gcc (and binutils) is
> flexible enough to let you pick an ABI at runtime, perhaps a RISCoid
> ABI for x86 could coexist with the SysV one, to be used initially for
> self-contained code like the kernel.  (And later, a possible transition
> in userspace.)

We tried once; at that point the register-based ABI support in gcc was
too buggy to be useful.  We might try again in 2.5 since we now have
increased the minimum gcc version for kernel compiles.  Binutils needs no
change.

	-hpa

-- 
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Re: Where is it written?

[Peter Samuelson]

[H. Peter Anvin <hpa@transmeta.com>]
> but the Alpha is a *much* more recent ABI... the x86 ABI really dates
> back to the 16-bit 8086-series CPUs.

Oh, right.  Xenix.  I'd forgotten.

> We might try again in 2.5 since we now have increased the minimum gcc
> version for kernel compiles.  Binutils needs no change.

You mean trivial changes to understand the ELF magic number for a
riscoid-ABI x86 object.  (You wouldn't lie to the linker and call them
SysV objects, now, would you?)  Also gdb and libbfd need to know about
stack frames.  Admittedly none of this is strictly necessary just to
link and boot a kernel.

Peter
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Re: Where is it written?

[Keith Owens]

On Sat, 11 Nov 2000 23:36:42 -0600 (CST), 
Peter Samuelson <peter@cadcamlab.org> wrote:
>You mean trivial changes to understand the ELF magic number for a
>riscoid-ABI x86 object.  (You wouldn't lie to the linker and call them
>SysV objects, now, would you?)  Also gdb and libbfd need to know about
>stack frames.  Admittedly none of this is strictly necessary just to
>link and boot a kernel.

Any ABI change needs to be externalised for modules.  Otherwise symbol
versions will not detect that the kernel was compiled with -mregparm=3
but the module was compiled with parameters on stack.  Good thing this
is 2.5 material.

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Re: Where is it written?

[Andi Kleen]

On Sat, Nov 11, 2000 at 09:21:01PM -0800, H. Peter Anvin wrote:
> 
> We tried once; at that point the register-based ABI support in gcc was
> too buggy to be useful.  We might try again in 2.5 since we now have
> increased the minimum gcc version for kernel compiles.  Binutils needs no
> change.

As far as I know egcs 1.1 still has fastcall bugs, they were only fixed in
2.95 (which unfortunately has other bugs). So to use it the minimum compiler 
would need to be bumped again.


-Andi

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Re: Where is it written?

[Andrea Arcangeli]

On Sat, Nov 11, 2000 at 05:17:49PM -0600, Peter Samuelson wrote:
> I'd say go for it -- set up a mailing list and flesh out a better x86
> ABI. [..]

I think it doesn't worth to break binary compatilibity at this late stage.

> design such.)  One issue: ideally you want to use 64-bit regs on AMD
> Hammer for long longs, but then you leave out all legacy x68s. :(

We can't in compatibilty mode because the rex regs are available _only_ in
64bit mode and even assuming the hardware would support that I would not
recommend that since as you said that binary would not run anymore on any other
x86 so causing pain.  Recompiling a program with native x86-64 gcc 64bit (that
uses the 64bit ABI) is the right way to go in that case (64bit mode uses 1
64bit register for long long as all other 64bit architectures of course).

> AIUI gcc can cope OK with multiple ABIs to be chosen at runtime, am I
> right?  IRIX, HP-UX and AIX all have both 32-bit and 64-bit ABIs.

Yes as in other systems, 32bit mode and 64bit mode needs different ABI and they
will coexist in the same system.

Andrea
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Re: Where is it written?

[H. Peter Anvin]

Followup to:  <20001112132328.C2366@athlon.random>
By author:    Andrea Arcangeli <andrea@suse.de>
In newsgroup: linux.dev.kernel
> 
> I think it doesn't worth to break binary compatilibity at this late stage.
> 
> > design such.)  One issue: ideally you want to use 64-bit regs on AMD
> > Hammer for long longs, but then you leave out all legacy x68s. :(
> 
> We can't in compatibilty mode because the rex regs are available _only_ in
> 64bit mode and even assuming the hardware would support that I would not
> recommend that since as you said that binary would not run anymore on any other
> x86 so causing pain.  Recompiling a program with native x86-64 gcc 64bit (that
> uses the 64bit ABI) is the right way to go in that case (64bit mode uses 1
> 64bit register for long long as all other 64bit architectures of course).
> 

Well, you *could* run REX32, but REX32 is not x86 (x86 code doesn't
run in REX32 mode, and REX32 code doesn't run on an x86.)

	-hpa
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Re: Where is it written?

[Olaf Titz]

> Well it's safer if you are lazy about prototyping varargs functions.
> But of course by doing that you're treading on thin ice anyway, in
> terms of type promotion and portability.  So I guess it's much better
> to say "varargs functions MUST be prototyped" and use the registers.

make -Wmissing-prototypes mandatory :-)

Olaf
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Re: Where is it written?

[H. Peter Anvin]

Followup to:  <200011110011.eAB0BbF244111@saturn.cs.uml.edu>
By author:    "Albert D. Cahalan" <acahalan@cs.uml.edu>
In newsgroup: linux.dev.kernel
> 
> Gee that looks old. Might there be better calling conventions
> for the Pentium 4 or Athlon? Memory latency, vector registers,
> and more direct access to floating-point registers may mean
> we ought to change the calling conventions. One would start
> with the kernel of course, because it stands alone.
> 

The main win would be passing arguments in registers -- at least three
such registers could be used (%eax, %edx, %ecx) without increasing
register pressure.  Doing this for nonvaradic functions probably would
be a win.  Similarly, floating-point arguments and SSE arguments could
be passed in registers, and _Bool output (a C99 feature) could at
least theoretically be returned in a flag.

	-hpa
-- 
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Re: Where is it written?

[H. Peter Anvin]

Followup to:  <3A0C2464.A7CDEFAB@mvista.com>
By author:    George Anzinger <george@mvista.com>
In newsgroup: linux.dev.kernel
>
> I thought this would be simple, but...
> 
> Could someone point me at the info on calling conventions to be used
> with x86 processors.  I need this to write asm code correctly and I
> suspect that it is a bit more formal than the various comments I
> have found in the sources.  Is it, perhaps an Intel doc?  Or a gcc
> thing?
>  

http://www.sco.com/developer/devspecs/abi386-4.pdf

	-hpa
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