Re: -mregparm=3 (was Re: [PATCH] i386 do_machine_check() is redundant.

[Mikulas Patocka]

On Mon, 29 Sep 2003, Daniel Jacobowitz wrote:

> On Mon, Sep 29, 2003 at 10:20:45PM +0200, Mikulas Patocka wrote:
> > > > > Use machine_check_vector in the entry code instead.
> > > >
> > > > This is wrong. You just lost the "asmlinkage" thing, which means that it
> > > > breaks when asmlinkage matters.
> > > >
> > > > And yes, asmlinkage _can_ matter, even on x86. It disasbles regparm, for
> > > > one thing, so it makes a huge difference if the kernel is compiled with
> > > > -mregparm=3 (which used to work, and which I'd love to do, but gcc has
> > > > often been a tad fragile).
> > >
> > > gcc 3.2 and later are supposed to be ok (eg during 3.2 development a
> > > long standing bug with regparm was fixed and now is believed to work)...
> > > since our makefiles check gcc version already... this can be made gcc
> > > version dependent as well for sure..
> >
> > They are still buggy. gcc 3.3.1 miscompiles itself with -mregparm=3
> > (without -O or -O2 it works). (I am too lazy to spend several days trying
> > to find exactly which function in gcc was miscompiled, maybe I do it one
> > day). gcc 2.95.3 compiles gcc 3.3.1 with -mregparm=3 -O2 correctly.
> > gcc 3.4 doesn't seem to be better.
> >
> > gcc 2.7.2.3 has totally broken -mregparm=3, even quite simple programs
> > fail.
>
> You can't build GCC with -mregparm=3.  It changes the interface to
> system functions.  So unless your libc happened to be built with
> -mregparm=3, and extensively hacked to expect arguments in registers to
> the assembly stubs, it can't work.

Of course I linked it with libc compiled with regparm=3.

> It's interesting for kernel code, whole distributions, or things which
> are careful to have a glue layer.

BTW. libc headers surround all function parameters with __P, like
extern int printf __P ((__const char* __format, ...));

so you can change in include/sys/cdefs.h
#define __P(x) x
into
#define __P(x) x __attribute__((regparm(0)))

and compile programs with -mregparm=3 -ffreestanding even on normal linux
distribution. I didn't try it for larger program, for simple it works. (it
works as long as program doesn't call libc function via pointer to
function).

(without -ffreestanding gcc sometimes emits calls to library functions on
its own, and uses calling convention from command line, not convention
from prototype).

Mikulas
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