Re: 32 bit vs 64 bit Cygwin, followup

[Sam Habiel <sam dot habiel at gmail dot com>]

On Thu, Nov 29, 2018 at 11:33 AM Corinna Vinschen
<corinna-cygwin@cygwin.com> wrote:
>
> On Nov 29 10:18, Sam Habiel wrote:
> > On Thu, Nov 29, 2018 at 3:58 AM Corinna Vinschen wrote:
> > > On Nov 28 11:06, Sam Habiel wrote:
> > > > On Wed, Nov 28, 2018 at 11:01 AM Yaakov Selkowitz wrote:
> > > > > On Mon, 2018-11-26 at 14:07 -0500, Sam Habiel wrote:
> > > > > > [...]
> > > > > >  GT.M contains a large
> > > > > > amount of assembly code, written to run on the x32 Linux ABI and the
> > > > > > AMD x64 ABI. It's was very easy to get the x32 Linux ABI to run on
> > > > > > Cygwin x32; Cygwin x64 on the other hand uses the Windows x64 ABI,
> > > > > > which is very different than the AMD ABI (more detail here:
> > > > > > https://eli.thegreenplace.net/2011/09/06/stack-frame-layout-on-x86-64/).
> > > > > > I don't have the expertise nor the time to rewrite a lot of assembly
> > > > > > code to use the Windows x64 ABI. There are about 100 source code files
> > > > > > that are in assembly.
> > > > >
> > > > > -mabi=sysv ?
> > > > >
> > > > Are you telling me that gcc has a flag to support AMD ABI on Cygwin
> > > > x64? The assembly code is not standalone; it gets called from C code
> > > > and calls C code.
> > >
> > > That's what he's telling you.  However, you have to interact with the MS
> > > ABI(*) as well as soon as you call external library functions so it
> > > makes sense to keep your C code in MS ABI.  For the assembler functions,
> > > you can just tell the compiler they are in SYSV ABI by adding a function
> > > attribute to the declaration:
> > >
> > > int asm_func (args) __attribute__ ((sysv_abi))
> > >
> > > Good luck,
> > > Corinna
> > >
> > > (*) Just keep in mind that Cygwin is LP64, not LLP64:
> > >     https://cygwin.com/faq/faq.html#faq.programming.64bitporting
> > > [...]
> > [...]
> > This sounds very promising, but I would like a clarification; because
> > I think you covered 50% of the issue:
> >
> > 1. There are frequent calls from the C code to Assembly.
> > 2. There are also frequent calls from Assembly to C code.
> >
> > Looks like compiling the .s files with the -mabi=sysv flag and
> > declaring the function in C with the __attribute__ ((sysv_abi)) will
> > fix #1.
>
> You shouldn't have to use the flag when building the assembler files,
> they are using SYSV ABI anyway.  In fact, while Yaakov is right,
> basically, I think in your scenario you should only use the GCC function
> attribute since that allows more fine-grained control.  Just stick to MS
> ABI by default and only perform the SYSV ABI juggle where required to
> interact with the assembler code.
>
> > How about #2? I don't see an easy solution. The assembly code puts
> > together the parameters in the registers in the sysv way (rdi, rsi,
> > rdx, rcx, r8, r9), not rcx, rdx, r8, and r9.
>
> One way is to create a SYSV wrapper for each C function called from
> assembler.  Assuming this simple scenario:
>
>   There's a C function foo(), which is called from assembler as
>   well as from other C functions.
>
>     extern long foo (long, double, int, long);
>
>   For the "normal" (i.e. MS ABI) C code add this in front of the above
>   declaration:
>
>     #define foo(a,b,c,d)        __foo((a),(b),(c),(d))
>
>   So the C function is renamed to __foo and C code will call __foo.
>
>   Add a wrapper C file to add a function foo with SYSV ABI, calling
>   __foo:
>
>     #undef foo
>     long __attribute__ ((sysv_abi))
>     foo (long a, double b, int c, long d)
>     {
>       return __foo (a,b,c,d);
>     }
>
> That should do it.  Of course there may be more complicated cases,
> but I leave them as excercise for the reader, and only you are in
> a position to know them ;)
>
>
> HTH,
> Corinna
>
> --
> Corinna Vinschen
> Cygwin Maintainer

Corinna et al.,

I and a colleague started the work to migrate the Linux x64 version to
Cygwin. The results have been very promising; but I think we found a
bug in gcc when dealing with va_start in sysv_abi compiled code. I
have a simple test case. Can somebody confirm? It works fine without
the attribute on PrintFloats.

/* va_start example */
#include <stdio.h>      /* printf */
#include <stdarg.h>     /* va_list, va_start, va_arg, va_end */

void __attribute__ ((sysv_abi)) PrintFloats (int n, ...)
{
  int i;
  double val;
  printf ("Printing floats:");
  va_list vl;
  va_start(vl,n);
  for (i=0;i<n;i++)
  {
    val=va_arg(vl,double);
    printf (" [%.2f]",val);
  }
  va_end(vl);
  printf ("\n");
}

int main ()
{
  PrintFloats (3,3.14159,2.71828,1.41421);
  return 0;
}

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