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List:       cygwin
Subject:    Re: 32 bit vs 64 bit Cygwin, followup
From:       Corinna Vinschen <corinna-cygwin () cygwin ! com>
Date:       2018-11-29 16:33:27
Message-ID: 20181129163327.GD30649 () calimero ! vinschen ! de
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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

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