Re: 32 bit vs 64 bit Cygwin, followup
- Date: Sat, 2 Feb 2019 09:39:08 -0500
- From: Sam Habiel <sam.habiel@xxxxxxxxx>
- Subject: Re: 32 bit vs 64 bit Cygwin, followup
On Thu, Nov 29, 2018 at 11:33 AM Corinna Vinschen
> 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
> #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
> #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 ;)
> 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, ...)
printf ("Printing floats:");
printf (" [%.2f]",val);
int main ()
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