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From: =?ISO-8859-1?Q?Markus_Sch=F6pflin?= <no.spam@spam.spam>
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Subject: Re: Importing C function with variable argument list
Date: Fri, 13 Apr 2012 10:04:01 +0200
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Date: 2012-04-13T10:04:01+02:00
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Am 13.04.2012 06:08, schrieb Tero Koskinen:

[...]

> So on amd64/x86_64 (unix) platform injecting asm code, which sets
> %rax/%eax to 0, works in most cases (= 0 vector registers used).

Yes, I did read a similar document (I think it's actually %al that is looked 
at, not the full register) which lead me to try using inline assembly like this:

---%<---
with SYSTEM.MACHINE_CODE;
with INTERFACES.C;

procedure TEST
is
    use SYSTEM.MACHINE_CODE;
    use INTERFACES.C;

    function PRINTF (FORMAT : in CHAR_ARRAY; N1 : in INT) return INT;
    pragma IMPORT (C, PRINTF, "printf");

    F1 : constant CHAR_ARRAY := TO_C("%08d" & ASCII.LF);
    DUMMY : INT;
begin
    ASM ("mov $0, %%al", CLOBBER => "al", VOLATILE => TRUE);
    DUMMY := PRINTF(F1, N1 => 123);
end TEST;
--->%---

Using -O0 when compiling results in the following assembly code:

---%<---
   401696:       b0 00                   mov    $0x0,%al
   401698:       48 8b 85 40 ff ff ff    mov    -0xc0(%rbp),%rax
   40169f:       48 89 55 c0             mov    %rdx,-0x40(%rbp)
   4016a3:       48 89 4d c8             mov    %rcx,-0x38(%rbp)
   4016a7:       48 89 45 b0             mov    %rax,-0x50(%rbp)
   4016ab:       48 8d 45 c0             lea    -0x40(%rbp),%rax
   4016af:       48 89 45 b8             mov    %rax,-0x48(%rbp)
   4016b3:       48 8b 45 b0             mov    -0x50(%rbp),%rax
   4016b7:       be 7b 00 00 00          mov    $0x7b,%esi
   4016bc:       48 89 c7                mov    %rax,%rdi
   4016bf:       e8 2c fb ff ff          callq  4011f0 <printf@plt>
   4016c4:       89 45 ec                mov    %eax,-0x14(%rbp)
--->%---

The first line is from the inline assembly, the rest is code generated by gnat 
for the call to printf and the handling of the return value. Still no luck, as 
%rax is used between the first line and the call to printf for different things.

Interestingly, compiling with -O2 gives:

---%<---
   40157c:       b0 00                   mov    $0x0,%al
   40157e:       be 7b 00 00 00          mov    $0x7b,%esi
   401583:       e8 68 fc ff ff          callq  4011f0 <printf@plt>
--->%---

Et voil�, the resulting binary really works and prints out the expected result.

> (Of course, it would be better to have some standard way to
> say this.)

As others have already said, a new calling convention for variadic C functions 
would be the way to go, I think.

Markus