Re: Floating-point variadic function call

[Andrew Pinski <pinskia@gcc.gnu.org>]

On Mon, Oct 10, 2016 at 12:03 AM, Damien Thébault <damien@dtbo.net> wrote:
> Hello,
>
> I am trying to use libffi to call various functions, and my first try
> was with printf(). As this is a variadic function, I used
> ffi_prep_cif_var(). Everything seems to be working, except with floats,
> which are not working, and are even generating wrong memory accesses
> (detected by valgrind).
>
> A similar function with ffi_prep_cif() and floats is working properly.
>
> Could type promotion from float to double be the issue here ?
> Does libffi handle type promotion in variadic calls or is it the
> caller's job ?
>
> I tested on a x86_64 computer both 64-bit and 32-bit versions (the
> latter compiled with -m32) as well as ARM under qemu.
> In all those cases, I didn't get the proper result with floats while
> doubles are ok, as well as chars.

That is because the variadic function ABI is different from the normal
argument ABI on ARM hard-float EABI.  Basically for variadic functions
float are passed via the integer registers while for normal functions,
they are passed via the vfp registers.

Basically you need to use variadic function support in libff.  Use
ffi_prep_cif_var instead of ffi_prep_cif and then don't do:

    for(i=1 ; i<10 ; i++)
    {
        arg_types[i] = &ffi_type_float;
    }


Thanks,
Andrew Pinski

> I've attached a sample source file that reproduces the problem and here
> is the output of one test run:
>
>  $ gcc $(pkg-config --cflags --libs libffi) -std=c99 -pedantic -Wall -Wextra libffi_variadic_printf.c -o libffi_variadic_printf
>  $ ./libffi_variadic_printf
> Non-variadic call:
> [1.100000,2.200000,3.300000,4.400000,5.500000,6.600000,7.700000,8.800000,9.900000]
> result is 83
> Variadic call:
> [0.000000,0.000000,0.000000,0.000000,0.000000,91750.390826,0.000000,0.000000,0.000000]
> result is 87
>
> Thanks,