Re: AARCH64 configure check for gas -mabi support

[Yufeng Zhang]

Hi Kugan,

Thanks for working on this issue.

On 12/04/13 21:03, Kugan wrote:
> Hi,
>
> gcc trunk aarch64 bootstrapping fails with gas version 2.23.2 (with
> error message similar to cannot compute suffix of object files) as this
> particular version does not support -mabi=lp64. It succeeds with later
> versions of gas that supports -mabi.

The -mabi option was introduced to gas when the support for ILP32 was 
added.  Initially the options were named -milp32 and -mlp64:

  http://sourceware.org/ml/binutils/2013-06/msg00178.html

and later on they were change to -mabi=ilp32 and -mabi=lp64 for 
consistency with those in the aarch64 gcc:

  http://sourceware.org/ml/binutils/2013-07/msg00180.html

The following gcc patch made the driver use the explicit option to drive 
gas:

  http://gcc.gnu.org/ml/gcc-patches/2013-07/msg00083.html

It is a neglect of the backward compatibility with binutils 2.23.

> Attached patch add checking for -mabi=lp64 and prompts upgradation. Is
> this Ok?

I think instead of mandating the support for the -mabi option, the 
compiler shall be changed able to work with binutils 2.23.  The 2.23 
binutils have a good support for aarch64 and the main difference from 
2.24 is the ILP32 support.  I think it is necessary to maintain the 
backward compatibility, and it should be achieved by suppressing the 
compiler's support for ILP32 when the -mabi option is not found 
available in gas during the configuration time.

I had a quick look at areas need to be updated:

* multilib support

In gcc/config.gcc, the default and the only accepted value for 
--with-multilib-list and --with-abi shall be lp64 when -mabi is not 
available.

* -mabi option

I suggest we keep the -mabi option, but reject -mabi=ilp32 in 
gcc/config/aarch64/aarch64.c:aarch64_override_options ()

* driver spec

In gcc/config/aarch64/aarch64-elf.h, the DRIVER_SELF_SPECS and ASM_SPEC 
shall be updated to not pass/specify -mabi for gas.

* documentation

I think it needs to be mentioned in gcc/doc/install.texi the constraint 
of using pre-2.24 binutils with aarch64 gcc that is 4.9 or later.

It is a quick scouting, but hopefully it has provided provide some 
guidance.  If you need more help, just let me know.


Yufeng

P.s. some minor comments on the attached patch.

> diff --git a/gcc/configure b/gcc/configure
> index fdf0cd0..17b6e85 100755
> --- a/gcc/configure
> +++ b/gcc/configure

Diff result of auto-generation is usually excluded from a patch.

> diff --git a/gcc/configure.ac b/gcc/configure.ac
> index 91a22d5..730ada0 100644
> --- a/gcc/configure.ac
> +++ b/gcc/configure.ac
> @@ -3532,6 +3532,15 @@ case "$target" in
>                 [Define if your assembler supports the -no-mul-bug-abort 
> option.])])
>       ;;
>
> + aarch64-*-*)

aarch64*-*-*

> +    gcc_GAS_CHECK_FEATURE([-mabi option],
> +      gcc_cv_as_aarch64_mabi,,
> +      [-mabi=lp64], [.text],,,)
> +    if test x$gcc_cv_as_aarch64_mabi = xno; then
> +       AC_MSG_ERROR([Assembler support for -mabi=lp64 is required. Upgrade the 
> Assembler.])
> +    fi
> +    ;;
> +
>     sparc*-*-*)
>       gcc_GAS_CHECK_FEATURE([.register], gcc_cv_as_sparc_register_op,,,
>         [.register %g2, #scratch],,