Re: gcc_GAS_FLAGS: Add more gcc_cv_as_flags overrides

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Wed, Sep 11, 2013 at 2:23 PM, Thomas Schwinge
<thomas@codesourcery.com> wrote:
> Hi!
>
> In a toolchain where GCC has been configured for
> --target=i686-pc-linux-gnu, and where an x86_64 multilib has also been
> configured, by default GCC will generate code for 32-bit mode, and for
> 64-bit mode only when -m64 is passed.  Per ASM_SPEC definition, the -m64
> option is translated to --64 when invoking the assembler.  This is
> working fine.
>
> What however is not working fine is GCC's very own configure testing.
> When using said toolchain for building on/for x86_64-linux-gnu by
> configuring a native GCC build with CC='gcc -m64' CXX='g++ -m64', I'm
> seeing the following happen in [GCC]/gcc/configure:
>
>     configure:23559: checking assembler for thread-local storage support
>     configure:23572: [...]/bin/as   --fatal-warnings -o conftest.o conftest.s >&5
>     conftest.s: Assembler messages:
>     conftest.s:5: Error: bad register name `%rax'
>     [...]
>     configure: failed program was
>
>             .section ".tdata","awT",@progbits
>     foo:    .long   25
>             .text
>             movq    %fs:0, %rax
>     [...]
>     configure:23586: result: no
>
> In this toolchain, an unadorned invocation of as will default to 32-bit
> mode, and enter 64-bit mode only when an explicit --64 is passed -- which
> is not done here.
>
> This is, from a quick glance, the "inverse" thing as has once been
> discussed in <http://gcc.gnu.org/PR42345>,
> <http://news.gmane.org/find-root.php?message_id=%3C20091210175824.GA32242%40lucon.org%3E>.
>
> Wanting to use this toolchain, do I have to configure GCC pointing to an
> assembler that defaults to 64-bit mode, or use something like
> --with-as='as --64' (not checked whether that works), or should the GCC
> configure system deduce from $CXX' -m64 that it needs to pass --64 to the
> assembler (à la ASM_SPEC as indicated above), or is the following patch
> the way to go, or something else?  H.J., the gnux32 change is just a
> guess; please comment if that's not right.  (And, of course, this is not
> a problem specific to *-*-linux*, but changing that is for another day.)
>
> gcc/
>         * acinclude.m4 (gcc_GAS_FLAGS): Add more gcc_cv_as_flags
>         overrides.
>         * configure: Regenerate.
>
> --- gcc/acinclude.m4
> +++ gcc/acinclude.m4
> @@ -444,8 +444,16 @@ AC_DEFUN([gcc_GAS_FLAGS],
>  [AC_CACHE_CHECK([assembler flags], gcc_cv_as_flags,
>  [ case "$target" in
>    i[[34567]]86-*-linux*)
> -    dnl Always pass --32 to ia32 Linux assembler.
> -    gcc_cv_as_flags="--32"
> +    dnl Override the default, which may be incompatible.
> +    gcc_cv_as_flags=--32
> +    ;;
> +  x86_64-*-linux-gnux32)
> +    dnl Override the default, which may be incompatible.
> +    gcc_cv_as_flags=--x32

I don't think it is necessary.  I configure x32 gcc with

CC="gcc -mx32" CXX="g++ -mx32" RUNTESTFLAGS=" .../configure
--with-multilib-list=m32,m64,mx32 --with-abi=mx32

There is no difference in as/ld features between x32 and x86-64.

> +    ;;
> +  x86_64-*-linux*)
> +    dnl Override the default, which may be incompatible.
> +    gcc_cv_as_flags=--64
>      ;;
>    powerpc*-*-darwin*)
>      dnl Always pass -arch ppc to assembler.
>
>


-- 
H.J.