Re: [PATCH] configure: Improve alias attribute check

[Thomas Huth <thuth@redhat.com>]

On 20/03/2021 05.27, Gavin Shan wrote:
> It's still possible that the wrong value is returned from the alias
> of variable even if the program can be compiled without issue. This
> improves the check by executing the binary to check the result.
> 
> If alias attribute can't be working properly, the @target_page in
> exec-vary.c will always return zeroes when we have the following gcc
> version.
> 
>    # gcc --version
>    gcc (GCC) 11.0.0 20210210 (Red Hat 11.0.0-0)
> 
> This abstracts the code from exec-vary.c and use it as indicator to
> enable gcc alias attribute or not.
> 
> Signed-off-by: Gavin Shan <gshan@redhat.com>
> ---
>   configure | 34 ++++++++++++++++++++++++++++++----
>   1 file changed, 30 insertions(+), 4 deletions(-)
> 
> diff --git a/configure b/configure
> index f7d022a5db..8321f380d5 100755
> --- a/configure
> +++ b/configure
> @@ -75,6 +75,7 @@ fi
>   
>   TMPB="qemu-conf"
>   TMPC="${TMPDIR1}/${TMPB}.c"
> +TMPC_B="${TMPDIR1}/${TMPB}_b.c"
>   TMPO="${TMPDIR1}/${TMPB}.o"
>   TMPCXX="${TMPDIR1}/${TMPB}.cxx"
>   TMPE="${TMPDIR1}/${TMPB}.exe"
> @@ -4878,13 +4879,38 @@ fi
>   
>   attralias=no
>   cat > $TMPC << EOF
> -int x = 1;
> +static int x;
>   extern const int y __attribute__((alias("x")));
> -int main(void) { return 0; }
> +extern int read_y(void);
> +void write_x(int val);
> +
> +void write_x(int val)
> +{
> +    x = val;
> +}
> +
> +int main(void)
> +{
> +    return read_y();
> +}
>   EOF
> -if compile_prog "" "" ; then
> -    attralias=yes
> +cat > $TMPC_B << EOF
> +extern const int y;
> +extern void write_x(int val);
> +int read_y(void);
> +
> +int read_y(void)
> +{
> +     write_x(1);
> +     return y;
> +}
> +EOF
> +
> +TMPC+=" ${TMPC_B}"
> +if compile_prog "" "" && ! $TMPE; then

What about cross-compiling? Running an executable won't work if QEMU gets 
cross-compiled...

  Thomas