Re: [PATCH v2] configure: Avoid using strings binary

[Paolo Bonzini <pbonzini@redhat.com>]

On 10/13/22 15:43, Michal Privoznik wrote:
> When determining the endiandness of the target architecture we're
> building for a small program is compiled, which in an obfuscated
> way declares two strings. Then, we look which string is in
> correct order (using strings binary) and deduct the endiandness.
> But using the strings binary is problematic, because it's part of
> toolchain (strings is just a symlink to
> x86_64-pc-linux-gnu-strings or llvm-strings). And when
> (cross-)compiling, it requires users to set the symlink to the
> correct toolchain.
> 
> Fortunately, we have a better alternative anyways. We can mimic
> what compiler.h is already doing: comparing __BYTE_ORDER__
> against values for little/big endiandness.
> 
> Bug: https://bugs.gentoo.org/876933
> Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
> ---
> 
> v2 of:
> 
> https://lists.gnu.org/archive/html/qemu-devel/2022-10/msg02054.html
> 
> diff to v1:
> - Fixed reversed logic
> - Ditched custom compiler macros in favor of __BYTE_ORDER__
> 
>   configure | 33 +++++++++++++++++----------------
>   1 file changed, 17 insertions(+), 16 deletions(-)
> 
> diff --git a/configure b/configure
> index 45ee6f4eb3..2ac26c6978 100755
> --- a/configure
> +++ b/configure
> @@ -1423,30 +1423,31 @@ if test "$tcg" = "enabled"; then
>       git_submodules="$git_submodules tests/fp/berkeley-softfloat-3"
>   fi
>   
> -# ---
> +##########################################
>   # big/little endian test
>   cat > $TMPC << EOF
> -#include <stdio.h>
> -short big_endian[] = { 0x4269, 0x4765, 0x4e64, 0x4961, 0x4e00, 0, };
> -short little_endian[] = { 0x694c, 0x7454, 0x654c, 0x6e45, 0x6944, 0x6e41, 0, };
> -int main(int argc, char *argv[])
> -{
> -    return printf("%s %s\n", (char *)big_endian, (char *)little_endian);
> -}
> +#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__

#if defined __BYTE_ORDER__ && ...

> +# error LITTLE
> +#endif
> +int main(void) { return 0; }
>   EOF
>   
>   if compile_prog ; then

And then you need to check if it *fails* (i.e. "if ! compile_prog ; then 
bigendian=no", and likewise below for the big-endian test).  This way if 
the macro is not defined both tests will compile successfully and you'll 
print the error here:

> +  if compile_prog ; then
> +    bigendian="no"
> +  else
>       echo big/little test failed
>       exit 1
> +  fi
>   fi

You can test it by misspelling __BYTE_ORDER__ intentionally.

Thanks,

Paolo