Re: [Qemu-devel] [PULL 3/3] configure: Fix -lm test, so that tools can be compiled on hosts that require -lm

[Peter Maydell]

On 1 July 2014 09:22, Paolo Bonzini <address@hidden> wrote:
> From: Alexey Kardashevskiy <address@hidden>
>
> The existing test whether "-lm" needs to be included or not is
> insufficient as it reports false negative on Fedora20/ppc64.
> This happens because sin(0.0) is a constant value which compiler
> can safely throw away and therefore there is no need to add "-lm".
> As the result, qemu-nbd/qemu-io/qemu-img tools cannot compile.
>
> This adds a global variable and uses it in the test to prevent
> from optimization.

> --- a/configure
> +++ b/configure
> @@ -3453,7 +3453,7 @@ fi
>  # Do we need libm
>  cat > $TMPC << EOF
>  #include <math.h>
> -int main(void) { return isnan(sin(0.0)); }
> +double x; int main(void) {return isnan(sin(x));}
>  EOF
>  if compile_prog "" "" ; then
>    :

This looks to me like we're leaving ourselves open for
a smarter compiler with linktime optimisation to complain
that x is used uninitialized.

thanks
-- PMM