Peter Maydell <peter.mayd...@linaro.org> writes:
> On 1 July 2014 09:22, Paolo Bonzini <pbonz...@redhat.com> wrote:
>> From: Alexey Kardashevskiy <a...@ozlabs.ru>
>>
>> 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.
x *is* initialized, to zero. A sufficiently smart compiler(TM) could
figure out that x is still zero in main(), and constant fold the test
away.
Suggest to use argc.
