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

[Markus Armbruster]

Peter Maydell <address@hidden> writes:

> 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.

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.