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

[Eric Blake <eblake@redhat.com>]

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On 07/01/2014 02:26 AM, Peter Maydell wrote:
> On 1 July 2014 09:22, Paolo Bonzini <pbonzini@redhat.com> wrote:
>> From: Alexey Kardashevskiy <aik@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.

If that's your worry, what about:

int main(int argc, char **argv) { return isnan(sin(argc > 1)); }

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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