-Wmaybe-uninitialized leads to build failures

There a multiple functions in Papi which when compiled with -O3 -Wall -Werror fail to build. This currently stops upgrading the papi package for Ubuntu.

The full build log is here: https://launchpadlibrarian.net/648311749/buildlog_ubuntu-lunar-ppc64el.papi_7.0.0-2_BUILDING.txt.gz

Let’s use src/components/sde/tests/Simple2/Simple2_Lib.c as an example. Here we find:

// This function allows the library to perform operations in order to compute the value of an SDE at run-time
long long counter_accessor_function( void *param ){
    long long *ll_ptr;
    double *dbl_ptr = (double *)param;

    // Scale the variable by a factor of two. Real libraries will do meaningful work here.
    double value = *dbl_ptr * 2.0;

    // Pack the bits of the result in a long long int.
    ll_ptr = (long long *)&value;

    return *ll_ptr;
}

As ll_ptr and value are of different type and none of both is a charlike array the sequence of the assignments to ll_ptr and to value is undefined behavior. This can be avoided by either adding a barrier or by using a union:

// This function allows the library to perform operations in order to compute the value of an SDE at run-time
long long counter_accessor_function( double *param ){
    double *dbl_ptr = param;
    union {
        double d;
        long long l;
    } value;

    // Scale the variable by a factor of two. Real libraries will do meaningful work here.
    value.d = *dbl_ptr * 2.0;

    return value.l;
}

Best regards

Heinrich

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