Three different definitions of a function, no linker error; how can that be?

Question

I have something which looks very impossible to me. I have three files which each seem to contain the exact same function declaration, albeit with a different definition.

inst_dp_vec2.cc:

void loadSOAFVec(InstVector &ivector,
                 const FVec &ret,
                 const Address *a,
                 int soanum,
                 int soalen,
                 string mask) {
    if (soalen == 2) {
        ivector.push_back(new LoadFVec(ret, a, string("")));
    } else {
        printf("SOALEN = %d not supported at %s:%d\n", soalen, __FILE__,
               __LINE__);
        exit(1);
    }
}

inst_dp_vec4.cc

void loadSOAFVec(InstVector &ivector,
                 const FVec &ret,
                 const Address *a,
                 int soanum,
                 int soalen,
                 string mask) {
    if (soalen == 4) {
        ivector.push_back(new LoadFVec(ret, a, string("")));
    } else if (soalen == 2) {
        ivector.push_back(new LoadHalfFVec(ret, a, soanum));
    } else {
        UNSUPPORTED_SOALEN(soalen);
    }
}

inst_dp_vec8.cc

void loadSOAFVec(InstVector &ivector,
                 const FVec &ret,
                 const Address *a,
                 int soanum,
                 int soalen,
                 string mask) {
    int mskbits = (((1 << soalen) - 1) << (soanum * soalen));
    stringstream mk;
    mk << "0x" << hex << mskbits;
    string localmask = mk.str();
    ivector.push_back(new LoadUnpackFVec(ret, a, localmask));
}

The linker command as executed from the Makefile (generated by GNU Autotools) seems to include all three compiled files:

g++ -O3 -g -DNO_HW_MASKING -DUSE_LDUNPK -DUSE_PKST -DUSE_PACKED_GAUGES
-DUSE_PACKED_CLOVER -DNO_GPREF_L1 -DNO_GPREF_L2 -DENABLE_STREAMING_STORES
-DSERIAL_SPIN -DSOALEN=8 -DVECLEN=4 -DPRECISION=2 codegen.o data_types.o
dslash.o dslash_common.o inst_dp_vec8.o inst_sp_vec16.o inst_dp_vec4.o
inst_sp_vec8.o inst_sp_vec4.o inst_dp_vec2.o inst_scalar.o -o codegen

From my knowledge of the one definition rule, this should give a linker error. Even more peculiar is that the version from inst_dp_vec8.o is not the one used, but the one from inst_dp_vec4.cc, although it appears first in the linker command line. I changed the code such that UNSUPPORTED_SOALEN throws an exception and with GDB I found that soalen = 8 is active. From what I know about the software, soalen = 8 only works with veclen ≥ 8 such that only the inst_dp_vec8.cc could contain the right definition.

My question: How can this possible link to a program that can be executed and only fails because of the explicit exception raised with UNSUPPORTED_SOALEN?

Answer 1

Because no diagnostic is required when the multiple definitions occur in multiple translation units.

From [basic.def.odr]:

No translation unit shall contain more than one definition of any variable, function, class type, enumeration type, or template.

and later

Every program shall contain exactly one definition of every non-inline function or variable that is odr-used in that program outside of a discarded statement (6.4.1); no diagnostic required.

So you get an error if they are multiple definitions within on source module, but don't have to get them if they're in different ones.