Return value on MIPS N64 ABI

[Heiher <admin at heiher dot info>]

Hi,

Looks the return value of TestNewA is passed on $f0/$f2 from
disassembly code.  I don't known why the return value of TestNewB is
passed on $v0/$v1? a bug?

229 0000000120000c40 <_Z8TestNewAv>:
230    120000c40:   3c030002        lui     v1,0x2
231    120000c44:   0079182d        daddu   v1,v1,t9
232    120000c48:   64638400        daddiu  v1,v1,-31744
233    120000c4c:   dc628050        ld      v0,-32688(v1)
234    120000c50:   67bdffe0        daddiu  sp,sp,-32
235    120000c54:   d4400e68        ldc1    $f0,3688(v0)
236    120000c58:   dc628050        ld      v0,-32688(v1)
237    120000c5c:   67bd0020        daddiu  sp,sp,32
238    120000c60:   03e00008        jr      ra
239    120000c64:   d4420e70        ldc1    $f2,3696(v0)
240
241 0000000120000c68 <_Z8TestNewBv>:
242    120000c68:   3c0307f9        lui     v1,0x7f9
243    120000c6c:   3c0207f7        lui     v0,0x7f7
244    120000c70:   34633333        ori     v1,v1,0x3333
245    120000c74:   34423333        ori     v0,v0,0x3333
246    120000c78:   00031cb8        dsll    v1,v1,0x12
247    120000c7c:   000214b8        dsll    v0,v0,0x12
248    120000c80:   3463cccd        ori     v1,v1,0xcccd
249    120000c84:   3442cccd        ori     v0,v0,0xcccd
250    120000c88:   67bdfff0        daddiu  sp,sp,-16
251    120000c8c:   00031c78        dsll    v1,v1,0x11
252    120000c90:   00021478        dsll    v0,v0,0x11
253    120000c94:   6463999a        daddiu  v1,v1,-26214
254    120000c98:   6442999a        daddiu  v0,v0,-26214
255    120000c9c:   03e00008        jr      ra
256    120000ca0:   67bd0010        daddiu  sp,sp,16

// test.cpp
// gcc -march=mips64r2 -mabi=64 -O3 -o test test.cpp
#include <stdio.h>

class TestA
{
public:
        double l;
        double h;

        TestA(double l, double h) : l(l), h(h) {}
};

class TestB : public TestA
{
public:
        TestB(const TestA& a) : TestA(a) {}
};

TestA
TestNewA(void)
{
        return TestA(0.1, 0.2);
}

TestB
TestNewB(void)
{
        return TestA(0.1, 0.2);
}

int
main(int argch, char *argv[])
{
        TestA a = TestNewA();
        printf("%lf, %lf\n", a.l, a.h);

        TestB b = TestNewB();
        printf("%lf, %lf\n", b.l, b.h);

        return 0;
}