Aw: Re: [MIPS r5900] libgcc floating point fixes

["JÃrgen Urban" <JuergenUrban at gmx dot de>]

Hello Richard,

> "JÃrgen Urban" <JuergenUrban@gmx.de> writes:
> > The problem happens with the r5900 hard float configurations, e.g.:
> > configure --target=mipsel-linux-gnu --with-float=hard --with-fpu=single
> > --with-arch=r5900
> > I created the attached patch which fixes this problem for r5900 and
> > another problem explained later.
> > The fixed code generates the following code which should be correct
> > (mipsr5900el-ps2-elf):
> >
> > 00105440 <__extendsfdf2>:
> >   105440:       27bdffc8        addiu   sp,sp,-56
> >   105444:       27a40028        addiu   a0,sp,40
> >   105448:       27a50018        addiu   a1,sp,24
> >   10544c:       afbf0034        sw      ra,52(sp)
> >   105450:       0c0417b5        jal     105ed4 <__unpack_f>
> >   105454:       e7ac0028        swc1    $f12,40(sp)
> >   105458:       8fa20024        lw      v0,36(sp)
> >   10545c:       8fa40018        lw      a0,24(sp)
> >   105460:       8fa5001c        lw      a1,28(sp)
> >   105464:       8fa60020        lw      a2,32(sp)
> >   105468:       00021882        srl     v1,v0,0x2
> >   10546c:       00021780        sll     v0,v0,0x1e
> >   105470:       afa20010        sw      v0,16(sp)
> >   105474:       0c041789        jal     105e24 <__make_dp>
> >   105478:       afa30014        sw      v1,20(sp)
> >   10547c:       8fbf0034        lw      ra,52(sp)
> >   105480:       03e00008        jr      ra
> >   105484:       27bd0038        addiu   sp,sp,56
> >
> > The default targets mipsr5900el and mips64r5900el are not affected by
> > the problem, because soft float is the default.
> >
> > It also seems that the same problem occurs with the following configuration:
> > configure --target=mipsel-linux-gnu --with-float=hard --with-fpu=single
> > I expected that this combination should work and a problem should
> > already be detected. Can somebody confirm that the problem also occurs
> > with default mipsel?
> 
> Although that configuration is in theory supported by GCC (said like that
> because I don't know the state of the glibc support for single-float),
> I don't think anyone uses it for any target other than r5900.  But you're
> right that this is a --with-fpu=single thing rather than an r5900 thing,
> so I don't think the patch is correct.  It should be keyed off whether
> the target is single-float or double-float, which you can test by
> checking whether the preprocessor macro __mips_single_float is defined.

Checking these macros seem to be too late, because it needs to be known at configure time and not at build time. The libgcc doesn't seem to be very flexible. It doesn't seem to provide different libraries for single and double float. The Linux kernel on the PS2 has support to switch between single and double float. For single float the hardware FPU is used and for double the FPU emulator gets activated. Currently it only checks whether the architecture is mips r5900 or not. For non r5900 the FPU emulator is activated. It seems that we also need to add something to the ELF file to specify the 32 bit or 64 bit for float. It would be good when it is not at a so complicated position as the soft vs hard float flag, because it needs to be read by the kernel when starting a ELF file. This way it would also be possible to emulate the r5900 FPU on a TX79 system. The TX79 is the same as r5900, but the FPU is 64 bit and compliant to IEEE 754.

> Note that this won't really be correct for r5900 anyway because of its
> non-IEEE FPU.  E.g. the soft-float routines will treat 0x7f800000 as
> infinity and 0x7fffffff as a NaN, whereas for r5900 they should be treated
> as normal numbers.

The code looked like it uses the configured floating point configuration for hard float, but you are right the conversion is not working in these cases.

I think there is no problem with the second part of the patch which disables t-mips16 for r5900. So could you commit the last part of the patch?

Best regards
JÃrgen