[MIPS] Avoiding FP operations/register usage

[Matthew Fortune]

> Matthew Fortune <Matthew.Fortune@imgtec.com> writes:
> > I'm still interested in how successfully the MIPS backend is managing
> > to avoid floating point but I am also convinced there are bugs in
> > ld.so entry points for MIPS.
> 
> It uses the standard mechanism to avoid it, which is marking uses of FP
> registers for integer moves, loads and stores with "*".  This tells the register
> allocator to ignore those alternatives.  AFAIK it is effective and I think any
> cases where it doesn't work would be fair bug reports.

I understand that '*' has no effect on whether reload/LRA will use the alternative though so I take that to mean they could still allocate FP regs as part of an integer move?

> It becomes a lot more difficult to define with things like the Loongson
> extensions though, since some of those are also useful as scalar integer
> operations.  And of course the same goes for MSA.

Indeed.

Avoiding FP registers 99.9% of time is fine for performance, it's the potential 0.1% I'm concerned about for correctness. I'm tending towards accounting for potential FPU usage even from integer only source just to be safe. I don't want to ever be the one debugging something like ld.so in the face of this kind of bug.

I'll move the discussion to glibc regarding ld.so.

Regards,
Matthew