powerpc: avoiding lfd/stfd for 64 bit integer load/stores

[Segher Boessenkool]

Hi!

On Thu, Nov 22, 2018 at 10:38:41AM +0000, Rasmus Villemoes wrote:
> On 21/11/2018 17.52, Peter Bergner wrote:
> > On 11/21/18 7:13 AM, Segher Boessenkool wrote:
> >> There is no such switch.  Implementing such a switch would be hard; for
> >> example, the convert-float-to-integer instructions need to store an integer
> >> in a floating point register.
> 
> Thanks for the answer. I accept that it would be hard to do such a
> switch, though I don't think I understand that example. Why would gcc
> ever create an int-to-float or float-to-int unless the source code
> actually used float or double?

One of the nastier problems is we need to allow some integer modes in the
floating point registers, for this and similar; so there is no way you can
say "do not put integers in FP regs" and still have a working compiler.

We also need to allow floating point modes in other than FPRs, because
-msoft-float works that way, if nothing else.

> >> Compiling translation units with different needs, with different flags, is
> >> the way to go.  -msoft-float means "do not use the FPRs".
> > 
> > I agree with Segher here.  I'll note though that -msoft-float and -mhard-float
> > are different ABIs.  However, if you don't call a function across the soft-float
> > to hard-float barrier that has floating point arguments or floating point
> > return value, you should be fine.
> 
> We may have to do it that way, then. But I would hope to have some help
> from the compiler to detect ABI violations, or generation of soft-float
> code. We could build everything with -msoft-float, and all such code
> should be forbidden from using float/double/long double in any way -
> calling functions taking those, defining variables, casting to/from, ...
> either via a new warning, or a hypothetical '-mno-float' [which I see
> actually exists for MIPS]. Then we could selectively turn on
> -mhard-float for the few TUs that actually need that.

For us, hard float is the normal case: PowerPC requires it (and always
has, but some embedded things do not have it, sigh).

Detecting problems should be easy...  Whenever your code tries to access
the floating point registers while you don't have access to them (which
is a flag per context), you get an exception (an interrupt).  So as long
as you keep MSR[FP] turned off for the tasks that should not touch FP
registers, you should be fine.


Segher