[LLVMdev] mips16 hard float puzzle

[Eli Friedman]

On Fri, Jan 4, 2013 at 4:08 PM, reed kotler <rkotler at mips.com> wrote:
> I'm working on mips16 hard float which at a first approximation is just soft
> float but calls different library routines. Those different library routines
> are just an implementation (in mips32 mode) of soft float using mips32
> hardware instructions. This part is already done. (mips16 mode has no
> floating point instructions).
>
> The next level of this that I am working on now is the ability to call
> mips32 mode functions from mips16. This is needed for example to call the
> math library (sin, cos), because those library
> routines are mips32 functions.
>
> This has an interesting problem.
>
> The mips16 functions do not know what they are calling when it's an external
> function; i.e. it could be a mips32 or mips16.
>
> On mips32, floating point arguments, if they are argument 1 and argument 2,
> are passed in floating point registers. If the return type is floating
> point, then it is returned in a floating point register.
>
> On mips 16, it is doing soft float essentially so the arguments are passed
> in integer registers.
>
> The way gcc mips16 does this is to call helper functions which do the actual
> call to the external function. The helper functions copy arg 1 and 2 into fp
> arg 1 and fp arg 2 and then do the actual call. So whether you are calling
> mips16 or mips32 from that point on does not matter.
>
> I'm simplifiying this a bit and you have to think about the mips abi to
> understand how this all works but it does.
>
> The question I have is how to find the original prototype.
>
> In order to know if I need to call a helper function, and which helper
> function to call, I need to know the return type and type of arguments 1 and
> 2. I need to know this pre soft float lowering.
>
> From what I can see, in the beginning of ISelLowering code, the DAG will
> still reflect the original prototype. If I do a DAG.viewGraph() there in the
> debugger, it knows about the proper prototype (pre soft float lowering).
>
> It would have been cool to get the function class that corresponds to this
> call and get the arguments that way but I don't see how I can get it.
>
> Is looking at the DAG the only (or best) way to do find the function
> prototype?
>
> I would be happy to add something to lowering calls methods but it was not
> clear how to do this too.

You have to do this as part of call lowering: once the call is
lowered, the type information is lost permanently.  It's not
conceptually difficult to do as part of call lowering, though: in the
MIPS-specific call lowering code, if you detect that a call requires
an argument in an FP register, you just need to generate a call to the
relevant helper rather than the function itself.

-Eli