[LLVMdev] mips16 hard float puzzle

[Eli Friedman]

On Fri, Jan 4, 2013 at 6:28 PM, reed kotler <rkotler at mips.com> wrote:
> On 01/04/2013 06:08 PM, Eli Friedman wrote:
>>
>> 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
>
> So just read the DAG in the beginning of MIPS-specific call lowering before
> it gets modified?

Take a look at CCValAssign etc. in MipsTargetLowering::LowerCall.
(The DAG node for the callee doesn't have the necessary information in
the general case, even though it might appear to work in simple
cases.)

-Eli