Re: Determining live registers on Cmm procedure entry?

David,

this reminds me, I have added a feature bugzilla to LLVM:

http://llvm.org/bugs/show_bug.cgi?id=14080

May be interesting to discuss it here first.

Cheers,

    Gabor

On 10/16/12, David Terei <[email protected]> wrote:
> What is the problem you face? That you want to allow passing a dynamic
> number of float or double arguments? I assume you are keeping it fixed
> at 6 still but want to allow those 6 to be either 6 floats, 6 doubles
> or any other combination. (Long term is anything that can be put in a
> xmm* register...)
>
> Why do you want Cmm liveness on entry? Is it simply to solve the above
> so you can generate specialized function types instead of the
> universal one the LLVM backend uses?
>
> If so, have you tried just using the most general universal type?
> e.g., how badly is the generated code affected if you have all
> arguments be Doubles and then do type casts on jumps and function
> entries?
>
> This is the original commit I wrote for tracking STG reg liveness:
> 4384e146640230399b38cd62e8e5df391f72c3a7
> This is the one Simon Marlow wrote to do the same but in the new code
> gen: c6a61235aa0baf6f9e8a41c5a771ccc7e32c23a5
>
> I'd suggest you look at these commits and they should show you where
> to get the information from. (I can't remember anymore sorry).
>
> Cheers,
> David
>
> On 16 October 2012 03:45, Geoffrey Mainland <[email protected]> wrote:
>> Hi all,
>>
>> I'm working on changing GHC's calling convention to support overlapping
>> register classes. On x86-64 this will mean that a function can receive up
>> to
>> six Float/Double arguments (any mixture) in registers. Right now only two
>> Double arguments can be passed in registers on x86-64, even if a function
>> takes zero Float arguments---the register classes for Floats and Doubles
>> cannot overlap even though both are passed in xmm* registers.
>>
>> This is working fine with the native back-end, but I'm not sure how to
>> get
>> it working with the LLVM back-end. Right now LLVM translates a Cmm
>> procedure
>> to an LLVM function of a single universal type( that is, all LLVM
>> functions
>> that the LLVM back-end produces for Cmm procedures have the same type)
>> that
>> takes all STG registers as arguments. A Cmm procedure call passes all the
>> STG registers, although the back-end takes advantage of the liveness
>> information attached to Cmm jumps to pass undefined values for non-live
>> registers.
>>
>> Now that register classes can overlap, I need to change this. Since jumps
>> have liveness information attached, I can simply pass the live registers.
>> But when translating Cmm procedures, I need to know which registers are
>> live
>> on entry to the procedure.
>>
>> The slightly longer-term goal is to add support for passing SSE vectors
>> in
>> registers. What I'd like to have ideally is the set of live registers
>> *and
>> their Cmm types* at each procedure entry and jump. I envision having a
>> single STG register class, say X16, for all 128-bit-wide SSE registers,
>> but
>> I'd like the Cmm type information when generating LLVM type signatures so
>> I
>> can generate the appropriate LLVM types---a 128-bit-wide vector of 32-bit
>> integers does not have the same type as a 128-bit-wide vector of double
>> precision floats.
>>
>> I might be able to do without the Cmm type information by inserting
>> appropriate casts in the generated LLVM code, but I really need the set
>> of
>> live registers on entry to a Cmm procedure. I don't see how to get this
>> unfortunately. Any hints?
>>
>> Thanks,
>> Geoff
>>
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