[LLVMdev] 64 bit special purpose registers

[Reed Kotler]

Here is the problem explained more.

Normally there is a 64 bit register that is the result of certain 
multiply and divide instructions.
It's really 2 32 bit registers.

This is like  HI[0]/Lo[0]

In fact there are four such pairs, only the 0th pair available to basic 
multiply and divide.

But DSP instructions have access to 4 , Hi[i],Lo[i], i=0..3

We want the register allocator to allocate them for us but also we need 
to have them paired,
i.e. Hi[1],Lo[1]

So in principle if you have a 64 bit register you can have two 32 bit 
registers inside.

If you tell the register allocator that you have 64 bit registers, then 
it wants to assume that 64 bit
is a legal operand type and then llvm assumes that you have native 
instructions for all the 64 bit
types, and we don't have that in mips32, for example. So you would have 
to lower them all yourself.


On 09/06/2012 05:06 AM, Ivan Llopard wrote:
> Hi Akira, Micah,
>
> On 05/09/2012 21:44, Akira Hatanaka wrote:
>> Micah,
>>
>> Do you mean we should make GPR64 available to register allocator by 
>> calling addRegisterClass?
>>
>> addRegisterClass(MVT::i64, &GPR64RegClass)
>
> I have a related question to this thread. Does the RA use target 
> lowering information?
> Because if it doesn't, you don't need to register your i64 reg class.
>
> Ivan
>
>>
>> If we add register class GPR64, type legalization will stop expanding 
>> i64 operations because i64 is now a legal type.
>> Then we will probably have to write lots of code to custom-lower 
>> unsupported 64-bit operations during legalization. Note that 
>> mips32/16 lacks support for most of the basic 64-bit instructions 
>> (add, sub, etc.).
>>
>> I don't think setting operation action by calling 
>> setOperationAction(... ,MVT::i64, Expand) would work either. Judging 
>> from the code I see in Legalize.cpp, operation legalization doesn't 
>> seem to do much to expand unsupported i64 operations.
>>
>> On Tue, Aug 7, 2012 at 9:24 AM, Villmow, Micah <Micah.Villmow at amd.com 
>> <mailto:Micah.Villmow at amd.com>> wrote:
>>
>>     This can be done by declaring a register class with these
>>     registers and only using that register class as an operand in the
>>     instructions where it is legal.
>>     You then set as sub registers what you want to represent as the
>>     hi and lo registers for those 64bit registers.
>>
>>     So something like this:
>>     def lo_comp : SubRegIndex;
>>     def hi_comp : SubRegIndex;
>>     def R1 : Register<1>;
>>     def R2 : Register<2>;
>>     def R3 : Register<1>;
>>     def R4 : Register<2>;
>>     def D1 : RegisterWithSubRegs<1, [R1, R2], [lo_comp, hi_comp]>;
>>
>>     This says that D1 is a register with two components, lo and hi.
>>     When you allocate D1, you also use R1/R2.
>>     def GPR32 : RegisterClass<..., [i32], [32], (add (sequence "R%u",
>>     1, 4))> ...
>>     def GPR64 : RegisterClass<..., [i64], [64], (add D1)> ...;
>>
>>     So in your instruction it would be something like:
>>     def mul : Inst<(dst GPR64:$dst), (src GPR32:$src0, GPR32:$src1),
>>     ...>;
>>
>>     This would mean you take in two inputs and you have 64bit output.
>>     When D1 is not being used, R1/R2 will get allocated to
>>     instructions that use GPR32 register class, otherwise they will
>>     be seen as used and not get allocated.
>>
>>     Hope this helps,
>>     Micah
>>
>>     > -----Original Message-----
>>     > From: llvmdev-bounces at cs.uiuc.edu
>>     <mailto:llvmdev-bounces at cs.uiuc.edu>
>>     [mailto:llvmdev-bounces at cs.uiuc.edu
>>     <mailto:llvmdev-bounces at cs.uiuc.edu>]
>>     > On Behalf Of reed kotler
>>     > Sent: Monday, August 06, 2012 4:52 PM
>>     > To: llvmdev at cs.uiuc.edu <mailto:llvmdev at cs.uiuc.edu>
>>     > Subject: [LLVMdev] 64 bit special purpose registers
>>     >
>>     > On Mips 32 there is traditionally a 64 bit HI/LO register for
>>     the result
>>     > of multiplying two 64 bit numbers.
>>     >
>>     > There are corresponding instructions to load the LO and HI
>>     parts into
>>     > individual 32 registers.
>>     >
>>     > On Mips with the DSP ASE (an application specific extension),
>>     there are
>>     > actual 4 such pairs of registers.
>>     >
>>     > Is there a way to have special purpose 64 bit registers without
>>     actually
>>     > having to tell LLVM that you have a 64 bit processor?
>>     >
>>     > But it's still possible to use the individual parts of the 64
>>     register
>>     > as temporaries.
>>     >
>>     > The only true 64 bit operation is multiplying two 32 bit numbers.
>>     >
>>     >
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