[LLVMdev] 64 bit special purpose registers

Villmow, Micah Micah.Villmow at amd.com
Wed Sep 5 12:50:51 PDT 2012

From: Akira Hatanaka [mailto:ahatanak at gmail.com]
Sent: Wednesday, September 05, 2012 12:44 PM
To: Villmow, Micah
Cc: reed kotler; llvmdev at cs.uiuc.edu
Subject: Re: [LLVMdev] 64 bit special purpose registers


Do you mean we should make GPR64 available to register allocator by calling addRegisterClass?

addRegisterClass(MVT::i64, &GPR64RegClass)

If we add register class GPR64, type legalization will stop expanding i64 operations because i64 is now a legal type.
[Villmow, Micah] You'll have to set everything that you don't support to 'Expand' and everything you do support into 'Legal'.

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,

> -----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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