[LLVMdev] 64 bit special purpose registers

[Villmow, Micah]

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]
> On Behalf Of reed kotler
> Sent: Monday, August 06, 2012 4:52 PM
> To: 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.
> 
> 
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev