[LLVMdev] MachineOperand SubReg

[Jakob Stoklund Olesen]

On Apr 19, 2013, at 7:18 AM, <dag at cray.com> wrote:

> Jakob Stoklund Olesen <stoklund at 2pi.dk> writes:
> 
>>> A MachineOperand has both a getReg() and a getSubReg() interface.
>>> For a physical register operand, is getReg() guaranteed to be the
>>> "most super" register with getSubReg() providing the specific
>>> subregister information for the operand?  If so then for my current
>>> purposes it seems I don't need to worry about subregisters at all.
>> 
>> For physregs, getSubReg() is guaranteed to be 0. It is only used for
>> virtual registers.
> 
> Aha!  So for physregs, getSubReg() will simply be the exact physical
> register defined.  Cool, that'll work.

Yes, s/SubReg/Reg/

>> Yep, there is no such thing as a 'most super' register. The register
>> aliasing graph is not a tree. The graph of sub/super-register
>> relationships is also not a tree, it is a DAG.
> 
> Hmm.  I don't doubt it but can you give me an example of a case where
> there is no "most super" register?  I'm having a hard time thinking up
> how one would design such an ISA.

The ARM NEON D-registers are 64 bits each. NEON has vld1 instructions that can load 2, 3, or 4 consecutive D-registers.

Two consecutive D-registers is represented by the D0_D1, D1_D2, D2_D3, ... super-registers. As you can see, D1 has two super-registers, neither is more super than the other.

We similarly define triples and quads of consecutive D-registers.

NEON also has 128-bit vector instructions operating on even-odd pairs of D-registers, so the actual register names we use are: Q0, D1_D2, Q1, D3_D4, Q2, D5_D6, ... Here, Q0 is the name we use for D0_D1 (which doesn't exist).

This register structure also means that the complete set of aliasing registers can get quite large. Some NEON registers have more than 40 aliases. The register units help control that complexity. Each physreg has an associated set of regunits, and two physregs alias if and only if their regunit sets overlap. See TRI::regsOverlap().

The regunits usually correspond to the leaf sub-registers - the registers that have no sub-registers themselves. However, the numbering is different, and if you use 'Aliases = ...' in your register file, they no longer correspond exactly to the leaf registers.

/jakob