[LLVMdev] Simpler subreg ops in machine code IR

[Jakob Stoklund Olesen]

On Jun 15, 2010, at 11:57 PM, Evan Cheng wrote:

> 
> On Jun 15, 2010, at 2:48 PM, Jakob Stoklund Olesen wrote:
> 
>> I am considering adding a new target independent codegen-only COPY instruction to our MachineInstr representation. It would be used to replace INSERT_SUBREG, EXTRACT_SUBREG, and virtual register copies after instruction selection. Selection DAG still needs {INSERT,EXTRACT}_SUBREG, but they would not appear as MachineInstrs any longer.
>> 
>> The COPY instruction handles subreg operations with less redundancy:
>> 
>> 	%reg1045<def> = EXTRACT_SUBREG %reg1044<kill>, 4
>> 	%reg1045<def> = COPY %reg1044:sub_32bit<kill>
>> 
>> 	%reg1045<def> = INSERT_SUBREG %reg1045, %reg1044<kill>, 4
>> 	%reg1045:sub_32bit<def> = COPY %reg1044<kill>
>> 
>> 	%reg1050:ssub_0<def> = EXTRACT_SUBREG %reg1060:dsub_1<kill>, ssub_0
>> 	%reg1050:ssub_0<def> = COPY %reg1060:ssub_2<kill>
>> 
>> It will also replace the TargetInstrInfo::copyRegToReg hook when copying virtual registers:
>> 
>> 	%reg1050 = COPY %reg1044<kill>
>> 
>> It will be lowered with a TII.copyRegToReg() call in LowerSubregsInstructionPass (which may need renaming).
> 
> Ok. An immediate concern is this will make it harder to understand machine instructions. For example, it's hard for me to read
> %reg1045:sub_32bit<def> = COPY %reg1044<kill>
> as updating part of reg1045. It's solvable by pretty printing COPY instructions that include register class information.

It is safe to assume that a <def> operand with a subreg index represents a partial update to a subregister on any instruction. You can't have a noop subreg index, for instance TRI.getSubReg(%EAX, sub_32bit) returns 0. X86 has some weird idempotent subreg indices, TRI.getSubReg(%XMM0, sub_ss) returns %XMM0, but that is an exception.

So this is an insert_subreg:

	%reg1045:sub_32bit<def> = COPY %reg1044<kill>

and this is a normal copy:

	%reg1045<def> = COPY %reg1044<kill>

But decorating with register classes is a good idea. The question is how to do it without too much clutter. I thought about listing them after the operands:

	%reg1045:sub_32bit<def> = COPY %reg1044<kill>	; GR64 GR32_ABCD

Inline is tricky:

	(GR64)%reg1045:sub_32bit<def> = COPY (GR32_ABCD)%reg1044<kill>

[...]

>> 3. The register class arguments to TII.copyRegToReg() can be eliminated - it will only ever be called for physical registers. This
> 
> I am not sure I follow this. Why would the regclass arguments be eliminated?

If you look in LowerSubRegs.cpp, copying physical registers goes like this:

    const TargetRegisterClass *TRCS = TRI->getPhysicalRegisterRegClass(DstReg);
    const TargetRegisterClass *TRCD = TRI->getPhysicalRegisterRegClass(SrcReg);
    bool Emitted = TII->copyRegToReg(*MBB, MI, DstReg, SrcReg, TRCD, TRCS,
                                     MI->getDebugLoc());
    (void)Emitted;
    assert(Emitted && "Subreg and Dst must be of compatible register class");

The register class arguments provide no added value in this case. If copyRegToReg wants register classes, it can call getPhysicalRegisterRegClass() itself.

When copying virtual registers, the regclass arguments are essential, but copyRegToReg won't be used for copying virtual registers any longer.

>> 5. copyRegToReg() can insert multiple instructions, avoiding fun pseudo-instructions like VMOVQQ and VMOVQQQQ.
> 
> The reason for VMOVQQ etc. is actually to keep scavenger happy by pretending the whole QQ register is updated / referenced. With sub-register indices on the operands, it's possible for current copyRegToReg to expand to multiple instructions. But then the scavenger can't determine a register is partially defined.

Good point.

>> 1. copyRegToReg() won't be able to use register classes to pick a copy opcode. For instance, an XMM register will no longer be copied by MOVSS or MOVSD. Given just the physical register, MOVAPS will be used. Is that a problem?
> 
> I don't see this being a big problem. That said, I am not sure I totally follow this. Do you propose we eliminate register classes arguments even when virtual register copies are being created?

No, copying of virtual registers is done with the generic COPY instruction. It would be an error to pass a virtual register to copyRegToReg.

COPY is lowered to native instructions with copyRegToReg after register allocation where all registers are physical.

A transitional implementation would look like this:

  virtual void copyRegToReg(MachineBasicBlock &MBB,
                            MachineBasicBlock::iterator MI,
                            unsigned DstReg, unsigned SrcReg,
                            DebugLoc DL) const {
    assert(TargetRegisterInfo::isPhysicalRegister(DstReg));
    assert(TargetRegisterInfo::isPhysicalRegister(SrcReg));
    const TargetRegisterInfo *TRI = MBB.getParent()->getTarget().getRegisterInfo();
    const TargetRegisterClass *TRCS = TRI->getPhysicalRegisterRegClass(DstReg);
    const TargetRegisterClass *TRCD = TRI->getPhysicalRegisterRegClass(SrcReg);
    bool Emitted = copyRegToReg(*MBB, MI, DstReg, SrcReg, TRCD, TRCS, DL));
    (void)Emitted;
    assert(Emitted && "Nobody uses the copyRegToReg return value");
  }

>> 2. What else?
> 
> Every corner case that we can't anticipate right now. :-)

I suspect that our handling of partial redefines in not yet complete enough that we can do without the explicit use operand on INSERT_SUBREG. We'll see.