[LLVMdev] VLIW Ports
Carlos Sánchez de La Lama
carlos.delalama at urjc.es
Tue Oct 25 02:29:38 PDT 2011
> >> I think any implementation that makes a "bundle" a different entity from MachineInstr is going to be difficult to use. All of the current backend passes will have to taught to know about bundles.
> > The approach in the patch I sent (and I believe Timo's code works similar, according to his explanations) is precisely to make "bundles" no different from MachineInstructions. They are MIs (a class derived from it), so all other passes work transparently with them. For example, in my code register allocator does not know it is allocating regs for a bundle, it sees it just as a MI using a lot of registers. Of course, normal (scalar) passes can not "inspect" inside bundles, and wont be able for example to put spilling code into bundles or anything like that.
> > But the good point is that bundles (which are MIs) and regular MIs can coexist inside a MachineBasicBlock, and bundles can easily be "broken back" to regular MIs when needed for some pass.
> Ok, so in your proposal a bundle is just a special MachineInstr? That sounds good. How are the MachineInstr's embedded inside a bundle? How are the cumulative operands, implicit register defs and uses represented?
Yes, it is a special MachineInstruction (but again, not an
MachineInstruction object directly, I have this MachineInstructionBundle
class which is derived from MachineInstruction, but as a derived class
all existing passes can handle it directly).
The MIB has a vector of MIs inside. Basically your scheduler (in the
patch I provided just an example very trivial scheduler) will get some
MIs, remove them from the MachineBasicBlock, put them into a MIB, and
add the MIB to the basic block. At some point (when scheduling VLIW
scheduling is finished, but can also be before, as the target requires),
you call MIB->pack() for all your bundles, which has the effect of
"copying" the registers and other flags of internal instructions to the
bundle. From this point on, all the passes will see only the bundle. If
needed, you can "unpack" at anytime and return the inner MIs to the
basic block directly.
> >> I think what we need is a concept of a sequence of fixed machine instructions. Something that represent a number of MachineInstr's that are scheduled as a unit, something that is never broken up by MI passes such as branch folding. This is something that current targets can use to, for example, pre-schedule instructions. This can be useful for macro-fusing optimization. It can also be used for VLIW targets.
> > There might be something I am missing, but I do not see the advantage here. Even more, if you use sequences you need to find a way to tell the passes how long a sequence is. On the other hand, if you use a class derived from MI, the passes know already (from their POV their are just dealing with MIs). You have of course to be careful on how you build the bundles so they have the right properties matching those of the inner MIs, and there is where the pack/unpack methods come in.
> A "sequence" would not be actually a sequence of MachineInstr's. I'm merely proposing you using a generic concept that is not tied to VLIW. In the VLIW bundle, there are no inter-dependencies between the instructions. However, I'm looking for a more generic concept that may represent a sequence of instructions which may or may not have dependencies between them. The key is to introduce a concept that can be used by an existing target today.
> Sounds like what you are proposing is not very far what I've described. Do you have patches ready for review?
Sure, I already sent it couple of weeks ago (is where this thread
started, actually). The original message is here:
> >> On Oct 21, 2011, at 4:52 PM, Stripf, Timo wrote:
> >>> Hi all,
> >>> I worked the last 2 years on a LLVM back-end that supports clustered and non-clustered VLIW architectures. I also wrote a paper about it that is currently within the review process and is hopefully going to be accepted. Here is a small summary how I realized VLIW support with a LLVM back-end. I also used packing and unpacking of VLIW bundles. My implementations do not require any modification of the LLVM core.
> >>> To support VLIW I added two representations for VLIW instructions: packed and unpacked representation. Within the unpacked representation a VLIW Bundle is separated by a NEXT instruction like it was done within the IA-64 back-end. The pack representation packs all instructions of one Bundle into a single PACK instruction and I used this representation especially for the register allocation.
> >>> I used the following pass order for the clustered VLIW back-end:
> >>> DAG->DAG Pattern Instruction Selection
> >>> ...
> >>> Clustering (Not required for unicluster VLIW architectures)
> >>> Scheduling
> >>> Packing
> >>> ...
> >>> Register Allocation
> >>> ...
> >>> Prolog/Epilog Insertion & Frame Finalization
> >>> Unpacking
> >>> Reclustering
> >>> ...
> >>> Rescheduling (Splitting, Packing, Scheduling, Unpacking)
> >>> Assembly Printer
> >>> In principle, it is possible to use the LLVM scheduler to generate parallel code by providing a custom hazard recognizer that checks true data dependencies of the current bundle. The scheduler has also the capability to output NEXT operations by using NoopHazard and outputting a NEXT instruction instead of a NOP. However, the scheduler that is used within "DAG->DAG Pattern Instruction Selection" uses this glue mechanism and that could be problematic since no NEXT instructions are issued between glued instructions.
> >>> Within my back-end I added a parallelizing scheduling after "DAG->DAG Pattern Instruction Selection" by reusing the LLVM Post-RA scheduler together with a custom hazard recognizer as explained. The Post-RA scheduler works very well with some small modifications (special PHI instruction handling and a small performance issue due to the high virtual register numbers) also before register allocation.
> >>> Before register allocation the Packing pass converts the unpacked representation outputted by the scheduler into the pack representation. So the register allocation sees the VLIW bundles as one instruction. After "Prolog/Epilog Insertion & Frame Finalization" the Unpack pass converts the PACK instruction back to the unpacked representation. Thereby, instructions that were added within the Register Allocation and Prolog/Epilog Insertion are recognized and gets into one bundle since they are not parallelized.
> >>> At the end (just before assembly output) I added several passes for doing a rescheduling. First, the splitting pass tries to split a VLIW bundle into single instructions (if possible). The Packing pass packs all Bundles with more the one instruction into a single PACK instruction. The scheduler will recognize the PACK instruction as a single scheduling unit. Scheduling is nearly the same as before RA. Unpacking establishes again the unpacked representation.
> >>> If anyone is interested in more information please send me an email. I'm also interested in increasing support for VLIW architectures within LLVM.
> >>> Kind regards,
> >>> Timo Stripf
> >>> -----Ursprüngliche Nachricht-----
> >>> Von: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] Im Auftrag von Carlos Sánchez de La Lama
> >>> Gesendet: Donnerstag, 6. Oktober 2011 13:14
> >>> An: LLVM Dev
> >>> Betreff: Re: [LLVMdev] VLIW Ports
> >>> Hi all,
> >>> here is the current (unfinished) version of the VLIW support I mentioned. It is a patch over svn rev 141176. It includes the MachineInstrBundle class, and small required changes in a couple of outside LLVM files.
> >>> Also includes a modification to Mips target to simulate a 2-wide VLIW MIPS. The scheduler is really silly, I did not want to implement a scheduler, just the bundle class, and the test scheduler is just provided as an example.
> >>> Main thing still missing is to finish the "pack" and "unpack" methods in the bundle class. Right now it manages operands, both implicit and explicit, but it should also manage memory references, and update MIB flags acording to sub-MI flags.
> >>> For any question I would be glad to help.
> >>> BR
> >>> Carlos
> >>> On Tue, 2011-09-20 at 16:02 +0200, Carlos Sánchez de La Lama wrote:
> >>>> Hi,
> >>>>> Has anyone attempted the port of LLVM to a VLIW architecture? Is
> >>>>> there any publication about it?
> >>>> I have developed a derivation of MachineInstr class, called
> >>>> MachineInstrBundle, which is essnetially a VLIW-style machine
> >>>> instruction which can store any MI on each "slot". After the
> >>>> scheduling phase has grouped MIs in bundles, it has to call
> >>>> MIB->pack() method, which takes operands from the MIs in the "slots"
> >>>> and transfers them to the superinstruction. From this point on the
> >>>> bundle is a normal machineinstruction which can be processed by other
> >>>> LLVM passes (such as register allocation).
> >>>> The idea was to make a framework on top of which VLIW/ILP scheduling
> >>>> could be studies using LLVM. It is not completely finished, but it is
> >>>> more or less usable and works with a trivial scheduler in a synthetic
> >>>> MIPS-VLIW architecture. Code emission does not work though (yet) so
> >>>> bundles have to be unpacked prior to emission.
> >>>> I was waiting to finish it to send a patch to the list, but if you are
> >>>> interested I can send you a patch over svn of my current code.
> >>>> BR
> >>>> Carlos
> >>> _______________________________________________
> >>> LLVM Developers mailing list
> >>> LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu
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