[LLVMdev] VLIW Ports

[Sergei Larin]

Evan, Timo, Carlos (and everyone else),

  I have somewhat similar interest. 
  What would you say to a some sort of a "global cycle" field/marker to
determine all instructions scheduled at a certain "global" cycle. That way
the "bundle"/packet/multiop can be identified at any time via a common
"global cycle" value. I could see that being set first in pre-ra scheduler
and refined later (just as Timo is describing). No changes required to any
existing passes, unless a pass "needs" to know about "bundle"/packet. New
instructions added but not yet scheduled would have some invalid cycle (-1
for example). 
  Exact placement of this marker - I am not sure yet, but MachineInstr sure
must have one, but as early as SUnit in DAG->DAG Pattern Instruction
Selection we should be able to do that.

  This is a trivial solution that seems to provide required functionality in
the least intrusive way, and useful to non-VLIW architectures to convey
pre-schedule information down flow. 

Sergei Larin

-----Original Message-----
From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] On
Behalf Of Carlos Sánchez de La Lama
Sent: Monday, October 24, 2011 4:38 PM
To: Evan Cheng
Cc: Stripf, Timo; LLVM Dev
Subject: Re: [LLVMdev] VLIW Ports

Hi Evan (and all),

> 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.

> 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.

BR

Carlos

> 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
>> 
>> 
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> 


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