[LLVMdev] VLIW Ports

Sergei Larin slarin at codeaurora.org
Wed Oct 26 13:01:51 PDT 2011


  What would change if tomorrow we got a VLIW target/back end with some
certain properties - let's say no intra-packed deps - would it sway your
opinion in either direction? Would it be a natural prerogative to implement
it certain way for such hypothetical contributor/submitter? 


Sergei Larin

-----Original Message-----
From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] On
Behalf Of Evan Cheng
Sent: Wednesday, October 26, 2011 2:08 PM
To: Stripf, Timo
Cc: LLVM Dev
Subject: Re: [LLVMdev] VLIW Ports

On Oct 25, 2011, at 1:59 AM, Stripf, Timo wrote:

> Hi all,
>> 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?
> I attached the packing and unpacking pass I used within my backend. In my
solution multiple MachineInstruction are packed into one variadic "PACK"
MachineInstruction. The opcode and operands of the original instruction are
encoded as operands of the PACK instruction. The opcode is added as
immediate following by the operands of the original instructions. Within the
operands one instruction is terminated by an "EndOfOp" operand. The implicit
defs/uses are also added to the PACK instruction but not used for unpacking.
Unpacking reconstructs them from the TargetDescriptionInfo. 
> I took a look at the packing/unpacking solution of Evan and I think it is
more elegant to use a derived class of MachineInstr for storing multiple
instructions into one.

Here are my thoughts on instruction bundle.

First, let's talk about the prerequisite for adding a codegen level IR
extension. A MachineInstr bundle should be generic enough to support the
followings 1) VLIW bundles (where there are no intra-dependencies between
instructions in a bundle), 2) bundles for other targets where there may be
intra-dependencies between instructions in a bundle. #2 is very important
for the extension to be accepted into LLVM mainline today since there are no
proper VLIW targets.

Now let's look at the options.

1. Extend MachineInstr to represent a bundle. This can be achieved either a
derived class or add a pointer in MachineInstr that points to the next
instruction in the bundle.
2. Add a bit to MachineInstr that indicates it is part of a bundle /

The advantage of #1 is this requires minimum change to register allocator
and many other codegen passes. However, that's only true for VLIW targets
with no intra-bundle dependencies. For other targets or for use of
optimizations which model a sequence of instructions, this is not true. The
register allocator and scheduler need to know the cumulative properties of a
bundle. For example, the register allocator needs to know what are the input
operands, what are the outputs. The scheduler needs to know the cumulative
latency of the bundle. Other passes that examine individual instruction
properties (e.g. is it a load / store, control flow) will need to know the
combined properties of individual instructions in a bundle.

Of course, this is a solvable problem. The pass that combine instructions
into bundles can construct the bundle MachineInstr properly so it presents
the right information. The down size is this will add memory overhead and it
needs to be carefully studied.

The advantage of #2 is the low overhead. Adding a bit won't add much if any
memory overhead. Packing / unpacking are both very easy. This is especially
good for register allocator, which can still model register liveness even
when there are intra-bundle dependencies. The downsize of #2 is also
obvious. Every pass that operates on MachineInstr will have to be aware of
bundles. This is the only real downsize that I can think of, but it's a big


> Best regards,
> Timo Stripf
> -----Ursprüngliche Nachricht-----
> Von: Evan Cheng [mailto:evan.cheng at apple.com] 
> Gesendet: Dienstag, 25. Oktober 2011 01:55
> An: Carlos Sánchez de La Lama
> Cc: Stripf, Timo; LLVM Dev
> Betreff: Re: [LLVMdev] VLIW Ports
> On Oct 24, 2011, at 2:38 PM, Carlos Sánchez de La Lama wrote:
>> 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.
> 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?
>>> 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?
> Evan
>> 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
>>>> 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
>>>> If anyone is interested in more information please send me an email.
I'm also interested in increasing support for VLIW architectures within
>>>> 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
>>>>> 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
>>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> <TS1VLIWPacking.cpp><TS1VLIWUnpacking.cpp>

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