[LLVMdev] Re: LLVM Compiler Infrastructure Tutorial

[Yiping Fan]

Vikram, 
    I also agree with you. I understand that target-independent representation is very valuable 
and important for software compilation. 

    However, when we are doing high-level synthesis (also called behavioral/architectural synthesis), 
the targeting architecture is also changing. That is, we need to do architecture exploration 
and the IR transfromation simultaneously. For example, after a particular pass, we may need 4 
ALUs to execute the program, under a certain latency constraint; then, after another optimization pass, 
we may end up with only 3 ALUs. The instruction-to-ALU binding will be different after this pass. 
The samething could happen for register allocation and binding, and other synthesis pass. 
    Also, there are lots of synthesis passes there, such as scheduling, functional unit binding, 
register binding, (FU) port binding, re-scheduling and re-binding, interconnect reduction, DSP 
specific optimization, ...
   Therefore, we need some architectural information (both on-disk and in-memory) associated with 
the instructions after every synthesis pass. 

   In addition, in hardware description, there are lots of bit-vector manipulations (e.g., bit extraction,
concatenatoin ...), which are not represented in LLVM. We are thinking about to add some intrinsic 
functions to LLVM, so that the original instruction set can be untouched. 

   Currently we use a simple CDFG (CFG, whose every node is a DFG) representation to do our 
synthesis. Of cource, it is not as powerful as LLVM, and we also want to use many of your 
transformation/analysis passes. That is why we want to move our project on top of your IR. 


Thanks,

-Yiping

  ----- Original Message ----- 
  From: Vikram Adve 
  To: LLVM Developers Mailing List 
  Cc: 'Zhiru Zhang' ; Guoling Han ; Yiping Fan 
  Sent: Wednesday, October 20, 2004 3:08 PM
  Subject: Re: [LLVMdev] Re: LLVM Compiler Infrastructure Tutorial


  Yiping,

  Could you describe in a little more detail what your goals are? I agree with Reid and Misha that modifying the instruction definition is usually not advisable but to suggest alternatives, we would need to know more. Also, for some projects it could make sense to change the instruction set.

  --Vikram
  http://www.cs.uiuc.edu/~vadve
  http://llvm.cs.uiuc.edu/

  On Oct 20, 2004, at 2:41 PM, Misha Brukman wrote:


    On Wed, Oct 20, 2004 at 11:59:45AM -0700, Yiping Fan wrote:

      Yeah. We need to have more extra fields in the instruction. Fo
      example, during high-level synthesis, we must schedule an instruction
      to a certain control step (or cycle), and bind it to be execute on a
      certain functional unit, etc. 


    Since we're talking about "execution" and "scheduling", you are creating
    a back-end for LLVM, correct? In that case, we're talking about code
    generation.

    LLVM is a target-independent representation. Note that with LLVM, we
    are trying to separate WHAT the program is doing (the meaning of the
    program) from HOW it does it (which specific instructions get executed
    and when, and this includes scheduling). What you are trying to add to
    it is target-dependent (e.g. scheduling). That is not advisable on
    several levels, one of which is breaking the target abstraction that we
    have tried hard to maintain. 

    Take a look at the X86, PowerPC, and Sparc target code generators
    (llvm/lib/Target/*). They are using a different representation,
    specifically, MachineInstr, MachineBasicBlock, and MachineFunction
    classes that are target-dependent (for example, they include asm opcodes
    and target registers). Something target-dependent such as scheduling
    and assignment to functional units would be done in this representation,
    after code generation (LLVM -> Machine code).

    Presumably, this (e.g. scheduling) information is not provided from the
    C/C++ front-end, but computed by a pass that you would write, correct?
    Then you can always compute this information on the fly, before any pass
    that needs to do something with this information needs to use it. As
    Reid mentioned, take a look a the Analysis interfaces and see if you can
    implement this as an Analysis that could be required by a pass and
    transparently ran for you by the PassManager.


      Besides the in-memory exchange of the information, we also want
      on-disk exchange. That introduces the write-out/parse-in problem.


    Again, if this is information that's computable from bytecode alone, you
    do not need to store it every time -- an Analyser pass can compute it
    dynamically. Also, as a reminder, if you change the LLVM
    representation, your new version may or may not be able to use the
    current set of analyses and optimizations, thus forcing you to
    "reinvent the wheel" in that respect.

    -- 
    Misha Brukman :: http://misha.brukman.net :: http://llvm.cs.uiuc.edu

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