[LLVMdev] Re: LLVM Compiler Infrastructure Tutorial

[Vikram Adve]

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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