[LLVMdev] LLVM related question

Eli Friedman eli.friedman at gmail.com
Sat May 9 14:48:06 PDT 2009

On Sat, May 9, 2009 at 8:37 AM, Rotem Varon <varonrotem at gmail.com> wrote:
> Its not x86 IR nor  x86 assembly. I have x86 IR "like" instructions ( add
> R1, R2, R3    and so...)
>>  If it's really
>> x86 assembly, you can use LLVM's code generator to get x86 assembly,
> Do you mean the "llvm-gcc -S file.c" , or can i have the same effect with
> C++ code (API) ?

Everything in LLVM is accessible through the C++ APIs.

>> but it's extremely difficult to get back to LLVM IR.
> Please, its vary important for me that you elaborate why it will
> be extremely difficult ( i can imagine it would hard, but if you inlight me
> i would probably create a better design...)?

A lot of information gets lost when converting to assembly, like
types, switches, the SSA form, etc.  It's not impossible, especially
if you make some assumptions about the compiler, but it's difficult
enough that I wouldn't suggest it.  It looks like this doesn't apply
here, though.

>>  If it's
>> something higher-level, it might not be so bad.
>  Do you mean, higher than IR ? like c or C++ code ...

Your IR looks high-level enough that it wouldn't be so bad.  It would
still be a pain to write, though.

>> If you're going to need conversions both ways, my guess is that it'll
>> be faster to just rewrite the pass as an LLVM IR pass.
> Can you tall me where can i fined more information about the  LLVM IR pass?

http://llvm.org/docs/WritingAnLLVMPass.html . Also, a bunch of other
stuff in http://llvm.org/docs/ is likely to be useful.

>>  It depends on
>> how complicated the pass is, though.
> Lets say its only a project (academic)  ...

Then it's probably easier to rewrite as an LLVM IR pass.  There's less
to worry about when you're not converting back and forth.

> If i have a pointer to Instruction, how can i get or set the VALUE
> of  Instruction's operands ?

I->getOperand(0) gets the first operand, I->setOperand(0) sets it,
etc.  Note that in general, the best way to get used to the API is to
look at existing passes, espcecially if there's already a pass that's
does stuff that's related to what you're doing.


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