[LLVMdev] LLVM related question

[Rotem Varon]

Thank your answer.

On Sun, May 10, 2009 at 5:39 PM, Rotem Varon <varonrotem at gmail.com> wrote:

> Thank you you answer.
>
>
> On Sun, May 10, 2009 at 12:48 AM, Eli Friedman <eli.friedman at gmail.com>wrote:
>
>> 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.
>>
>> -Eli
>>
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>
>
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