[LLVMdev] Question to Chris

[Vikram S. Adve]

Bill,

Depending on what Seung's problem is, converting *out* of SSA form may  
actually be the wrong thing to do.  Seung needs to explain exactly  
problem he is unable solve using SSA form.

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



On Jan 28, 2008, at 1:10 AM, Bill Wendling wrote:

> Hi Seung,
>
> It should be fairly straight-forward to do in LLVM. Once you identify
> the loops, then identify the PHI nodes that you need to convert, then
> apply the transformation below. The fine details on how to create an
> instruction and replace one instruction with another are documented
> in the docs section and in other code. :-) One thing to be careful
> of, if you convert a variable like I showed, then you need to make
> sure that it's not used in any other PHI nodes (if it is, then you'll
> need to perform some other type of transformation).
>
> -bw
>
> On Jan 27, 2008, at 8:50 AM, Seung Jae Lee wrote:
>
>> Thank you, Bill.
>> Seems to be better.
>> Anyway...Is there a way I can do what you showed for me?
>>
>> Thanks,
>> Seung J. Lee
>>
>> ---- Original message ----
>>> Date: Sat, 26 Jan 2008 22:10:01 -0800
>>> From: Bill Wendling <isanbard at gmail.com>
>>> Subject: Re: [LLVMdev] Question to Chris
>>> To: LLVM Developers Mailing List <llvmdev at cs.uiuc.edu>
>>>
>>> On Jan 26, 2008, at 9:48 AM, Seung Jae Lee wrote:
>>>
>>>> Dear Dr.Lattner
>>>>
>>>> Hello, Dr.Lattner.
>>>> You may find your reply at http://lists.cs.uiuc.edu/pipermail/
>>>> llvmdev/2007-August/010479.html and other your replies to me right
>>>> up and down at the list.
>>>> You had suggested me to read the "structural analysis" section in
>>>> Muchnick's book.
>>>> Thank you for this. I bought and read it, which was very helpful
>>>> but...
>>>> I still don't have any idea about how to deal with phi-nodes in
>>>> LLVM Intermediate Representation systemically to resolve my  
>>>> problem.
>>>> In order to construct high-level 'for' from LLVM IR, it is critical
>>>> to move Phi-nodes hither and thither or split them but... I can't
>>>> find any material about this from anywhere.
>>>> So could you reply to me briefly about this if you are fine?
>>>>
>>>> Thank you very much and have a good day.
>>>> Seung J. Lee
>>>>
>>>> P.S: In fact, I am thinking about an alternative way for doing this
>>>> by using reverse engineering. Now that LLVM IR has phi-nodes which
>>>> is tricky to handle for this issue, I just slightly changed my way
>>>> to use the machine assembly which does not have phi-nodes. Already
>>>> someone (like Doug Simon in Sun microsystems) got high-level C code
>>>> "which is quite same with the original including loops,
>>>> conditionals and so on" from Sparc assembly by using de- 
>>>> compilation.
>>>> Therefore, if you reply "it is difficult to handle phi-nodes for
>>>> constructing high-level loops", I am almost ready to go the other
>>>> way using the machine assembly.
>>>> Anyway, could you shed some lights on me?
>>>> Thank you very much
>>>
>>> Hi Seung,
>>>
>>> It would appear to me that you would simply need to perform a
>>> modified form of "un-SSAification". For instance, if you have PHI
>>> nodes whose values come from the back edges of a loop, then you  
>>> could
>>> perhaps store those values to memory and then load them inside of  
>>> the
>>> loop in place of the PHI node.
>>>
>>> Meta-code:
>>>
>>> BB1:
>>>    %v1 = ...
>>>    ...
>>>    br label %Loop
>>>
>>>
>>> Loop:
>>>    %v2 = phi i32 [%v1, %BB1], [%v3, %BB2]
>>>    ...
>>>    br label %BB2
>>>
>>>
>>> BB2:
>>>    %v3 = ...
>>>    br label %Loop
>>>
>>>
>>> into something like:
>>>
>>> Entry:
>>>    %ptr = alloca i32
>>> ...
>>> BB1:
>>>    %v1 = ...
>>>    store i32 %v1, %32* %ptr
>>>    ...
>>>    br label %Loop
>>>
>>> Loop:
>>>    %v2 = load i32* %ptr
>>>    ...
>>>    br label %BB2
>>>
>>> BB2:
>>>    %v3 = ...
>>>    store i32 %v3, i32* %ptr
>>>    ...
>>>    br label %Loop
>>>
>>> What do you think?
>>>
>>> -bw
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