[LLVMdev] choice between SSAPRE and bitvector aporach

[Vikram S. Adve]

On Apr 4, 2008, at 4:51 PM, Daniel Berlin wrote:

> On Fri, Apr 4, 2008 at 2:38 AM, Bill Wendling <isanbard at gmail.com>  
> wrote:
>> On Apr 2, 2008, at 10:11 PM, Xuehai Qian wrote:
>>> Hi LLVMers,
>>>   I am a PHD student in CS dept in UIUC, I am doing a project for
>>> Vikram's course, it is about PRE. I would like to know why you  
>>> didn't
>>> choose SSAPRE in LLVM, since it seems to be more suitable for LLVM  
>>> (it
>>> can operate directly on SSA form and avoid the conversion between  
>>> SSA
>>> and bit-vector). Can anyone tell me the reason?
>>
>> Hi Xuehai,
>>>
>>
>> If I remember correctly, there were several details that the paper
>> assumed that made adapting it to work in LLVM very difficult.
>
> It would at least require side-data structures to store the
> occurrences and expression phis.


Dan,

Doesn't the paper also assume the invariant that phi operands are  
effectively dead after the Phi, which is true right after SSA is  
constructed, but potentially not after transformations?

--Vikram