[LLVMdev] Computing live values

[Chris Lattner]

On Wed, 11 May 2005, Alkis Evlogimenos wrote:
> On Wed, 2005-05-11 at 13:17 -0500, Chris Lattner wrote:
>> On Wed, 11 May 2005, Vladimir Prus wrote:
>>> Say I want to find all LLVM Value*-es that a live on exit from a basic block.
>>> What's the best way?
>>>
>>> - The 'LiveRange', 'LiveVariables' and 'LiveIntervals' classes seem to be tied
>>> to register allocation.
>>> - The ./lib/Target/SparcV9/LiveVar/FunctionLiveVarInfo.h file seem to provide
>>> what I need, but it's no a public header.
>>
>> This is overkill.  I would suggest something like this:
>>
>> bool LiveOutsideOfBlock(Instruction *I) {
>>    for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E; ++UI)
>>      if (cast<Instruction>(*UI))->getParent() != I->getParent()) return true;
>>    return false;
>> }
>
> Is this really going to work? What if the value is used in a loop and it
> is used by a phi-node in the same block it is defined?

Good point. :)  If PHI nodes matter (depends on your application), it 
would turn into something like this:

bool LiveOutsideOfBlock(Instruction *I) {
   for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E; ++UI)
        if (cast<Instruction>(*UI))->getParent() != I->getParent() ||
            isa<PHINode>(*UI))
          return true;
     return false;
}

This assumes that it is okay to treat phi uses in successor blocks as uses 
outside of the block, which, again, depends on the application.

-Chris

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