[LLVMdev] Value liveout (uses)

[Evan Cheng]

I believe Dan has added a pass to compute livein / liveout values.

Evan

Sent from my iPhone

On May 30, 2009, at 5:03 AM, Rotem Varon <varonrotem at gmail.com> wrote:

> Thank you.
>
> Is it possible to determine the liveout of the operands (see example  
> bellow) ?
>
>         %5 = add i32 %4, %3
>
> For '%5': i can simply use " i->isUsedOutsideOfBlock() "
> For '%3' and '%4' : this is the question ...
>
> From your answer, is it possible to determine which value is liveout  
> ( in binary instruction)?
>
>
> On Sat, May 30, 2009 at 2:57 AM, John McCall <rjmccall at apple.com>  
> wrote:
> On May 29, 2009, at 11:37 PM, Rotem Varon wrote:
> > How can i know, if a value have uses outside of the current basic
> > block (liveout), without iterating through all the basic block ?
>
> If the value is created within the basic block in question, and the
> block doesn't loop to itself, then you can just iterate through the
> uses and note if the use is an instruction in a different block:
>
> bool isLiveOut(Instruction* I) {
>        BasicBlock* I_BB = I->getParent():
>        for (Value::use_iterator ui = I->use_begin(), ue = I->use_end 
> (); ui !
> = ue; ++ui)
>                if (cast<Instruction>(ui)->getParent() != I_BB)
>                        return true;
>        }
> }
>
> If the value is created within the block but the block loops to
> itself, you can get away with a very slight adjustment:
>
>                if (cast<Instruction>(ui)->getParent() != I_BB && ! 
> isa<PHINode>(ui))
>
> If the value is created in one block and you want to know if it's live
> out of some dominated block, that's a lot more complicated (and
> expensive).
>
> John.
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