[LLVMdev] Value liveout (uses)

[Rotem Varon]

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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