[LLVMdev] Still Another LiveInterval Question

Evan Cheng evan.cheng at apple.com
Thu Jan 17 15:25:01 PST 2008

On Jan 17, 2008, at 2:06 PM, David Greene wrote:

> Ok, so I'm delving much more deeply into this stuff.  :)
> On a target like x86, is it true to say that for virtual registers  
> post
> phi-elimination, the only  time their live intervals should have  
> more than one
> value number is if they participate in 2-address instructions where  
> a copy had
> to be introduced to satisfy the read-write requirements of the  
> instruction?
> And further, in that  case, LiveIntervals for virtual registers  
> should only
> have at most two value numbers?

That's true only before coalescing. After coalescing, you will  
frequently see intervals but more valno's.

> How about on targets without these reuiqrements?  Should  
> LiveIntervals for
> virtual registers only ever have one value number?

Again, true but only before coalescing.


>                                               -Dave
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