Re: register allocation: basic blocks, liveness and next use

Gene <gene.ressler@gmail.com> writes:


> On Mar 22, 8:22 pm, kphillips <kevin.phillip...@yahoo.com> wrote:
>> ... Am I missing something here?
>
> Perhaps the definition of a basic block.


Actually, the original poster doesn't seem to have missed the
definition of a basic block. The CALL instructions do create basic
block boundaries. And what you say in the remainder of your message
doesn't contradict this. Instead, you seem to be suggesting that he
missed the definition of a temporary.


> The definition of a temporary requires that none of its defs can reach
> the end of the corresponding block. Obviously your t1..3 don't meet
> this requirement.


I'm not convinced that "temporary" has a consensus meaning, the way
"basic block" does. I've seen plenty of authors other than the
original poster use "temporary" to mean "a name introduced by the
compiler, as opposed to appearing in the source code."


But even if we accept your definition of "temporary" as meaning
"local" (to a basic block), the way you would do that would not be by
stipulating that the definitions can't reach the end of the block, but
rather by stipulating that they not be live at the end of the block.
The last of the definitions for any given name will always reach the
end of the block. (Recall, a definition "reaches" a point if control
must have passed through the definition on the way to the point
without passing through any other definition of the same name in
between.)

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