[llvm-dev] Multi-Threading Compilers

[Chris Lattner via llvm-dev]

On Mar 25, 2020, at 4:14 PM, Doerfert, Johannes <jdoerfert at anl.gov> wrote:
> 
>> Today, you can walk the use-list of any operand to an instruction.
>> This would be broken by this change,
> 
> First, you would be able to walk the use-list of any operand. So nothing
> breaks just yet.

If I understand correctly, you are suggesting that you can walk it, but you don’t get all the uses.  This is extremely dangerous, it would be better to abort.

>> which would make it much easier to write buggy/incorrect compiler code
>> and passes.
> 
> I argued (implicitly) above that the uses of a Constant[Int/Float/...]
> are really not interesting if no-one ever walks them.

There is a difference between “no one ever walks them in practice” and “no one can walk them.    :-)

> Let's assume you
> walk the uses and we "removed" the use list so there are none, what does
> that mean. I'd say, nothing much. If you inspect the Value and see it's
> a Constant you have to assume it has Aliases that denote the same value
> so direct uses are not really interesting anyway. If you inspect further
> and see ConstantInt/Float/... you can deal with the missing use list. If
> you don't inspect the Value you cannot really make much of an empty use
> list, or can you? I doubt we ever call RAUW on a ConstantInt/Float/...
> 
> Feel free to elaborate on your concerns.


Have you tested your thesis that no one walks the use-def chains?  You could just add an assertion to the compiler (the methods like use_begin() hasOneUse() etc), that aborts on constants, then run the test suite.

-Chris