[llvm-dev] Multi-Threading Compilers

Doerfert, Johannes via llvm-dev llvm-dev at lists.llvm.org
Wed Mar 25 16:14:48 PDT 2020

On 3/25/20 3:23 PM, Chris Lattner wrote:
 >> On Mar 25, 2020, at 12:52 AM, Doerfert, Johannes <jdoerfert at anl.gov> 
 >> Some random thoughts:
 >> If no pass ever walks the use list of a constant, except globals which
 >> we could handle differently, we could get rid of their use-list or
 >> overwrite their use-list interface functions to make them no-ops.
 > This would be really problematic because it breaks orthogonality in
 > the compiler.

I doubt it is as simple as that and I we should differentiate more
before we make blanked statements.

 > 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.

 > 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. 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.


 > -Chris

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