[llvm-dev] Multi-Threading Compilers

[Nicolai Hähnle via llvm-dev]

On Fri, Mar 27, 2020 at 9:55 PM David Blaikie <dblaikie at gmail.com> wrote:
> On Fri, Mar 27, 2020 at 1:53 PM Nicolai Hähnle via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>>
>> On Fri, Mar 27, 2020 at 5:22 PM Chris Lattner via llvm-dev
>> <llvm-dev at lists.llvm.org> wrote:
>> > On Mar 27, 2020, at 12:23 AM, Johannes Doerfert <johannesdoerfert at gmail.com> wrote:
>> >
>> >
>> > > Getting to a multithread clean optimizer is not one bug fix away -
>> > > this is a multiyear journey, and I would prefer not to see big changes
>> > > with unclear tradeoffs made in an effort to get a quick win.
>> >
>> > I think I missed the suggestion of "big changes with unclear tradeoffs
>> > made in an effort to get a quick win". So far I thought this was a
>> > discussion of ideas, methods to gather data, and potential pitfalls.
>> >
>> >
>> > Making use-def chains work differently based on the dynamic type of a Value* is very problematic to me.  It breaks the liskov substitution principle and is likely to lead to widespread bugs.
>>
>> That's why I'm also wary of the idea of just having use lists empty
>> for certain types without any other special handling. However, I would
>> argue that if Value::use_begin() etc. contain an assertion that fails
>> when called on one of the value types that don't have use lists, then
>> the Liskov substition principle is de facto not broken. It basically
>> leads to a situation that is as-if Value didn't have use lists in the
>> first place, and only certain sub-types had use lists.
>
>
> But it doesn't - it means you write some generic code, test it with some cases & looks like it generalizes to other cases (you don't want to/can't test generic code with all possible generic arguments - that's why substitutability is important) then the code breaks when it hits a constant Value because it doesn't conform to the contract.

It's not a break of the Liskov substitution principle, though. For
example: In the proposal, you can iterate over uses of an Instruction,
and you can iterate over uses of all sub-types of an Instruction. You
can't iterate over Constant, but then Constant isn't a sub-type of
Instruction.

It's a bit of an academic point. What you're really trying to say is
that with this particular version of the proposal, it is not
**statically** checked at compile-time whether a Value has iterable
use lists, and that makes it easier to write incorrect code
accidentally. Yes, that's a weakness, and precisely the reason why I
suggested that we may want to re-organize the inheritance hierarchy of
Values.

Cheers,
Nicolai


>
>>
>> If we go down that route, maybe the inheritance hierarchy could be
>> reorganized in a way that makes that more obvious.
>>
>> Cheers,
>> Nicolai
>>
>>
>> >
>> > > Instead, let’s break down the problems and fix them (the right way!)
>> > > one at a time.  For example, it seems that the thread agrees that the
>> > > overall design of constants are a problem: let's talk about
>> > > (incrementally!) moving constants to the right architecture. There
>> > > are good suggestions about this upthread.
>> >
>> > I'm not sure why you think anyone so far suggested anything to "fix it
>> > all" or to do it "not-incrementally". For example, I think we are
>> > talking about the design of constants and how it can be incrementally
>> > improved (for different purposes). I also think there are interesting
>> > suggestions upthread but I'm not sure sure there was any kind of
>> > agreement on "the right architecture”.
>> >
>> >
>> > It sounds like there is agreement that whole module/llvmcontext use-def chains for constant are a bad idea.  There is one specific proposal for how to fix that that is incremental and avoids the problem I mention above: use instructions to materialize them, so Constant stops deriving from Value.  I would love to see this be explored as a step along the way, instead of spending time pondering how bad a break to the core APIs in the compiler would be in practice.
>> >
>> > -Chris
>> >
>> >
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>>
>>
>>
>> --
>> Lerne, wie die Welt wirklich ist,
>> aber vergiss niemals, wie sie sein sollte.
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-- 
Lerne, wie die Welt wirklich ist,
aber vergiss niemals, wie sie sein sollte.