[llvm-dev] Multi-Threading Compilers

Nicholas Krause via llvm-dev llvm-dev at lists.llvm.org
Fri Mar 27 20:41:50 PDT 2020

On 3/27/20 11:31 PM, Chris Lattner via llvm-dev wrote:
>> On Mar 27, 2020, at 1:55 PM, David Blaikie <dblaikie at gmail.com 
>> <mailto:dblaikie at gmail.com>> wrote:
>>     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.
> David is exactly right.  To say the same thing in another way, such a 
> design point would break library based design, one of the core 
> principles of LLVM that makes it so powerful.
> To make this explicit, a library that walks use-lists internally would 
> have implicit dependencies in its APIs that some things are not 
> allowed to be constants.
> -Chris
I'm not sure who is removing me from this discussion but please keep me 
CCed to it.
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