[LLVMdev] Guidance on using pointers vs. references for function arguments

[Chris Lattner]

On May 26, 2014, at 9:15 PM, Chandler Carruth <chandlerc at google.com> wrote:

> 
> On Mon, May 26, 2014 at 8:45 PM, Chris Lattner <clattner at apple.com> wrote:
> Chandler, Andy's recollection is correct.
> 
> I was personally infatuated by this idea and pushed it through a ton of the LLVM IR APIs (this was many years ago, (in the pre-1.0 days, and then again early in Clang's development because I'm a slow learner).  It led to all sorts of weird cases where some arguments to IR objects would be taken by reference and some by pointer.  It made it impossible to remember whether a given function took a pointer or a reference, and made for absolute madness trying to program against the APIs.  It got to the point where literally you had to do a build and just add the missing &/*'s to get the build to go through.
> 
> I didn't doubt any of Andy's recollection here, and it seemed like we were largely in agreement...
> 
> Anyways, while inconsistency such as you describe between pointer and reference arguments can be quite frustrating (as I see it was for you), at least LLVM is still pretty stunningly inconsistent to this day. I also don't find it to be a significant burden.

It is true in parts, but not broadly.  If you look at LLVM, for example, pretty much all of the IR stuff takes pointers, even in places where they should "obviously" be references: IRBuilder, ConstantArray::get and other factories, and many others.  Likewise pretty much the entire Clang AST library is defined in terms of pointers.

It is true that passes and other local stuff is wildly inconsistent, but the damage of that is far less than affecting the core IR/AST.  

> Ultimately, (as I said in my prior email) I don't think there is a problem to solve here because I don't think that any of these things are actively "bad". Maybe its just the devs I know who don't really have trouble here, but its not one of the complaints I hear from others or have myself in day-to-day development.
> 
> If mixing pointers and references for the same type in LLVM's APIs is a serious problem in your mind, what is your concrete suggestion? What should the end state look like?

I'm not really sure that we need to "solve" this problem.  Can you restate exactly what the problem is?  We don't all need the compiler to have perfectly identical code in every pass, for example.

That said, if I were to lay down a rule, I think the right general rule would be: pointers for "classes" everywhere, references for value types, and pass by value when it is >= the efficiency of pass by value for value types.

That said, I still don't see a huge problem being solved here...

-Chris
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