[LLVMdev] RFC: Separate machine IR from lib/CodeGen into lib/MIR

[Duncan P. N. Exon Smith]

> On 2015 May 27, at 10:24, Chandler Carruth <chandlerc at google.com> wrote:
> 
>> On Wed, May 27, 2015 at 8:15 AM Chris Lattner <clattner at apple.com> wrote:
>>> On May 26, 2015, at 11:20 PM, Quentin Colombet <qcolombet at apple.com> wrote:
>>> 
>>> +1.
>>> 
>>> Could those two be subdirectories of one “Machine-Related-Stuff” directory?
>>> E.g.,
>>> MachineStuff/IR
>>> MachineStuff/CodeGen
>>> 
>>> Where MachineStuff is something meaningful :).
>>> 
>>> That way, they keep a logic bound, more formal than the naming convention.
>> 
>> Something like?
>> 
>> lib/Machine/IR
>> lib/Machine/Passes
>> 
>> Unless there will be many subdirectories, it seems slightly better to flatten out the layer.
> 
> I strongly prefer breaking it out into subdirectories. There are a bunch of reasons:
> 
> 1) It will grow.
> 2) Without it, we cannot have separate libraries, which will lose some options for shrinking the size of libraries.
> 3) Without separate libraries we can't as easily enforce the layering separations between these things. For example, making this split will be *extremely* hard currently because there is a lot of inappropriate dependencies that will block splitting things out.
> 
> However, "IR" and "Passes" cover only two of the things in CodeGen. There is also the implementation of a lot of common infrastructure used by targets' code generators.
> 
> My initial suggestion would be to just sink CodeGen into Machine/CodeGen, add the .../IR and .../Passes directories, and then start extracting things from CodeGen into the two more narrow directories. I think there is likely some stuff that should continue to live in a "code generator" infrastructure directory as it is neither part of the machine IR, nor is it part of any particular pass.
> 
> My suggested layering would be:
> 
> Passes depend on IR, CodeGen depends on both Passes and IR. The idea is that anything passes require should be embedded into the IR.
> 

(Oops, missed this until after I sent my own response.

One thing I'd add to this from my email is that I think
lib/Machine/IR is likely to get confused with lib/IR for the same
reasons that lib/CodeGen is confusing between LLVM and Clang.  IMO
lib/Machine/MIR is "safer".)

> However, this won't currently work. There are things that seem to be parallel but independent of the machine IR and are used by any machine passes. There are also things that clearly use the machine passes. Currently, I'm not sure how to cleanly divide this library up without really significant refactoring of every part of the code generator.
> 
> While I would like to see this happen, is it really a good idea to put this in the critical path of getting MIR serialized and deserialized?

Not if it's as hard as you're saying.  My impression was that Alex
was able to move the IR stuff he needed into a separately library
pretty trivially (based on the P.O.C. patch he posted), but if it's
not trivial he should just move on.

>> Also, if we’re getting crazy here, CodeGen in clang should be renamed to IRGen, AsmPrinter should be renamed to MCGen, and SelectionDAG should be replaced ;-)
> 
> I'm happy to actually do the CodeGen -> IRGen rename. I actually built the change but didn't submit it because of the concerns of some out-of-tree users of Clang. I still have all the perl scripts and such I used sitting around.

I'm a really big fan of this.  If you supply the perl scripts
somehow (attached to a PR that you reference in the commit?) then
out-of-tree users shouldn't have a problem.  Unless I'm missing
something?