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

[Alex L]

2015-05-27 10:59 GMT-07:00 Duncan P. N. Exon Smith <dexonsmith at apple.com>:

> > 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.
>

While it's true that there are things that prevent a straightforward
library division, I don't think that they will pose such a big problem.
I have a brief list of things that have to be done before I can move the
Machine IR stuff from CodeGen to some other library:
- Move the SplitCriticalEdge method from MachineBasicBlock (
http://reviews.llvm.org/D10064).
- Move the UnpackMachineBundles and FinalizeMachineBundles passes from
MachineInstrBundle.cpp. (http://reviews.llvm.org/D10070 + 1 upcoming patch).
- Refactor SlotIndexes.h: keep the SlotIndexes pass and move the rest to
SlotIndex.h. Introduce a new container class in SlotIndex.h that will
extract the map
  between machine instructions and slot indexes from the SlotIndexes pass
and remove the dependency on the pass from MachineBasicBlock's print method.
- WinEHFuncInfo - Either move parseEHInfo from WinEHPrepare.cpp to new file
WinEHFuncInfo.cpp or move the declaration of parseEHInfo from
WinEHFuncInfo.h
  to a new header WinEHPrepare.h
- Get rid of several unused #includes in MachineIR cpp files that include
passes.

After that it should be possible to move the Machine IR files out of
CodeGen without them actually including any CodeGen headers.
And all the Machine IR passes like MachineFunctionPass,
MachineFunctionAnalysis, etc. will remain in CodeGen.

Alex.



>
> >> 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?
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20150527/3780fc14/attachment.html>