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

[Alex L]

2015-05-27 16:53 GMT-07:00 Pete Cooper <peter_cooper at apple.com>:

>
> > On May 27, 2015, at 2:18 PM, Duncan P. N. Exon Smith <
> dexonsmith at apple.com> wrote:
> >
> >
> >> On 2015 May 27, at 14:01, Alex L <arphaman at gmail.com> wrote:
> >>
> >>
> >>
> >> 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.
> >>
> >
> > Something Chandler pointed out on IRC is that #include dependencies
> > aren't enough -- we also need to root out any symbol dependencies.
> > Have you checked for symbols (mostly, functions) that are declared
> > in a header you're planning to move, but defined in a source file
> > you're *not* planning to move?
> >
> > (Apparently there have been failed attempts in the past to bring
> > sanity here that I wasn't aware of…)
> The solution to this is to have a tool which links only the parts we
> need.  So for example, write a tool which only verifies machine IR.  We
> parse it and verify it, and exit.  That only needs the MIR and parser and
> will root out any dependencies on IR/CodeGen.
>

This might be a bit of a problem as such a tool would need the
LLVMTargetMachine which is currently implemented in CodeGen.
As well as that, every target depends on CodeGen, thus we won't be able to
create any machine IR tools without linking in CodeGen given the current
proposal.
Such a tool would require much bigger changes IMHO.

Cheers,
Alex



>
> +1 for everything else BTW, not that we needed another +1.
>
> Cheers,
> Pete
> >
> >>
> >>
> >>>> 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?
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