llvm dev - May 2015 - [LLVMdev] RFC: Separate machine IR from lib/CodeGen into lib/MIR

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.

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?

>
> 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.
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> 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?

----- Original Message -----
> From: "Duncan P. N. Exon Smith" <dexonsmith at apple.com>
> To: "Chandler Carruth" <chandlerc at google.com>
> Cc: "LLVM Developers Mailing List" <llvmdev at cs.uiuc.edu>
> Sent: Wednesday, May 27, 2015 12:59:23 PM
> Subject: Re: [LLVMdev] RFC: Separate machine IR from lib/CodeGen into
lib/MIR
> 
> > 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".)
+1 -- Please only use IR to refer to LLVM IR. We should use some different
abbreviation for any other IR we have in the backend.
> 
> > 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. 
+1 -- Me too.

 -Hal

 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
> 
-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory

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
>
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