llvm dev - Jun 2017 - Should we split llvm Support and ADT?

> On May 30, 2017, at 3:49 PM, Pete Cooper via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
>> 
>> On May 30, 2017, at 12:05 PM, Zachary Turner via llvm-dev <llvm-dev
at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>> 
>> Let me try starting over without mentioning Tablegen :)
> Aw, but i had a list of issues, such as:
> - tablegen doesn’t generate .d files
> - CMake is set up with far reaching deps on .td files, so changing
Options.td rebuilds the backends, and adding an ARM intrinsic rebuilds X86 or
vice versa
> - All the tablegen backends are linked in to a single binary, so if you are
hacking on tablegen and change the disassemblers backend, you rebuild the
SelectionDAG code
>> 
>> Is Support too big, and if so should we consider breaking it up?
> Yes please.  Even if it didn’t have a signifiant impact on build time, its
become something of a dumping ground for code which would otherwise cause
layering violations if put somewhere else.
> 
> The way I see it, there’s a bunch of functionality which effectively just
wraps the platform, (i.e., anything which abstracts away posix/unix vs Windows
vs macOS).  This is often stuff like file systems, streams, the host itself and
probably a few others.  Then there is obviously ADT.  There are utilities which
basically everyone uses like Error, Debug, Statistic and stack trace.  There are
utilities which only tools *should* use, like CommandLine, and utilities only
used by a few projects right now, but maybe more in future, like Threading.
> 
> Then there are things which should now live elsewhere because perhaps other
libraries exist now which didn’t before.  ELF.h and others should be in
libObjectFile.  If they can’t be there, then we should have a very good reason
why not.
> 
> BranchProbability shouldn’t be there at all but probably is due to
layering, and likely the same goes for TargetRegistry.  We should fix that. 
Same goes for ARM* which should all be in the ARM backend.
>  Dwarf should be in libDebugInfoDWARF.
I think the reason why it isn't there is because lib/DebugInfo is for
*consuming* debug info, but these definitions are useful for both reading and
writing DWARF. That is not to say that they couldn't be moved somewhere else
of course (but probably not lib/DebugInfo in its current form).

-- adrian
>  *DeltaAlgorithm aren’t even used outside of tests!
> 
> Anyway, sorry for the brain dump.  But yes, this has grown in to something
which can be cleaner.  Perhaps Support will still exist to wrap posix and
friends, hopefully with ADT separately, but we should also move a bunch of
things to better suited libraries if we can.
> 
> Cheers,
> Pete
>> 
>> So far it sounds like the general consensus is "yes", with
some quibbles about precisely how to split it.
>> 
>> I'm not necessarily opposed to smaller libraries like Triple as you
suggest, but i think that only addresses what to with largely disparate set of
stuff which I'm calling "narrowly useful".  It doesn't really
address the fact that even after doing it at every logical division point, there
will still be a set of stuff that doesn't belong with other stuff.
>> 
>> To address that, I'm proposing something like "Base"
which is similar in spirit to Support as it exists today, but more restrictive,
and must satisfy the "would this be useful outside of LLVM" bar.
>> 
>> 
>> 
>> 
>> On Tue, May 30, 2017 at 11:05 AM Matthias Braun via llvm-dev
<llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>>
wrote:
>> In my experience the buildsystem works fine in combination with
tablegen (at least this aspect of it). The real problem here is that tablegen is
just slow. Some of the X86 tables take indeed
>> 
>> Last time I looked at it tablegen had still room to optimize the way it
resolves class hierarchies and the variables within which it did basically one
at a time, so it needed to traverse the hierarchies again for each variables.
>> 
>>> On May 27, 2017, at 12:54 PM, David Blaikie via llvm-dev
<llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>>
wrote:
>>> 
>>> Maybe we do and build systems aren't respecting/noticing this?
I'm not sure.
>>> 
>>> On Sat, May 27, 2017 at 12:50 PM Craig Topper <craig.topper at
gmail.com <mailto:craig.topper at gmail.com>> wrote:
>>> I thought we already did write tablegen output to temporary files
like X86GenAsmWriter1.inc.tmp first and then diffed them with the real .inc file
and conditionally copied.
>>> 
>>> ~Craig
>>> 
>>> On Sat, May 27, 2017 at 11:02 AM, David Blaikie via llvm-dev
<llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>>
wrote:
>>> 
>>> 
>>> On Fri, May 26, 2017 at 8:06 PM Zachary Turner via llvm-dev
<llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>>
wrote:
>>> It would be better, because a debug tablegen is slower than an
optimized tablegen, but it's still slow and it doesn't address the
problem that tablegen runs *at all* when it doesn't really need to. I think
if tablegen wasn't running all the time we could incremental builds down
from 15 minutes (and that's on my really powerful machine) to under 5, which
seemed like a big productivity boost averaged out over all users x time
>>> 
>>> Is that 10 minutes of running tablegen, though? I think this
touches on Hal's comment:
>>> "Maybe we should use a diff-and-update approach (I thought,
however, that we already did that)."
>>> 
>>> In that the 10 minutes is presumably from timestamp-based
invalidation chaining down from running tablegen, to then rebuilding everything
that depends on tablegen's output.
>>> 
>>> That could be addressed by either the build system or tablegen
writing output to a temporary file, comparing the temp to the current file, and
not modifying the current file if they're the same - thus not invalidating
everything down the chain that depends on the tablegen output (not sure if
that's actually enough to convince a build system not to rebuild other
things - but worth a shot). This is a bit of a hackaround what the build system
itself could/should be doing, but in the absence of a content based invalidation
scheme - addressing this one particularly critical point in LLVM's build
seems like it could be worth such a workaround. (this would speed up the build
for even the times when tablegen's real dependencies change - add a new
function to ArrayRef, or STLExtras, etc... and tablegen runs, but it doesn't
cascade to everything else).
>>> 
>>> (plus, opt build of tablegen, so that conclusion can be reached
more quickly)
>>> 
>>> As Hal also mentioned, I'd be OK with this if there's a
good logical division that can be taken more than "whatever is/isn't
used in tablegen".
>>>  
>>> 
>>> We've got a lot of random stuff in Support, not all really
related and not all relevant to every project. It seems like we should be able
to do better.
>>> On Fri, May 26, 2017 at 6:48 PM Hal Finkel <hfinkel at anl.gov
<mailto:hfinkel at anl.gov>> wrote:
>>> 
>>> 
>>> On 05/26/2017 07:59 PM, Zachary Turner wrote:
>>>> It's that TableGen depends on Support, so if you change one
file in support, support gets recompiled into a new static archive, which
triggers a rerun of tablegen on all the tablegen inputs, which is extremely
slow.
>>> 
>>> I thought that we had a "build an optimized tablegen even in
debug mode" setting to work around this problem. Would that avoid this
problem?
>>> 
>>> 
>>>  -Hal
>>> 
>>> 
>>>> 
>>>> On Fri, May 26, 2017 at 5:56 PM Hal Finkel <hfinkel at
anl.gov <mailto:hfinkel at anl.gov>> wrote:
>>>> 
>>>> 
>>>> 
>>>> On 05/26/2017 07:47 PM, Zachary Turner via llvm-dev wrote:
>>>>> Changing a header file somewhere and having to spend 10
minutes waiting for a build leads to a lot of wasted developer time.
>>>>> 
>>>>> The real culprit here is tablegen.  Can we split support
and ADT into two - the parts that tablegen depends on and the parts that it
doesn't?
>>>> 
>>>> What's the actual problem here? Is it that TableGen
regenerates different files and so we then need to rebuild all dependencies of
those files? Maybe we should use a diff-and-update approach (I thought, however,
that we already did that).
>>>> 
>>>>  -Hal
>>>> 
>>>>> 
>>>>> From what I can gather, Tablegen currently depends on these
headers and all of their transitive dependencies.
>>>>> 
>>>>> #include "llvm/Support/Casting.h"
>>>>> #include "llvm/Support/CommandLine.h"
>>>>> #include "llvm/Support/Compiler.h"
>>>>> #include "llvm/Support/DataTypes.h"
>>>>> #include "llvm/Support/Debug.h"
>>>>> #include "llvm/Support/Error.h"
>>>>> #include "llvm/Support/ErrorHandling.h"
>>>>> #include "llvm/Support/Format.h"
>>>>> #include "llvm/Support/FormattedStream.h"
>>>>> #include "llvm/Support/LEB128.h"
>>>>> #include "llvm/Support/LowLevelTypeImpl.h"
>>>>> #include "llvm/Support/ManagedStatic.h"
>>>>> #include "llvm/Support/MathExtras.h"
>>>>> #include "llvm/Support/MemoryBuffer.h"
>>>>> #include "llvm/Support/PrettyStackTrace.h"
>>>>> #include "llvm/Support/Regex.h"
>>>>> #include "llvm/Support/SMLoc.h"
>>>>> #include "llvm/Support/ScopedPrinter.h"
>>>>> #include "llvm/Support/Signals.h"
>>>>> #include "llvm/Support/SourceMgr.h"
>>>>> #include "llvm/Support/raw_ostream.h"
>>>>> 
>>>>> #include "llvm/ADT/APInt.h"
>>>>> #include "llvm/ADT/ArrayRef.h"
>>>>> #include "llvm/ADT/BitVector.h"
>>>>> #include "llvm/ADT/CachedHashString.h"
>>>>> #include "llvm/ADT/DenseSet.h"
>>>>> #include "llvm/ADT/IndexedMap.h"
>>>>> #include "llvm/ADT/IntEqClasses.h"
>>>>> #include "llvm/ADT/MapVector.h"
>>>>> #include "llvm/ADT/Optional.h"
>>>>> #include "llvm/ADT/PointerUnion.h"
>>>>> #include "llvm/ADT/STLExtras.h"
>>>>> #include "llvm/ADT/SetVector.h"
>>>>> #include "llvm/ADT/SmallPtrSet.h"
>>>>> #include "llvm/ADT/SmallSet.h"
>>>>> #include "llvm/ADT/SmallVector.h"
>>>>> #include "llvm/ADT/SparseBitVector.h"
>>>>> #include "llvm/ADT/Statistic.h"
>>>>> #include "llvm/ADT/StringExtras.h"
>>>>> #include "llvm/ADT/StringMap.h"
>>>>> #include "llvm/ADT/StringRef.h"
>>>>> #include "llvm/ADT/StringSet.h"
>>>>> #include "llvm/ADT/StringSwitch.h"
>>>>> #include "llvm/ADT/TinyPtrVector.h"
>>>>> #include "llvm/ADT/Twine.h"
>>>>> 
>>>>> 
>>>>> Is this something worth putting effort into?  If so, I
volunteer.
>>>>> 
>>>>> 
>>>>> 
>>>> 
>>>>> _______________________________________________
>>>>> LLVM Developers mailing list
>>>>> llvm-dev at lists.llvm.org <mailto:llvm-dev at
lists.llvm.org>
>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
<http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev>
>>>> 
>>>> 
>>>> -- 
>>>> Hal Finkel
>>>> Lead, Compiler Technology and Programming Languages
>>>> Leadership Computing Facility
>>>> Argonne National Laboratory
>>> 
>>> -- 
>>> Hal Finkel
>>> Lead, Compiler Technology and Programming Languages
>>> Leadership Computing Facility
>>> Argonne National Laboratory
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>>> 
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>>> 
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>> 
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On Thu, Jun 1, 2017 at 2:15 PM Pete Cooper via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
>
> On May 31, 2017, at 9:09 PM, Adrian Prantl <aprantl at apple.com>
wrote:
>
>  Dwarf should be in libDebugInfoDWARF.
>
> I think the reason why it isn't there is because lib/DebugInfo is for
> *consuming* debug info, but these definitions are useful for both reading
> and writing DWARF. That is not to say that they couldn't be moved
somewhere
> else of course (but probably not lib/DebugInfo in its current form).
>
> Ah, didn’t know that.  I’m sure a bunch of the others I mentioned also
> have similar reasons for being where they are.
>
> Ultimately its a judgement call, but i’d say that libDebugInfoDWARF is a
> less bad place for this file than libSupport.  Not a great reason to move
> it, but if someone wants to I still wouldn’t personally object.
>
That would seem to have a concrete disadvantage, though - that'd make many
things dependent on libDebugInfo that aren't currently - so there'd be
real
changes in build time, etc building all the DWARF parsing/dumping/etc API
code for users who only generate DWARF but don't parse it.

If it were moved, it'd probably want to be a separate library that both
DWARF reading libraries and DWARF writing libraries could be dependent on.
(unless the reading/writing could be tightened up to the point where it was
mostly a common library used for both sides)

>
> Cheers,
> Pete
>
>
> -- adrian
>
>
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I would probably just make a new library called ObjectDef or ObjectFormat
and put ELF.h, COFF.h, DWARF.h, etc all there. Doesn't even need to produce
a .a file
On Thu, Jun 1, 2017 at 2:15 PM Pete Cooper <peter_cooper at apple.com>
wrote:
>
> On May 31, 2017, at 9:09 PM, Adrian Prantl <aprantl at apple.com>
wrote:
>
>  Dwarf should be in libDebugInfoDWARF.
>
> I think the reason why it isn't there is because lib/DebugInfo is for
> *consuming* debug info, but these definitions are useful for both reading
> and writing DWARF. That is not to say that they couldn't be moved
somewhere
> else of course (but probably not lib/DebugInfo in its current form).
>
> Ah, didn’t know that.  I’m sure a bunch of the others I mentioned also
> have similar reasons for being where they are.
>
> Ultimately its a judgement call, but i’d say that libDebugInfoDWARF is a
> less bad place for this file than libSupport.  Not a great reason to move
> it, but if someone wants to I still wouldn’t personally object.
>
> Cheers,
> Pete
>
>
> -- adrian
>
>
>
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