[llvm] r188188 - Misc enhancements to LTO:

[Nick Lewycky]

On 12 August 2013 15:00, Shuxin Yang <shuxin.llvm at gmail.com> wrote:

>  On 8/12/13 2:32 PM, Nick Lewycky wrote:
>
> On 12 August 2013 13:29, Shuxin Yang <shuxin.llvm at gmail.com> wrote:
>
>>   On 8/12/13 1:03 PM, Nick Lewycky wrote:
>>
>> On 12 August 2013 12:22, Shuxin Yang <shuxin.llvm at gmail.com> wrote:
>>
>>> On 8/12/13 12:16 PM, Nick Lewycky wrote:
>>>
>>>> Shuxin Yang wrote:
>>>>
>>>>> Author: shuxin_yang
>>>>> Date: Mon Aug 12 13:29:43 2013
>>>>> New Revision: 188188
>>>>>
>>>>> URL: http://llvm.org/viewvc/llvm-project?rev=188188&view=rev
>>>>> Log:
>>>>> Misc enhancements to LTO:
>>>>>
>>>>>    1. Add some helper classes for partitions. They are designed in a
>>>>>       way such that the top-level LTO driver will not see much
>>>>> difference
>>>>>       with or without partitioning.
>>>>>
>>>>>    2. Introduce work-dir. Now all intermediate files generated during
>>>>>       LTO phases will be saved under work-dir. User can specify the
>>>>> workdir
>>>>>       via -lto-workdir=/path/to/dir. By default the work-dir will be
>>>>>       erased before linker exit. To keep the workdir, do -lto-keep, or
>>>>> -lto-keep=1.
>>>>>
>>>>>      TODO: Erase the workdir, if the linker exit prematurely.
>>>>>        We are currently not able to remove directory on signal. The
>>>>> support
>>>>>        routines simply ignore directory.
>>>>>
>>>>>    3. Add one new API lto_codegen_get_files_need_remove().
>>>>>       Linker and LTO plugin will communicate via this API about which
>>>>> files
>>>>>      (including directories) need to removed before linker exit.
>>>>>
>>>>
>>>> Please revert. Adding new flags to libLTO is the wrong direction (in
>>>> spite of the ones that exist -- consider those grandfathered in).
>>>>
>>>  It dose not make sense. Without flags, how do you manage to triage the
>>> correctness and performance problem?
>>>
>>
>>  Something else has flags,
>>
>>
>>  What are "something else"?  As far as I know, there are only two fall
>> into this category:
>>
>>    - Apple linker, and
>>    - GNU gold.
>>
>>   The former communicate with the libLTO directly with these APIs, while
>> GNU gold communicate
>> with the libLTO via, which I called adapter.
>>
>
>  You committed the patch, you tell me. What did this patch intend to
> change the behaviour of?
>
> As I already told you, the only "something else" I'm aware of are Apple
> linker and GNU linker.
>
> How can change their behavior, they are released as binary.
>
> I did a new API, which dose not affect Apple ld. Of course, it dose not
> change GNU ld either.
> However, we need to change gold-plugin (tool/gold/gold-plugin.cpp) a lit,
> such that,
>
>    it get a list of files to be removed from libLTO.xxx.
>
>    Prior to my change, the gold-plugin.cpp delete the intermediate object
> file as it has only
> one intermediate file. It was working ok, but not anymore.
>
>   The linker see no difference.
>
>
>
>
>     Directly calling these APIs is really bad idea. I manage to convince
>> the black-belt guru Nick @ Apple to
>> not directly calling these APIs in the new ld design. The linker's should
>> be LTO oblivious, the linker
>> should expose symbol-related interface instead of LTO-control interfaces.
>>
>
>  Directly calling which APIs?
>
>  What do you mean by "LTO oblivious"? Oblivious towards whether
> optimization is being performed?
>
>
> The linker dose not have any functions regarding lto,  take a look of GNU
> gold API or tool/gold/*.cpp
>
>
>
>   >which in turn drives libLTO through the API.
>>
>>  Depending on the what kind of info "something" else need to drive the
>> libLTO.
>> In general it is very bad idea, if "something else" need micro-management.
>>
>
>  libLTO is part of the linker that uses it.
>
>
> No! Absolutely not!
>

Fair enough. I meant "libLTO is part of the linker that uses it" in the
same sense that a networking library is part of the web browser that uses
it. The library shouldn't be off deciding to do things of its own accord,
it should provide an API that allows something else to accomplish its task.

I don't see this as a very bad idea or as micro-management. In this regard,
I don't see a difference between libLTO and other libraries like libPNG or
netlib or freetype. (There is a difference in that we want libLTO to be a
very high-level interface instead of exposing the details of .bc files,
entirely unlike what libPNG does for PNG or freetype does for font files.)

>   Having a default setting with the ability to override it is a sensible
> convenience for users of libLTO.
>
>
>
>
>  Take Apple ld as example,  if I want to change LTO in a way such that I
>> don't want to load all module,
>> I just want to load summary info. Current APIs are not sufficient. I have
>> to modify the API, or add new APIs
>> to that matter, in the mean time, I need release the new ld to the user
>> in order to accomodate the change.
>> that is nightmare.
>>
>
>  The point of libLTO is to provide an ABI-fixed library, isolating the
> linker from llvm's internals.
>
> It is not "fixed", it is changing constantly.
>

The only reason libLTO exists at all is to give the linker something to
link against which will have a fixed ABI. Same with "libclang" on the clang
side.

Thus far it's LLVM policy that the *whole and entire* C API is ABI-fixed
forever, and I've argued a few times on the mailing list that this can't be
right, and that only libLTO and libClang ought to be ABI locked.

E.g. the APIs used to take for granted the libLTO return only one objects,
> now I need to return multiple.
>

Yes, and that's a problem. Not your problem really, except to the degree
that you inherited it. The existing APIs in libLTO weren't nearly
forwards-compatible enough, and now we're in trouble.

Unless we figure out something clever, we may have to add a whole new set
of functions to libLTO, and not deprecate the existing ones (at least, not
unless we get consensus on llvm-dev that it's okay to break our previous
ABI promise).

> That in turn leads to a few design decisions. The API is designed to refer
> to high-level concepts instead of the details of llvm's actual behaviour.
> Things like module lazy loading or setting the datalayout are excluded from
> the API. Flags are even more private, surely we should be able to change
> flags in LLVM's libraries without worrying about breaking linkers.
>
>  If the linker needs to do something where it matters how llvm is
> implemented -- you mention loading summary info, I'll assume you mean
> lazy-loading the module such that function bodies aren't loaded -- then the
> linker doesn't use libLTO at all, but uses llvm directly. Conversely,
> libLTO knows all about llvm and will lazy-load .bc files without being
> asked to.
>
>  Sure, "something else" can control the libLTO, if it want. In my case,
>> if "something else" want specify
>>  a workdir, then go ahead. Otherwise, the libLTO use default one. Is
>> there any wrong here?
>>
>
>  At a high level that sounds fine to me. The wrong part is using flags to
> do it.
>
> then how to change the behavior for say, debugging purpose.
>

Debugging is special. In theory, you don't even need to commit to upstream
for debugging, but it's fine to add features that are helpful. We have that
sort of thin all over llvm. libLTO has addDebugOptions to permit this sort
of debugging usage, but it shouldn't be used in the non-debugging case.

>  Adding flags to linker instead, I think that is wrong direction. Linker
>>> dose not have data structure which libLTO dose.
>>
>>
>>  This is the discussion to have. What things do you need here which you
>> don't think should be exposed through the API, and yet you want to be
>> exposed for you?
>>
>>  I actually discuss with Nick @ Apple before.  The conclusion is linker
>> must be LTO oblivious,
>> it should think in symbol-way, and talk in symbol way (as with GNU gold).
>> It would otherwise
>>  very very troublesome both for linker and libLTO.
>>
>
>  And now you're discussing it with me. I also agree that the linker
> should communicate primarily in symbols and about symbols with libLTO.
>
>  On the other hand, we now have two linkers support LTO. There are
>> different way to control
>> the libLTO (even for simple task, like save intermediate files), how
>> messy?
>>
>> I'd like to move all these stuff to libLTO to have a unified control.
>>
>
>  I have no problem with a unified control.
>
>>     libLTO is intended to be used as a library, it may not get a chance
>>> to parse flags.
>>>  It has to. Prior to my change, linkers (GNU linker and Apple ld) pass
>>> arch to linker, via a function
>>> confusingly called, something like "add.*debug.*options".
>>
>>
>>  Can't. If we allow this, every flag in every part of LLVM that libLTO
>> links against is baked into the C ABI forever.
>>
>>  Of course addDebugOptions does allow this, but it's named (and I
>> thought documented in the comments) such that anybody using it knows
>> they're using a non-stable non-production debugging API. Anybody using
>> addDebugOptions for something other than debugging libLTO is living outside
>> the ABI guarantees.
>>
>>  addDebugOptions is misnomer. It is also passes essential flags like
>> -arch=x86.  Without such flags,
>> the LTO dose not even compile.
>>
>
>  That sounds like a nice bug you've got there! Wouldn't want anything to
> happen to it. It'd be a shame if breaks before you manage to add a
> liblto_set_arch() function for it.
>
>
>   * Honestly, I looked and couldn't find a -arch flag that libLTO would
> interpret. How sure are you about this?
>
> Perhaps not -arch flags.
> But at least some flags are passed this way.  I remember we use this way
> to pass -fast-math before Bill's attribute-stuff is working.
>
>
>   In case it isn't completely clear, flags are absolutely right out.
> Either you will revert this patch, or I will revert it for you.
>
> I have no alternative.  If I introduce a workdir, I need to have to way to
> inform linker-plugin to get rid of way.
> This is another example why those API sucks.
>

You don't have the source code to the linker?

Let's focus on this, it sounds like this is the key problem. What's wrong
with modifying the linker if you want to change the behaviour of your
linker?

>    I'm sorry you decided to land three things together in one patch,
> please remember not to do that in the future.
>
>   Ok, tell me how to create temp workding directory right. How to save
> temp files right both for gold and Apple ld.
>

*Why*? Are you implementing this as a linker feature you intend to ship in
the real linker? Or is this to debug the innards of libLTO?

The only case I *am* okay with flags is when we all agree they're flags for
debugging the internals of libLTO, and that we don't ship products that
rely on them. I explicitly called that out. If the only purpose of these
was to implement debugging features, then I'm sorry for the
miscommunication!

If the intention is to let libLTO run on machines that don't have /tmp
(this is what I thought), we should give libLTO an API that lets the linker
decide where the files go. Maybe it wants to do smart things like putting
it in a directory with the right permissions, or which is scheduled for
cleanup in the event of a crash.

If the intention is to implement something like -r, or to implement some
new feature like -shared but which emits a shared-bitcode file instead of a
shared-object, then I don't think that controlling the working directory
then pulling out intermediate files is the right design anyways.

Or are you doing something else?

Nick
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