[llvm-dev] [RFC] Clean up the way we store optional Function data

Vedant Kumar via llvm-dev
Wed, 21 Oct 2015 21:25:45 -0000

I've done some measurements on this.

The test program I have just calls Function::Create(), F->setPersonalityFn(),
and then F->eraseFromParent() in a loop 2^20 times.

Results:

    pre-patch  --- min: 1.10s    max: 1.13s    avg: 1.11s
    post-patch --- min: 1.26s    max: 1.35s    avg: 1.29s

So we expect to lose 0.2 seconds per 1 million functions (with personality
functions) in a project. I've only tested on my machine, and I haven't
accounted for performance variations in other parts of the codebase where code
was removed.

I'm happy enough with the patch and think that this is a reasonable
tradeoff. Should we go with this?

http://reviews.llvm.org/D13829

vedant


> On Oct 16, 2015, at 5:56 PM, Duncan P. N. Exon Smith <dexonsmith at
apple.com> wrote:
>
>
>> On 2015-Oct-16, at 13:54, Vedant Kumar <vsk at apple.com> wrote:
>>
>> Here is a WIP patch as promised:
>>
>>   http://reviews.llvm.org/D13829
>>
>> It uses a hungoff uselist to store optional data as needed.
>>
>> Some early objections from Duncan:
>>
>>   - An extra one-time malloc() is required to set personality
functions.
>>   - We get and set personality functions frequently. This patch
introduces a level of indirection which slows the common case down.
>>
>> Is this overhead acceptable?
>
> If the overhead isn't bad, then this SGTM.  (I haven't looked at
the
> patch.)
>
> I guess I was thinking about large applications that use C++ exceptions
> and build with LTO, but even there it's a per-function overhead.  I
> doubt it really matters.
>
>> If not, maybe we could leave personality function handling untouched
and add a "Constant **OptionalData" array to Function.
>
> The `IndirectUser` approach you outlined initially might be better
> than this?
>
>> vedant
>>
>>
>>> On Oct 14, 2015, at 3:12 PM, Vedant Kumar <vsk at apple.com>
wrote:
>>>
>>> I like the idea of using hung off uses.
>>>
>>> We can keep using SubclassData to indicate whether or not some
optional data is present.
>>>
>>> Benefits: zero space overhead until some optional data is set, we
can get rid of the DenseMaps in LLVMContextImpl, and RAUW just works (so no
clang changes are needed).
>>>
>>> I'll have a patch up before the end of the week.
>>>
>>> thanks
>>> vedant
>>>
>>>
>>>> On Oct 12, 2015, at 12:15 PM, Sanjoy Das via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>>>>
>>>>
>>>>
>>>> David Majnemer wrote:
>>>>>
>>>>>
>>>>> On Mon, Oct 12, 2015 at 11:00 AM, Duncan P. N. Exon Smith
via llvm-dev
>>>>> <llvm-dev at lists.llvm.org <mailto:llvm-dev at
lists.llvm.org>> wrote:
>>>>>
>>>>>
>>>>>> On 2015-Oct-12, at 10:41, Sanjoy Das
>>>>> <sanjoy at playingwithpointers.com
>>>>> <mailto:sanjoy at playingwithpointers.com>> wrote:
>>>>>>
>>>>>>
>>>>>>
>>>>>> Vedant Kumar wrote:
>>>>>>>>> That's a neat idea. To summarize: make
Function have 3 optional
>>>>> operands. (For context -- Function currently has 1 optional
operand,
>>>>> and my proposal is to move to 0.)
>>>>>>>>>
>>>>>>>>> Could someone else chime in on what
they'd like to see?
>>>>>>>> Sanjoy's idea makes sense to me, but only
if we never need to add
>>>>>>>> prefix/prologue data after functions are
created.  Are there any
>>>>> places
>>>>>>>> where we need/want to add them after the fact?
>>>>>>>
>>>>>>> I think so. I see:
>>>>>>>
>>>>>>> LinkModules.cpp:
>>>>> Dst.setPrefixData(MapValue(Src.getPrefixData(), ValueMap,
>>>>>>> BitcodeReader.cpp:
>>>>>
FunctionPrologueWorklist.back().first->setPrologueData(C);
>>>>>>> InlineFunction.cpp:
Caller->setPersonalityFn(CalledPersonality);
>>>>>>>
>>>>>>> Some of these sites could be refactored so that the
Functions are
>>>>> created with the prefix/prologue data they need. I
don't think
>>>>> that's possible for personality functions (see my third
example).
>>>>>>>
>>>>>>> Would we inhibit any future patches which add
prefix/prologue
>>>>> data to Functions on the fly by taking this approach?
>>>>>>
>>>>>> You should always be able to create a new
`llvm::Function`
>>>>> instance (and RAUW it in) if you want to add
prefix/prologue data to
>>>>> functions after they've been created; just like you
have to do today
>>>>> for any other `llvm::User`s that do not have hung off uses.
>>>>>
>>>>> It's possible, but a lot more involved with
`Function`s.  Besides
>>>>> RAUW, you need to transfer over all the basic blocks.
>>>>>
>>>>> This seems kind of wrong to me, if we expect it to happen.
>>>>>
>>>>>> Which brings me to -- can you use hung off uses for
this?  These
>>>>> use lists can be resized on the fly, so you should be able
to add
>>>>> and remove prologue data on the fly.  If you're using
hung off uses,
>>>>> you'll probably still need a descriptor to remember
whether / which
>>>>> operands are prologue data etc.
>>>>>
>>>>> Sure, this is another option.  It might be simplest.
I'd be
>>>>> tempted to start with a 0/3 choice (if we allocate any
hung-off
>>>>> uses, allocate enough for all three operands) to simplify
the
>>>>> logic.  Although I can't remember right now whether
that's
>>>>> legal (having nullptr operands followed by real ones)...
>>>>>
>>>>>>>>>>> Personalities are stored as
``optional`` Function operands.
>>>>> We actually always
>>>>>>>>>>> allocate the space for this
``optional`` operand: there's a
>>>>> FIXME in the
>>>>>>>>>>> destructor for Function about this.
>>>>>
>>>>> Makes me wonder, why didn't we use hung off uses to
begin with?
>>>>> Do functions "usually" have personality
functions, for some
>>>>> definition of?
>>>>>
>>>>>
>>>>> Depends. In C++? It's pretty common to have objects
which have
>>>>> non-trivial destructors on the stack which means calling a
function will
>>>>> be an invoke which will require the function to have a
personality. In
>>>>> C? It's pretty rare. You'd need something like
__attribute__((cleanup))
>>>>> to do it, the most common source of this will be something
>>>>> like pthread_cleanup_push. If I recall correctly, Julia
sets the
>>>>> personality on functions regardless of whether or not there
are any
>>>>> invokes, they need the AsmPrinter to scribble something
down.  I can't
>>>>> say for other languages (Rust, etc.).  From what I
understand, Swift
>>>>> doesn't use landingpad for EH so they wouldn't need
the personality set.
>>>>
>>>> Most functions we emit from our Java frontend have
personalities.
>>>>
>>>> -- Sanjoy
>>>>
>>>>>
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lists.llvm.org>
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>>>>>
>>>>>
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