llvm dev - Apr 2016 - RFC [ThinLTO]: Promoting more aggressively in order to reduce incremental link time and allow sharing between linkage units

> On Apr 7, 2016, at 10:58 AM, Xinliang David Li <davidxl at
google.com> wrote:
> 
> 
> 
> On Wed, Apr 6, 2016 at 9:53 PM, Mehdi Amini <mehdi.amini at apple.com
<mailto:mehdi.amini at apple.com>> wrote:
> 
>> On Apr 6, 2016, at 9:40 PM, Teresa Johnson <tejohnson at google.com
<mailto:tejohnson at google.com>> wrote:
>> 
>> 
>> 
>> On Wed, Apr 6, 2016 at 5:13 PM, Peter Collingbourne <peter at
pcc.me.uk <mailto:peter at pcc.me.uk>> wrote:
>> 
>> 
>> On Wed, Apr 6, 2016 at 4:53 PM, Mehdi Amini <mehdi.amini at
apple.com <mailto:mehdi.amini at apple.com>> wrote:
>> 
>>> On Apr 6, 2016, at 4:41 PM, Peter Collingbourne <peter at
pcc.me.uk <mailto:peter at pcc.me.uk>> wrote:
>>> 
>>> Hi all,
>>> 
>>> I'd like to propose changes to how we do promotion of global
values in ThinLTO. The goal here is to make it possible to pre-compile parts of
the translation unit to native code at compile time. For example, if we know
that:
>>> 
>>> 1) A function is a leaf function, so it will never import any other
functions, and
>> 
>> It still may be imported somewhere else right?
>> 
>>> 2) The function's instruction count falls above a threshold
specified at compile time, so it will never be imported.
>> 
>> It won’t be imported, but unless it is a “leaf” it may import and
inline itself.
>> 
>>> or
>>> 3) The compile-time threshold is zero, so there is no possibility
of functions being imported (What's the utility of this? Consider a program
transformation that requires whole-program information, such as CFI. During
development, the import threshold may be set to zero in order to minimize the
incremental link time while still providing the same CFI enforcement that would
be used in production builds of the application.)
>>> 
>>> then the function's body will not be affected by link-time
decisions, and we might as well produce its object code at compile time.
>> 
>> Reading this last sentence, it seems exactly the “non-LTO” case?
>> 
>> Yes, basically the point of this proposal is to be able to split the
linkage unit into LTO and non-LTO parts.
>> 
>> 
>>> This will also allow the object code to be shared between linkage
units (this should hopefully help solve a major scalability problem for
Chromium, as that project contains a large number of test binaries based on
common libraries).
>>> 
>>> This can be done with a change to the intermediate object file
format. We can represent object files as native code containing statically
compiled functions and global data in the .text,. data, .rodata (etc.) sections,
with an .llvmbc section (or, I suppose, "__LLVM, __bitcode" when
targeting Mach-O) containing bitcode for functions to be compiled at link time.
>>> 
>>> In order to make this work, we need to make sure that references
from link-time compiled functions to statically compiled functions work
correctly in the case where the statically compiled function has internal
linkage. We can do this by promoting every global value with internal linkage,
using a hash of the external names (as I mentioned in [1]).
>> 
>> 
>> Mehdi - I know you were keen to reduce the amount of promotion. Is that
still an issue for you assuming linker GC (dead stripping)?
> 
> Yes: we do better optimization on internal function in general. 
> 
> Inliner is one of the affected optimization -- however this sounds like a
matter of tuning to teach inliner about promoted static functions.
The inliner compute a tradeoff between pseudo runtime cost and binary size, the
existing bonus for static functions is when there is a single call site because
it makes the binary increase inexistant (dropping the static after inline). We
promote function because we think we are likely to introduce a reference to it
somewhere else, so “lying” to the inliner is not necessarily a good idea.
That said we (actually Bruno did) prototyped it already with somehow good
results :)
I’m not convinced yet that it should be independent of promoted or not promoted
though.

Assuming we solve the inliner issue, then remain the “optimizations other than
inliner”. We can probably solve most but I suspect it won’t be “trivial” either.

— 
Mehdi
>  
> 
> David
> 
>  
> Our benchmarks showed that it can really make some difference, and many
cases were ThinLTO didn’t perform as well as FullLTO were because of this
promotion.
> (binary size has never been my concern here)
>  
> 
>> With this proposal we will need to stick with the current promote
everything scheme.
> 
> I don’t think so: you would need do it only for “internal functions that a
leaf and aren’t likely to be imported/inlined”.
> That said any function that we emit the binary at compile time instead of
link time will contribute to inhibit optimizations for LTO/ThinLTO. The gain in
compile time has to be really important to make it worth it.
> (Of course the CFI use-case is a totally different tradeoff).
> 
> Peter: have you thought about debug info by the way?
> 
> — 
> Mehdi
> 
> 
> 
>>  
>>> 
>>> I imagine that for some linkers, it may not be possible to deal
with this scheme. For example, I did some investigation last year and discovered
that I could not use the gold plugin interface to load a native object file if
we had already claimed it as an IR file. I wouldn't be surprised to learn
that ld64 has similar problems.
>> 
>> I suspect ld64 would need to be update to handle this scheme. Somehow
it need to be able to extract the object from the section.
>> 
>> Do you mean the bitcode object? There's already support in LLVM for
this (http://llvm-cs.pcc.me.uk/lib/Object/IRObjectFile.cpp#269
<http://llvm-cs.pcc.me.uk/lib/Object/IRObjectFile.cpp#269>). I suspect the
tricky part (which I was unsuccessful at doing with gold) will be convincing the
linker that the bitcode object and the regular object are two separate things.
>> 
>> Otherwise this should work, but I suspect the applicability (leaving
CFI aside) may not concern that many functions, so I’m not sure about the
impact.
>> 
>> I'm also curious about the applicability in the regular ThinLTO
case. One thing that may help with applicability is that I suspect that not only
leaf functions but also functions that only call (directly or indirectly)
functions in the same TU, as well as functions that only make calls via function
pointers or vtables may fall under the criteria for non-importing.
>> 
>> With indirect call profiling the function pointer case should not be a
blocker for importing.
>> 
>> Teresa
>>  
>> 
>> Peter
>> 
>> 
>> 
>> -- 
>> Teresa Johnson |	 Software Engineer |	 tejohnson at google.com
<mailto:tejohnson at google.com> |	 408-460-2413 <tel:408-460-2413>
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On Thu, Apr 7, 2016 at 11:26 AM, Mehdi Amini <mehdi.amini at apple.com>
wrote:
>
> On Apr 7, 2016, at 10:58 AM, Xinliang David Li <davidxl at
google.com> wrote:
>
>
>
> On Wed, Apr 6, 2016 at 9:53 PM, Mehdi Amini <mehdi.amini at
apple.com> wrote:
>
>>
>> On Apr 6, 2016, at 9:40 PM, Teresa Johnson <tejohnson at
google.com> wrote:
>>
>>
>>
>> On Wed, Apr 6, 2016 at 5:13 PM, Peter Collingbourne <peter at
pcc.me.uk>
>> wrote:
>>
>>>
>>>
>>> On Wed, Apr 6, 2016 at 4:53 PM, Mehdi Amini <mehdi.amini at
apple.com>
>>> wrote:
>>>
>>>>
>>>> On Apr 6, 2016, at 4:41 PM, Peter Collingbourne <peter at
pcc.me.uk>
>>>> wrote:
>>>>
>>>> Hi all,
>>>>
>>>> I'd like to propose changes to how we do promotion of
global values in
>>>> ThinLTO. The goal here is to make it possible to pre-compile
parts of the
>>>> translation unit to native code at compile time. For example,
if we know
>>>> that:
>>>>
>>>> 1) A function is a leaf function, so it will never import any
other
>>>> functions, and
>>>>
>>>>
>>>> It still may be imported somewhere else right?
>>>>
>>>> 2) The function's instruction count falls above a threshold
specified
>>>> at compile time, so it will never be imported.
>>>>
>>>>
>>>> It won’t be imported, but unless it is a “leaf” it may import
and
>>>> inline itself.
>>>>
>>>
>>>> or
>>>> 3) The compile-time threshold is zero, so there is no
possibility of
>>>> functions being imported (What's the utility of this?
Consider a program
>>>> transformation that requires whole-program information, such as
CFI. During
>>>> development, the import threshold may be set to zero in order
to minimize
>>>> the incremental link time while still providing the same CFI
enforcement
>>>> that would be used in production builds of the application.)
>>>>
>>>> then the function's body will not be affected by link-time
decisions,
>>>> and we might as well produce its object code at compile time.
>>>>
>>>>
>>>> Reading this last sentence, it seems exactly the “non-LTO”
case?
>>>>
>>>
>>> Yes, basically the point of this proposal is to be able to split
the
>>> linkage unit into LTO and non-LTO parts.
>>>
>>>
>>>> This will also allow the object code to be shared between
linkage units
>>>> (this should hopefully help solve a major scalability problem
for Chromium,
>>>> as that project contains a large number of test binaries based
on common
>>>> libraries).
>>>>
>>>> This can be done with a change to the intermediate object file
format.
>>>> We can represent object files as native code containing
statically compiled
>>>> functions and global data in the .text,. data, .rodata (etc.)
sections,
>>>> with an .llvmbc section (or, I suppose, "__LLVM,
__bitcode" when targeting
>>>> Mach-O) containing bitcode for functions to be compiled at link
time.
>>>>
>>>> In order to make this work, we need to make sure that
references from
>>>> link-time compiled functions to statically compiled functions
work
>>>> correctly in the case where the statically compiled function
has internal
>>>> linkage. We can do this by promoting every global value with
internal
>>>> linkage, using a hash of the external names (as I mentioned in
[1]).
>>>>
>>>>
>> Mehdi - I know you were keen to reduce the amount of promotion. Is that
>> still an issue for you assuming linker GC (dead stripping)?
>>
>>
>> Yes: we do better optimization on internal function in general.
>>
>
> Inliner is one of the affected optimization -- however this sounds like a
> matter of tuning to teach inliner about promoted static functions.
>
>
> The inliner compute a tradeoff between pseudo runtime cost and binary
> size, the existing bonus for static functions is when there is a single
> call site because it makes the binary increase inexistant (dropping the
> static after inline). We promote function because we think we are likely to
> introduce a reference to it somewhere else, so “lying” to the inliner is
> not necessarily a good idea.
>
It is not lying to the inliner. If a static (before promotion) function is
a candidate to be inlined in the original defining module, it is probably
more likely to inlined in other importing modules where more context is
available. In other words, the inliner can apply the same bonus to
'promoted' static functions as if references in other modules will also
disappear.  Of course, we can not assume it has single callsite.

Comdat functions can be handled similarly.


> That said we (actually Bruno did) prototyped it already with somehow good
> results :)
> I’m not convinced yet that it should be independent of promoted or not
> promoted though.
>
Generally true (see the comdat case).

>
> Assuming we solve the inliner issue, then remain the “optimizations other
> than inliner”. We can probably solve most but I suspect it won’t be
> “trivial” either.
>

Any such optimizations in mind?

David

>
> —
> Mehdi
>
>
>
> David
>
>
>
>> Our benchmarks showed that it can really make some difference, and many
>> cases were ThinLTO didn’t perform as well as FullLTO were because of
this
>> promotion.
>> (binary size has never been my concern here)
>>
>
>
>>
>> With this proposal we will need to stick with the current promote
>> everything scheme.
>>
>>
>> I don’t think so: you would need do it only for “internal functions
that
>> a leaf and aren’t likely to be imported/inlined”.
>> That said any function that we emit the binary at compile time instead
of
>> link time will contribute to inhibit optimizations for LTO/ThinLTO. The
>> gain in compile time has to be really important to make it worth it.
>> (Of course the CFI use-case is a totally different tradeoff).
>>
>> Peter: have you thought about debug info by the way?
>>
>> —
>> Mehdi
>>
>>
>>
>>
>>
>>>
>>>> I imagine that for some linkers, it may not be possible to deal
with
>>>> this scheme. For example, I did some investigation last year
and discovered
>>>> that I could not use the gold plugin interface to load a native
object file
>>>> if we had already claimed it as an IR file. I wouldn't be
surprised to
>>>> learn that ld64 has similar problems.
>>>>
>>>>
>>>> I suspect ld64 would need to be update to handle this scheme.
Somehow
>>>> it need to be able to extract the object from the section.
>>>>
>>>
>>> Do you mean the bitcode object? There's already support in LLVM
for this
>>> (http://llvm-cs.pcc.me.uk/lib/Object/IRObjectFile.cpp#269). I
suspect
>>> the tricky part (which I was unsuccessful at doing with gold) will
be
>>> convincing the linker that the bitcode object and the regular
object are
>>> two separate things.
>>>
>>> Otherwise this should work, but I suspect the applicability
(leaving CFI
>>>> aside) may not concern that many functions, so I’m not sure
about the
>>>> impact.
>>>>
>>>
>>> I'm also curious about the applicability in the regular ThinLTO
case.
>>> One thing that may help with applicability is that I suspect that
not only
>>> leaf functions but also functions that only call (directly or
indirectly)
>>> functions in the same TU, as well as functions that only make calls
via
>>> function pointers or vtables may fall under the criteria for
non-importing.
>>>
>>
>> With indirect call profiling the function pointer case should not be a
>> blocker for importing.
>>
>> Teresa
>>
>>
>>>
>>> Peter
>>>
>>
>>
>>
>> --
>> Teresa Johnson |  Software Engineer |  tejohnson at google.com |
>> 408-460-2413
>>
>>
>
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> On Apr 7, 2016, at 11:59 AM, Xinliang David Li <davidxl at
google.com> wrote:
> 
> 
> 
> On Thu, Apr 7, 2016 at 11:26 AM, Mehdi Amini <mehdi.amini at apple.com
<mailto:mehdi.amini at apple.com>> wrote:
> 
>> On Apr 7, 2016, at 10:58 AM, Xinliang David Li <davidxl at
google.com <mailto:davidxl at google.com>> wrote:
>> 
>> 
>> 
>> On Wed, Apr 6, 2016 at 9:53 PM, Mehdi Amini <mehdi.amini at
apple.com <mailto:mehdi.amini at apple.com>> wrote:
>> 
>>> On Apr 6, 2016, at 9:40 PM, Teresa Johnson <tejohnson at
google.com <mailto:tejohnson at google.com>> wrote:
>>> 
>>> 
>>> 
>>> On Wed, Apr 6, 2016 at 5:13 PM, Peter Collingbourne <peter at
pcc.me.uk <mailto:peter at pcc.me.uk>> wrote:
>>> 
>>> 
>>> On Wed, Apr 6, 2016 at 4:53 PM, Mehdi Amini <mehdi.amini at
apple.com <mailto:mehdi.amini at apple.com>> wrote:
>>> 
>>>> On Apr 6, 2016, at 4:41 PM, Peter Collingbourne <peter at
pcc.me.uk <mailto:peter at pcc.me.uk>> wrote:
>>>> 
>>>> Hi all,
>>>> 
>>>> I'd like to propose changes to how we do promotion of
global values in ThinLTO. The goal here is to make it possible to pre-compile
parts of the translation unit to native code at compile time. For example, if we
know that:
>>>> 
>>>> 1) A function is a leaf function, so it will never import any
other functions, and
>>> 
>>> It still may be imported somewhere else right?
>>> 
>>>> 2) The function's instruction count falls above a threshold
specified at compile time, so it will never be imported.
>>> 
>>> It won’t be imported, but unless it is a “leaf” it may import and
inline itself.
>>> 
>>>> or
>>>> 3) The compile-time threshold is zero, so there is no
possibility of functions being imported (What's the utility of this?
Consider a program transformation that requires whole-program information, such
as CFI. During development, the import threshold may be set to zero in order to
minimize the incremental link time while still providing the same CFI
enforcement that would be used in production builds of the application.)
>>>> 
>>>> then the function's body will not be affected by link-time
decisions, and we might as well produce its object code at compile time.
>>> 
>>> Reading this last sentence, it seems exactly the “non-LTO” case?
>>> 
>>> Yes, basically the point of this proposal is to be able to split
the linkage unit into LTO and non-LTO parts.
>>> 
>>> 
>>>> This will also allow the object code to be shared between
linkage units (this should hopefully help solve a major scalability problem for
Chromium, as that project contains a large number of test binaries based on
common libraries).
>>>> 
>>>> This can be done with a change to the intermediate object file
format. We can represent object files as native code containing statically
compiled functions and global data in the .text,. data, .rodata (etc.) sections,
with an .llvmbc section (or, I suppose, "__LLVM, __bitcode" when
targeting Mach-O) containing bitcode for functions to be compiled at link time.
>>>> 
>>>> In order to make this work, we need to make sure that
references from link-time compiled functions to statically compiled functions
work correctly in the case where the statically compiled function has internal
linkage. We can do this by promoting every global value with internal linkage,
using a hash of the external names (as I mentioned in [1]).
>>> 
>>> 
>>> Mehdi - I know you were keen to reduce the amount of promotion. Is
that still an issue for you assuming linker GC (dead stripping)?
>> 
>> Yes: we do better optimization on internal function in general. 
>> 
>> Inliner is one of the affected optimization -- however this sounds like
a matter of tuning to teach inliner about promoted static functions.
> 
> The inliner compute a tradeoff between pseudo runtime cost and binary size,
the existing bonus for static functions is when there is a single call site
because it makes the binary increase inexistant (dropping the static after
inline). We promote function because we think we are likely to introduce a
reference to it somewhere else, so “lying” to the inliner is not necessarily a
good idea.
> 
> It is not lying to the inliner. If a static (before promotion) function is
a candidate to be inlined in the original defining module, it is probably more
likely to inlined in other importing modules where more context is available. In
other words, the inliner can apply the same bonus to 'promoted' static
functions as if references in other modules will also disappear.  Of course, we
can not assume it has single callsite.
> 
> Comdat functions can be handled similarly. 
> 
>  
> That said we (actually Bruno did) prototyped it already with somehow good
results :)
> I’m not convinced yet that it should be independent of promoted or not
promoted though.
> 
> Generally true (see the comdat case).
>  
> 
> Assuming we solve the inliner issue, then remain the “optimizations other
than inliner”. We can probably solve most but I suspect it won’t be “trivial”
either.
> 
> 
> Any such optimizations in mind?
I don’t have the details, but in short:

For promoted functions: IPSCCP, dead arg elimination
For promoted global variables: anything that is impacted somehow by aliasing

— 
Mehdi

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