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

Tue May 3 22:53:23 PDT 2016

On Tue, May 3, 2016 at 10:35 PM, Mehdi Amini <mehdi.amini at apple.com> wrote:

>
> On May 3, 2016, at 10:25 PM, Peter Collingbourne <peter at pcc.me.uk> wrote:
>
> On Tue, May 3, 2016 at 10:04 PM, Mehdi Amini <mehdi.amini at apple.com>
> wrote:
>
>>
>> On May 3, 2016, at 10:01 PM, Peter Collingbourne <peter at pcc.me.uk> wrote:
>>
>>
>>
>> On Tue, May 3, 2016 at 9:01 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
>>> 2) The function's instruction count falls above a threshold specified at
>>> compile time, so it will never be imported.
>>> 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. 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.
>>>
>>>
>>> I was wondering why can't the "precompiled" function be embedded in the
>>> IR instead of the bitcode embedded in the object file?
>>> The codegen would still emit a single object file out of this IR file
>>> that contains the code for the IR and the precompiled function.
>>>
>>> It seems to me that this way the scheme would work with any existing
>>> existing LTO implementation.
>>>
>>
>> You'd still have the same problem. No matter whether you put the native
>> object inside the IR file or vice versa, you still have a file containing a
>> native object and some IR. That's the scenario that I found that the gold
>> plugin interface wouldn't support.
>>
>>
>> It is not clear to me why it is a problem for gold: it does not need to
>> know that the IR file contains some native precompiled code: it only need
>> to know that this is an "LLVM file", that will be passed to LLVM for LTO
>> and it will get a single object file in return.
>> Can you elaborate why the linker need to know beforehand and
>> differentiate?
>>
>
> (There wouldn't just be one object file, there would be N native objects
> and 1 (or N if ThinLTO) combined LTO objects.)
>
>
> No really, that's not what I described. I described a mode where even llc
> would be able to process this input IR file with an embedded precompiled
> function and spit out a single object file.
> Conceptually, it should be similar to a naked IR function with inline
> assembly for instance, or module level inline assembly, i.e. totally
> transparent to the linker.
>

That doesn't seem like a good idea to me. Each embedded precompiled
function would need relocations, debug info, unwind info etc. in some form
or another. Given that the whole point of embedding precompiled code is to
reduce the work required by the linker, they would probably need to be a
form close to what will appear in the native object file. At that point
you're basically just inventing another native object format, and we
already have enough of them.

Peter

> --
> Mehdi
>
>
>
> In principle, it doesn't need to know. In practice, I found that in my
> prototype I couldn't persuade gold to accept what I was doing without
> giving undefined symbol errors.
>
> I suppose I could have debugged it further, but I couldn't justify
> spending more time on it, since the projects I care about are interested in
> switching to lld for other reasons.
>
> Peter
>
>
>
>>
>> --
>> Mehdi
>>
>>
>> Supporting IR embedded in a native object section inside a linker should
>> be pretty trivial, if you control the linker. My prototype implementation
>> in lld is about 10 lines of code.
>>
>> Peter
>>
>>
>>> --
>>> Mehdi
>>>
>>>
>>>
>>>
>>> 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]).
>>>
>>> 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.
>>>
>>> In cases where we completely control the linker (e.g. lld), we can
>>> easily support this scheme, as the linker can directly do whatever it
>>> wants. But for linkers that cannot support this, I suggest that we promote
>>> consistently under ThinLTO rather than having different promotion schemes
>>> for different linkers, in order to reduce overall complexity.
>>>
>>> Thanks for your feedback!
>>>
>>> Thanks,
>>> --
>>> --
>>> Peter
>>>
>>> [1] http://lists.llvm.org/pipermail/llvm-dev/2016-April/098062.html
>>>
>>>
>>>
>>
>>
>> --
>> --
>> Peter
>>
>>
>>
>
>
> --
> --
> Peter
>
>
>

-- 
-- 
Peter
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