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

[Mehdi Amini via llvm-dev]

> 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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