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

[Peter Collingbourne via llvm-dev]

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]).
>
> 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.

Peter
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