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

[Xinliang David Li via llvm-dev]

On Wed, 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).
>


3) sounds like a good use case (though very unique to CFI. Generally, a
function body will be affected by link time decisions including whole
program analyses).  In this mode, do we really need to emit the bit code
section at all? There does not any need for static promotions if we assume
no cross module transformations will happen in this mode.


David


>
> 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.
>
> 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
>
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