[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 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?

> 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.

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.

— 
Mehdi


> 
> 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 <http://lists.llvm.org/pipermail/llvm-dev/2016-April/098062.html>
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