[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 llvm-dev at lists.llvm.org
Tue May 3 22:01:55 PDT 2016

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.

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.


> --
> 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

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