[llvm-dev] ThinLTO Problem

[Teresa Johnson via llvm-dev]

Hi Mindong,

On Wed, Nov 27, 2019 at 3:29 AM chenmindong via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi,
>
> I'm working on enabling thinLTO for our custom backend on LLVM-8 with lld
> to get code size benefits from dead symbol elimination. The code in
> LTO::run() of LTO.cpp confuses me that, even though thinLTO is specified,
> runRegularLTO() will be run first and its return value determines whether
> runThinLTO() will be executed.
>
> My question is if it's clearly known that thinLTO is used, is it still
> necessary to execute runRegularLTO()?If it is, what's the reason behind?


This is to handle the case where the LTO link is given a mix of regular and
thin LTO bitcode - it should do regular LTO on that subset and ThinLTO on
the other. The other case this handles is when a bitcode object is split
into regular and thin LTO halves - this is enabled for things like CFI but
shouldn't be the default currently (you'd have to build with
-fsplit-lto-unit to get it unless you are building with CFI).

Whether the bitcode is added to the regular or Thin LTO partition is
determined in LTO::addModule, and is based on a flag set when the bitcode
is read which is based in turn on whether the bitcode has a summary block,
and whether that is a thinlto or regular (full) LTO summary block. How are
you creating your bitcode files? If you run llvm-dis on it does it have
summary entries? You can also see if you run llvm-bcanalyzer whether it has
a GLOBALVAL_SUMMARY_BLOCK or a FULL_LTO_GLOBALVAL_SUMMARY_BLOCK or neither.
If you are compiling with -flto=thin you should realistically have the
former, which would make it a ThinLTO bitcode. But it sounds like you have
some objects that either don't or have the full LTO summary.

For now our custom backend, where distributed thinLTO is adopted, it works
> fine as I removed the line executing runRegularLTO(). But if I preserve it,
> the code fails the `if (Conf.PostInternalizeModuleHook
> &&!Conf.PostInternalizeModuleHook(0, *RegularLTO.CombinedModule))`, which I
> also don't understand, and fall through to backend() and abort there. I
> believe something is missed during adding the target support but cannot
> figure it out. Could anyone help?
>

By default there should not be a PostInternalizeModuleHook set (it is set
to support cases like -save-temps), so it isn't surprising that it would
fail that test and fall through to the backend() call, which is what you
would want if there was a regular LTO partition. Where is it aborting in
the backend?

Teresa


> Regards,
> Mindong Chen
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>


-- 
Teresa Johnson |  Software Engineer |  tejohnson at google.com |
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