[llvm-dev] Emit LLVM bitcode after ThinLTO

[Teresa Johnson via llvm-dev]

Looks like I forgot to reply all so my reply and our follow on conversation
got lost. Copied it below. Basically, this is expected because -emit-llvm
only emits the result of the regular LTO link, which is typically empty for
a normal ThinLTO build.

Teresa

On Sun, Jul 12, 2020 at 10:16 AM Teresa Johnson <tejohnson at google.com>
wrote:

>
>
> On Sun, Jul 12, 2020 at 10:05 AM Stephan Z <stephan.yichao.zhao at gmail.com>
> wrote:
>
>> Hi Teresa,
>>
>> Thank you for your quick response.
>>
>> I have a program that analyzes the bitcode generated from "-flto
>> -Wl,-plugin-opt,emit-llvm" to analyze code across modules.
>>
>> "-flto=thin -Wl,-plugin-opt,save-temps" dumps those bc files for each
>> module. This does not provide a bitcode for all modules.
>>
>> I was trying ThinLTO because LTO uses too much memory and is slow, and ThinLTO
>> blog shows it uses less memory and time.
>> With ThinTLO, do we still have a way to get the entire bitcode after
>> link? This allows reusing my program.
>> Or does ThinLTO recommend other ways to do cross module analysis?
>>
>
> ThinLTO is fast and scalable because it does not combine all bitcode into
> a single module. It does whole program analysis on compact summaries of the
> input modules (during the "thin link"). You'd have to rewrite your analysis
> to operate on the combined summary index, either during the thin link, or
> on the serialized combined index (*.index.bc dumped by the save-temps
> option). But depending on what you are doing, you might need to enhance the
> summary info. See ModuleSummaryIndex.h for the info it currently has.
>
> Teresa
>
>
>> Thank you, S
>>
>>
>> On Sun, Jul 12, 2020 at 8:13 AM Teresa Johnson <tejohnson at google.com>
>> wrote:
>>
>>> -emit-llvm only emits the result of the regular LTO link, which is why
>>> it is very small for your ThinLTO link (which supports having some amount
>>> of regular LTO objects mixed in, and also split thin and regular LTO
>>> bitcode modules, but by default for ThinLTO this regular LTO combined
>>> module will be small and effectively empty).
>>>
>>> For ThinLTO there are going to be multiple modules. To get the bitcode
>>> for them you could use  -Wl,-plugin-opt,save-temps. This will give you
>>> more than you want, as it dumps the bitcode for each module at multiple
>>> places in the ThinLTO backends. The files end in .bc and the suffixes
>>> include the name of the phase where they were emitted:
>>> - preopt.bc  (before any optimization)
>>> - promote.bc (after ThinLTO local to global promotion)
>>> - internalize.bc (after internalization)
>>> - import.bc (after ThinLTO importing)
>>> - opt.bc (after optimization pipeline but before codegen)
>>> - precodegen.bc (usually the same as opt.bc)
>>>
>>> Are you debugging or trying to do something else?
>>>
>>> Teresa
>>>
>>
On Mon, Jul 20, 2020 at 12:42 PM David Blaikie <dblaikie at gmail.com> wrote:

> I think there is a way, but I'm not entirely sure.
>
> It's possible you've got the right thing & it's just that ThinLTO's
> optimized away most of the file you're looking at?
>
> On Sat, Jul 11, 2020 at 9:53 PM Stephan Z via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> >
> > Hi,
> >
> > I wanted to get the linked result in LLVM bitcode format.
> >
> > With LTO, this can be done by -flto -Wl,-plugin-opt,emit-llvm. Instead
> of generating native executables, it outputs a file with bitcode format.
> >
> > Does this still work with -flto=thin? -flto=thin
> -Wl,-plugin-opt,emit-llvm outputs a bitcode file, but its file size is too
> small, and does not contain all contents.
> >
> > What is the correct way to emit LLVM bitcode after ThinLTO?
> >
> > Thank you, S
> >
> > _______________________________________________
> > 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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