[llvm-dev] Where does LTO remove unused functions?

[Gábor Horváth via llvm-dev]

Thanks, this approach did work! I created https://reviews.llvm.org/D73597 in
case this is something we want to have upstream :)

On Tue, Jan 28, 2020 at 12:45 PM Teresa Johnson <tejohnson at google.com>
wrote:

>
>
>
> On Tue, Jan 28, 2020 at 11:32 AM Gábor Horváth <xazax at google.com> wrote:
>
>> I looked into this and printing the list of functions omitted from the
>> combined module is pretty straightforward. My only problem is that I am not
>> sure what is the most idiomatic way to surface this information for the
>> user. I initially wanted to add an optimization remark, but optimization
>> remarks are set up later on in the LTO backend (just before running opt).
>>
>> Do you think it is ok to set the remarks up earlier or should I plumb the
>> list of removed symbols and emit them later? Or is there another way to
>> surface this information?
>>
>
> +Peter for thoughts.
> That's a good question. A few possibilities come to mind. You could add a
> custom internal option to LTO.cpp to emit the list of dead functions.  If
> you do want to use the opt remarks infrastructure, which isn't a bad idea,
> you will need to add the setup earlier as you noted. However, note that
> this is only possible for regular LTO, not any ThinLTO modules, since we
> don't have those until later. What I would suggest in this case is to move
> the invocation of linkRegularLTO for modules with summaries into
> runRegularLTO and do the setup there, to minimize the handling along both
> LTO type paths. It looks like this should be doable.
>
>
>> Thanks,
>> Gabor
>>
>> On Mon, Jan 27, 2020 at 6:26 PM Gábor Horváth <xazax at google.com> wrote:
>>
>>> Thanks!
>>>
>>> On Mon, Jan 27, 2020 at 6:05 PM Teresa Johnson <tejohnson at google.com>
>>> wrote:
>>>
>>>> By default even regular LTO now has module summaries (like the kind
>>>> used for ThinLTO). LTO will then run index based dead symbol analysis here:
>>>> http://llvm-cs.pcc.me.uk/lib/LTO/LTO.cpp#923. Then when linkRegularLTO
>>>> is called here: http://llvm-cs.pcc.me.uk/lib/LTO/LTO.cpp#935, it
>>>> indicates that the index should be consulted for liveness, and that routine
>>>> skips even adding the dead symbols to the Keep set. So they never make it
>>>> into the combined module.
>>>>
>>>> Teresa
>>>>
>>>> On Mon, Jan 27, 2020 at 5:43 PM Gábor Horváth via llvm-dev <
>>>> llvm-dev at lists.llvm.org> wrote:
>>>>
>>>>> Hi!
>>>>>
>>>>> LLVM newbie here, I was mainly working on the frontend so far. We had
>>>>> a small hackathon project idea to piggyback on LTO to detect dead code
>>>>> (unused functions). The basic idea is to compile the code for every target
>>>>> and dump the removed functions. Intersect the function symbol names for
>>>>> each target and those functions should be safe to remove from the source
>>>>> code (unless there were some configuration that was not compiled). Is this
>>>>> reasonable?
>>>>>
>>>>> I started to play around with some toy examples and got stuck on the
>>>>> very beginning not being able to figure out where the unused functions are
>>>>> actually getting removed.
>>>>>
>>>>> Here is what I did:
>>>>> tu1.cpp:
>>>>> int unused(int a);
>>>>> int probably_inlined(int a);
>>>>> int main(int argc, const char *argv[]) {
>>>>>   return probably_inlined(argc);
>>>>> }
>>>>>
>>>>> tu2.cpp:
>>>>> int unused(int a) {
>>>>>   return a + 1;
>>>>> }
>>>>> int probably_inlined(int a) {
>>>>>   return a + 2;
>>>>> }
>>>>>
>>>>> I produced two object files with bitcode:
>>>>> clang -c -flto tu1.cpp -o tu1.o
>>>>>
>>>>> And I run LTO and attempted to dump the IR before each pass:
>>>>> clang -O2 -Wl,-mllvm -Wl,-print-before-all tu1.o tu2.o -o optimized
>>>>>
>>>>> In my dumps I saw the function unused removed even before the first
>>>>> pass. Where did that happen? I tried to invoke llvm-link manually and
>>>>> that did not remove unused.
>>>>>
>>>>> I also tried to dump optimization remarks and was no trace of a
>>>>> function being removed (I only saw a function being inlined).
>>>>>
>>>>> Thanks in advance,
>>>>> Gabor
>>>>> _______________________________________________
>>>>> 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 |
>>>>
>>>
>
> --
> Teresa Johnson |  Software Engineer |  tejohnson at google.com |
>
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