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

[Gábor Horváth via llvm-dev]

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 |
>
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