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

Teresa Johnson via llvm-dev llvm-dev at lists.llvm.org
Mon Jan 27 18:05:06 PST 2020

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.


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