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

[Gábor Horváth via llvm-dev]

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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