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

Alex Trebek via llvm-dev llvm-dev at lists.llvm.org
Tue Jan 28 18:49:45 PST 2020

you can find unused functions easily using static analysis tools; codeql comes to mind:


import cpp from Function f where not exists(FunctionCall fc | fc.getTarget() = f) select f, "This function is never called."

Sent from my iPhone

> On Jan 27, 2020, at 19:43, 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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