This means you can't then run it through opt and watch it pass by pass get optimized with opt --print-after-all -O3 xyz.ll -S -o abc.ll. Clang doesn't take the --print-after-all flag.
What is the purpose of the optnone attribute? Why is Clang preventing later optimization?
The purpose is to keep optimization flags around if you're, say, compiling with LTO or PGO--cases where the regular compilation process will keep bitcode files around.
The usual trick I do is clang -S -emit-llvm -O2 -Xclang --disable-llvm-passes.
> Clang doesn't take the --print-after-all flag.
You have to prefix it with -mllvm, e.g., clang -mllvm -print-after-all -O2 foo.cpp.
I’m not sure about the why, but it’s been that way for quite a while (since at least LLVM 5). You can explicitly override the optnone by passing `-disable-O0-optnone` to the underlying driver (i.e., you might need to pass `-Xclang` first.)
Edit: if I had to wager a guess, it’s because someone generated an IR module at some point with -O0 and then re-ran opt to perform a specific non-destructive analysis, at which point LLVM happily ran all of its default passes. So someone decided that initially building with -O0 is equivalent to saying “my code should never be optimized, even in subsequent invocations of the compiler.”
What is the purpose of the optnone attribute? Why is Clang preventing later optimization?