LLVM internals, part 4: attributes and attribute groups | Hacker News Reader

LLVM internals, part 4: attributes and attribute groups

blog.yossarian.net

119 pointslukastyrychtr7 comments

Author here; happy to answer any questions.

Hi, I had a general question regarding intrinsics which you might be able to answer. `llvm.stacksave` and `llvm.stackrestore` are used for implementing VLAs in C (to my knowledge). Is it required to emit those for variable size allocas, or could you skip them? If skipping is possible, how is the case handled when the basic block does not post-dominate an exit block?

AFAIK, LLVM doesn’t actually require those intrinsics for VLAs — the `alloca` instruction can take a non-immediate for the stack object size. I can’t remember off the top of my head whether I’ve seen LLVM consistently include the intrinsics as well.

As for the dominator case: it’s possible that it does need them then. But for a really trivial VLA (like at the level of the function’s frame) it shouldn’t.

The only case I recall seeing @llvm.{stacksave,stackrestore} use was when LLVM inlined a function with a non-entry-block-alloca into another function, where the intrinsics got added immediately before/after what used to be the function call.

clang -S -emit-llvm xyz.cpp generates an optnone attribute.

  attributes #0 = { noinline norecurse nounwind optnone ssp uwtable "darwin-stkchk-strong-link" "disable-tail-calls"="false" "frame-pointer"="all" "less-precise-fpmad"="false" "min-legal-vector-width"="0" "no-infs-fp-math"="false" "no-jump-tables"="false" "no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false" "no-trapping-math"="true" "probe-stack"="___chkstk_darwin" "stack-protector-buffer-size"="8" "target-cpu"="penryn" "target-features"="+cx16,+cx8,+fxsr,+mmx,+sahf,+sse,+sse2,+sse3,+sse4.1,+ssse3,+x87" "tune-cpu"="generic" "unsafe-fp-math"="false" "use-soft-float"="false" }

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?

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.”

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.

Reply on news.ycombinator.com