[llvm-dev] Revisiting/refining the definition of optnone with interprocedural transformations

[Roman Lebedev via llvm-dev]

There's 'noipa' attribute in GCC, currently it is not supported by clang.
Theoretically, how would one implement it?

With your proposal, clang `noipa` attribute could be lowered
to `optnone` on the whole function, To me that seems like
too much of a hammer, should that be the path forward.

Would it not be best to not conflate the two,
and just introduce the `noipa` attribute?

Roman

On Sun, Apr 18, 2021 at 7:37 PM David Blaikie <dblaikie at gmail.com> wrote:
>
> While trying to reproduce some debug info thing (I don't have the exact example at the moment - but I think it was more aggressive than the example I have now, but something like this:
>
> __attribute__((optnone)) int f1() {
>   return 3;
> }
> int main() {
>   return f1();
> }
>
>
> (actually I think in my case I had a variable to hold the return value from f1, with the intent that this variable's location couldn't use a constant - a load from a volatile variable would probably have provided similar functionality in this case)
>
> LLVM (& specifically Sparse Conditional Constant Propagation, llvm/lib/Transforms/Scalar/SCCP.cpp) optimizes this code noting that f1 always returns 3, so rather than using the return value from the call to f1, it ends up hardcoding the return value:
>
> define dso_local i32 @main() local_unnamed_addr #1 {
>
> entry:
>
>   %call = tail call i32 @_Z2f1v()
>
>   ret i32 3
>
> }
>
>
> I consider this a bug - in that optnone is used to implement -O0 for LTO, so it seemed to me that the correct behavior is for an optnone function to behave as though it were compiled in another object file outside the purview of optimizations - interprocedural or intraprocedural.
>
> So I sent https://reviews.llvm.org/D100353 to fix that.
>
> Florian pointed out that this wasn't quite specified in the LangRef, which says this about optnone:
>
> This function attribute indicates that most optimization passes will skip this function, with the exception of interprocedural optimization passes. Code generation defaults to the “fast” instruction selector. This attribute cannot be used together with the alwaysinline attribute; this attribute is also incompatible with the minsize attribute and the optsize attribute.
>
> This attribute requires the noinline attribute to be specified on the function as well, so the function is never inlined into any caller. Only functions with the alwaysinline attribute are valid candidates for inlining into the body of this function.
>
>
> So the spec of optnone is unclear (or arguably explicitly disallows) whether interprocedural optimizations should treat optnone functions in any particular way.
>
> So I was going to update the wording to rephrase this to say "Interprocedural optimizations should treat this function as though it were defined in an isolated module/object." (perhaps "interprocedural optimizations should treat optnone functions as opaque" or "as though they were only declarations")
>
> The choice of this direction was based on my (possibly incorrect or debatable) understanding of optnone, that it was equivalent to the function being in a separate/non-lto object. (this seems consistent with the way optnone is used to implement -O0 under lto - you could imagine a user debugging a binary, using -O0 for the code they're interested in debugging, and potentially using an interactive debugger to change some state in the function causing it to return a different value - which would get quite confusing if the return value was effectively hardcoded into the caller)
>
> What're folks thoughts on this?
>
> - Dave