[llvm-dev] Fix clang's 'flatten' function attribute: add depth to always_inline?

Jakub (Kuba) Kuderski via llvm-dev llvm-dev at lists.llvm.org
Wed Nov 6 18:22:14 PST 2019

(This is very loosely related, but the new attribute was something that
could be directly used by this patch for adding callsite-level inline
attributes from C/C++: https://reviews.llvm.org/D51200)

On Tue., Nov. 5, 2019, 13:25 Reid Kleckner via llvm-dev, <
llvm-dev at lists.llvm.org> wrote:

> I like the proposal to add a new call site attribute, either "flatten" or
> "always_inline_recursively".
> I wouldn't want to add an optional depth to the existing always_inline
> attribute. It already has a well understood meaning.
> Good luck. :)
> On Mon, Nov 4, 2019 at 2:12 PM LevitatingLion via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>> Hi everyone,
>> clang currently implements the 'flatten' function attribute by marking
>> all calls to not 'noinline' functions with 'always_inline'. In effect,
>> only the first level of calls is inlined, not all calls recursively
>> (like gcc does).
>> We briefly discussed possible solutions on IRC:
>> We could add an equivalent LLVM attribute for functions (e.g.
>> 'flatten'). The main problem with this approach occurs when we're
>> inlining the function marked with 'flatten'. In this case we could
>> either drop the attribute, move it to the function we're inlining into
>> (both would lose the scope of the original 'flatten' annotation), or
>> distribute it to all calls within the 'flatten' function (which would
>> require a new call site attribute).
>> The other approach is to add or modify a call site attribute. We could
>> add a specific attribute (e.g. 'flatten' or
>> 'always_inline_recursively'), but a more general solution is adding a
>> new 'depth' parameter to the existing 'always_inline' attribute. When a
>> call site marked 'always_inline' is inlined, the attribute will then be
>> duplicated to all new call sites (with decremented depth and only if the
>> depth is greater than zero).
>> With this solution, one problem remains: an 'always_inline' on the call
>> site is currently stronger than a 'noinline' on the callee. Thus, a
>> recursive 'always_inline' would ignore 'noinline'. To fix this, we can
>> make 'always_inline' weaker than 'noinline'. If that breaks backwards
>> compatibility too much, we could add a second parameter (boolean 'weak'
>> or 'strong') to 'always_inline'.
>> I've never worked on LLVM before, but if someone confirms what the
>> preferred solution to this is, I will start working on a patch.
>> Thanks for your time
>> LevitatingLion
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