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

LevitatingLion via llvm-dev llvm-dev at lists.llvm.org
Mon Nov 11 17:05:59 PST 2019

I have a working patch which adds the 'flatten' attribute almost ready, but it breaks the Bitcode/highLevelStructure.3.2.ll test. This is caused by the attribute definition in include/llvm/IR/Attributes.td:

def Flatten : EnumAttr<"flatten">;

llvm-dis seems to be unable to correctly decode attributes stored in the bitcode when the new attribute is added. Why does this happen, and how do I fix it?

Other attributes don't seem to have required any handling of this problem, see https://reviews.llvm.org/D62766 or https://reviews.llvm.org/D49165. Could that be because this is the 65th attribute (so a bitmask indicating all attributes doesn't fit in 64 bit anymore)?

On November 5, 2019 7:24:58 PM GMT+01:00, Reid Kleckner <rnk at google.com> wrote:
>I like the proposal to add a new call site attribute, either "flatten"
>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
>> all calls to not 'noinline' functions with 'always_inline'. In
>> 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
>> (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
>> 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
>> call site marked 'always_inline' is inlined, the attribute will then
>> duplicated to all new call sites (with decremented depth and only if
>> depth is greater than zero).
>> With this solution, one problem remains: an 'always_inline' on the
>> site is currently stronger than a 'noinline' on the callee. Thus, a
>> recursive 'always_inline' would ignore 'noinline'. To fix this, we
>> make 'always_inline' weaker than 'noinline'. If that breaks backwards
>> compatibility too much, we could add a second parameter (boolean
>> 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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>> llvm-dev at lists.llvm.org
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