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

Reid Kleckner via llvm-dev llvm-dev at lists.llvm.org
Thu Nov 14 10:32:24 PST 2019

I think this requires changes to the bitcode reader. This has been
discussed a bit, see
https://lists.llvm.org/pipermail/llvm-dev/2019-March/131216.html  for

On Mon, Nov 11, 2019 at 5:06 PM LevitatingLion <levitatinglion at web.de>

> 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" 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
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
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