[cfe-dev] libc++ and large stack frames

[Richard Smith via cfe-dev]

On Mon, 4 Jun 2018, 21:43 Duncan P. N. Exon Smith via cfe-dev, <
cfe-dev at lists.llvm.org> wrote:

>
>
> On May 31, 2018, at 15:35, Richard Smith via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
>
> On 31 May 2018 at 14:38, Friedman, Eli via cfe-dev <cfe-dev at lists.llvm.org
> > wrote:
>
>> On 5/31/2018 1:07 PM, Erik van der Poel via cfe-dev wrote:
>>
>>> This is a proposal to add a configuration option to disable
>>> _LIBCPP_INLINE_VISIBILITY, which causes large stack frames in non-optimized
>>> builds because it uses the always_inline attribute to force significant
>>> amounts of inlining of libc++ code.
>>>
>>> The new configuration option could be called
>>> _LIBCPP_DISABLE_INLINE_VISIBILITY.
>>>
>>> Note that _LIBCPP_INLINE_VISIBILITY and _LIBCPP_ALWAYS_INLINE have
>>> identical definitions. One could be renamed to the other or both could be
>>> renamed to _LIBCPP_HIDE_FROM_ABI as part of this work.
>>>
>>
> I think it's useful to have different names for these, since they have
> different intent.  It would be reasonable to change the definition of one
> without changing the other.
>
> Renaming _LIBCPP_INLINE_VISIBILITY to _LIBCPP_HIDE_FROM_ABI makes sense to
> me though.
>
> One option we'd like to investigate (but haven't yet) is changing
> _LIBCPP_INLINE_VISIBILITY/_LIBCPP_HIDE_FROM_ABI to drop the "always_inline"
> but keep the "hidden".  Have you considered
> _LIBCPP_DISABLE_ALWAYS_INLINE_IN_INLINE_VISIBILITY (or equivalent)?  If
> not, why not?
>

IIRC that results in missing definitions for all of the explicitly
instantiated template members, because the versions in the DSO are not
visible to their consumers.

Alternatives considered and rejected:
>>>
>>> 1. Remove __attribute__((__always_inline__)) from all include/__config
>>> macros.
>>> 2. Add _LIBCPP_DISABLE_ALWAYS_INLINE to control whether or not
>>> __always_inline__ is included in any __config macro.
>>> 3. Use the existing _LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS to control
>>> both __visibility__ and __always_inline__.
>>> 4. Use the existing _LIBCPP_ABI_UNSTABLE to control
>>> _LIBCPP_INLINE_VISIBILITY.
>>>
>>
>> 5. Start using __attribute__((internal_linkage)) instead, so a
>> configuration option isn't necessary.  See
>> http://lists.llvm.org/pipermail/cfe-dev/2016-October/051151.html .
>
>
> Yes, that'd solve the problem, but it would also result in duplicate
> definitions for all the INLINE_VISIBILITY member functions that don't get
> inlined. As a consequence, __attribute__((internal_linkage)) would be a
> worse solution than the one being proposed here for those who don't care
> about the ABI stability guarantees that libc++ is trying to provide.
> Replacing the current use of __always_inline__ with __internal_linkage__
> seems like it would probably be an improvement to me (conditional on
> whether the effects on code size are acceptable), but I think the change
> Erik is proposing and that one are largely orthogonal.
>
>
> I agree.
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