[llvm-dev] about __attribute__((artificial)) and inline pass

[Reid Kleckner via llvm-dev]

I thought `artificial` and `nodebug` were supposed to be synonyms, just
different names invented by different communities. The linked patch doesn't
implement the LLVM side of things, so I'm not sure when that got added.

On Fri, Jan 28, 2022 at 11:03 AM David Blaikie <dblaikie at gmail.com> wrote:

> Hmm, seems I missed this feature (or at least don't remember) when it went
> in.
>
> Looks like the GCC spec might be more accurate to the current
> implementation than what Clang's documentation says:
>
> "artificialThis attribute is useful for small inline wrappers which if
> possible should appear during debugging as a unit, depending on the debug
> info format it will either mean marking the function as artificial or using
> the caller location for all instructions within the inlined body."
> & the "marking the function as artificial" is the thing that this patch
> implemented in Clang. This did not have the same effect as nodebug, and
> doesn't "use the caller location for all instructions within the inlined
> body".
>
> It's possible we could implement the "artificial" attribute as a an alias
> for "nodebug" which would also be conforming with GCC's spec, but would be
> less expressive than the current implementation (though would save debug
> info size in a way the current Clang implementation does not).
>
> It looks like Clang's implementation is consistent with GCC's
> implementation for DWARF - "marking the function as artificial", not "using
> the caller location for instructions within the inlined body", but Clang's
> documentation is incorrect, since it documents the latter and not the
> former.
>
> On Fri, Jan 28, 2022 at 9:44 AM Fangqing Du via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Dear all,
>>
>> In clang/llvm 7.0, `__attribute__((artificial))` can be identified by
>> clang and attach `DIFlagArtificial` into function metadata in this
>> change. <https://reviews.llvm.org/D43259#change-jbvcnfxttyAz>
>>
>> And according to the specification
>> <https://clang.llvm.org/docs/AttributeReference.html#artificial> of
>> `artificial`, if I understand correctly, when the function (which is marked
>> with `artificial` attribute) is inlined, the inlined function instructions
>> should be associated with the callsite line number, instead of the line of
>> inlined callee.
>>
>> But from my observation, with and without this attribute, the inline pass
>> behavior is not affected, and inlined instructions debug location still
>> point to the callee instead of callsite.
>> Here is my experiment: godbolt experiment link
>> <https://godbolt.org/z/oW8Td5d4M>
>>
>> Is my understanding correct?
>> Thanks,
>> Fangqing
>> Xilinx Inc.
>>
>> == ------------ example -------------===
>>
>>   1 int bar(int);
>>
>>   2
>>
>>   3 inline int __attribute__((artificial))
>> __attribute__((always_inline)) foo(int x)
>>
>>   4 {
>>
>>   5     return bar(x + 1);
>>
>>   6 }
>>
>>   7
>>
>>   8 void baz(void)
>>
>>   9 {
>>
>>  10     auto x = foo(1);
>>
>>  11     x = foo(2);
>>
>>  12 }
>>
>> When function 'foo' is inlined, the debug location of call instruction
>> 'bar' still points to line #5, instead of line #10 and #11.
>> _______________________________________________
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>>
>
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