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

[David Blaikie via llvm-dev]

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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