nelhage added a comment.
In https://reviews.llvm.org/D40283#936088, @clayborg wrote:
> Jim will need to ok this. A few concerns follow. Most of the time when we
> don't get the DWARF -> AST conversion right, it can mean that we might code
> gen incorrect code. Is there not enough information for the GNU abi_tag in
> the DWARF to actually re-create these classes correctly in the AST?
Correct, as far as I can tell. Looking at a symbol definition from the test
program on the issue via `llvm-dwarfdump`, I see
0x00000076: DW_TAG_subprogram
DW_AT_linkage_name ("_ZN1A16helloWorld_cxx11B5cxx11Ev")
DW_AT_name ("helloWorld_cxx11")
DW_AT_decl_file ("/tmp/test_abi_tag.cc")
DW_AT_decl_line (5)
DW_AT_declaration (true)
DW_AT_external (true)
DW_AT_accessibility (DW_ACCESS_public)
0x00000082: DW_TAG_formal_parameter
DW_AT_type (cu + 0x009b "A*")
DW_AT_artificial (true)
0x00000087: NULL
The function of the `abi_tag` annotation is to append the `B5cxx11` to the
mangled name; I can't find any other indication in the DWARF of the ABI tag.
You can read more about `abi_tag` here:
https://gcc.gnu.org/onlinedocs/gcc/C_002b_002b-Attributes.html
Or an explanation of why it exists and how it is used here:
https://davmac.wordpress.com/2015/07/19/tale-of-two-abis/
The effect of this patch on the code generated by `expression` is to change the
names we emit into the LLVM IR; Previously, we would emit a call to
`_ZN1A16helloWorld_cxx11Ev`, which would then fail when resolving symbols. Now
we (correctly) emit a call to `_ZN1A16helloWorld_cxx11B5cxx11Ev`.
The test case I added in this patch also demonstrates that GCC and clang both
support arbitrarily overriding a symbol's mangled name in the source using
`asm("new_name")`. I can't recall having ever encountered a use of that in the
wild, so I don't care strongly about supporting it, but if we want to I think
this general approach is almost certainly the only approach that would do so.
https://reviews.llvm.org/D40283
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