[cfe-dev] Making lambda function name consistent with GCC?

[David Blaikie via cfe-dev]

On Mon, May 3, 2021 at 4:10 AM Nathan Sidwell via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> On 4/30/21 1:52 PM, Fāng-ruì Sòng via cfe-dev wrote:
> > Redirect to cfe-dev.
> >
> > On Fri, Apr 30, 2021 at 9:42 AM Xun Li via llvm-dev
> > <llvm-dev at lists.llvm.org> wrote:
> >>
> >> Hi,
> >>
> >> I noticed that when compiling lambda functions, the generated function
> >> names use different conventions than GCC.
> >> Example: https://godbolt.org/z/5qvqKqEe6
> >> The lambda in Clang is named "_Z3barIZ3foovE3$_0EvT_", while the one
> >> in GCC is named "_Z3barIZ3foovEUlvE_EvT_". Their demangled names are
> >> also different ("void bar<foo()::$_0>(foo()::$_0)" vs "void
> >> bar<foo()::{lambda()#1}>(foo()::{lambda()#1})").
> >> Lambdas are not covered by the ABI so this is OK.
>
> Actually, they are.  See 5.1.8 of the ABI doc
> (https://github.com/itanium-cxx-abi/cxx-abi)
>
> The reason is that these symbols do escape into object files with
> external linkage (not something originally anticipated).
>

Could you provide a quick example of this ABI break (where two files
compiled with matching compiler (GCC or Clang) link/run correctly, but
mismatching in either direction fails to link/run correctly)?


>
> >> However there are use-cases where I find it very inconvenient when
> >> they generate different names. For example, if we are to compare the
> >> performance difference of the same software compiled under Clang and
> >> GCC, the perf stack traces will look very different because of the
> >> naming differences, making it hard to compare.
> >> Is there any particular reason that Clang uses a different naming
> >> convention for lambdas, and would there be push-backs if we were to
> >> make it consistent with GCC?
>
> It would be good to have clang match the ABI.  I am not sure how much
> pain it would be for users to switch though -- perhaps having two
> manglings and therefore two distinct instances in the same executable.
> Other than code bloat most would not notice, unless someone put a static
> var into their lambda operator.
>
> >> Thanks.
> >>
> >> --
> >> Xun
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> >
>
>
> --
> Nathan Sidwell
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