libstdc++ "freestanding" ('--disable-hosted-libstdcxx') with '-fno-rtti', '-fno-exceptions': 'libstdc++-v3/libsupc++/tinfo.cc'

[Thomas Schwinge]

Hi!

In context of <https://gcc.gnu.org/PR101544> '[OpenMP][AMDGCN][nvptx]
C++ offloading: unresolved _Znwm = "operator new(unsigned long)"'
I'm looking into building GCN, nvptx offloading libstdc++ "freestanding"
('--disable-hosted-libstdcxx') with '-fno-rtti', '-fno-exceptions'.
(I've basically got these things wired up; details to be shared later.)

While there is some experimental/incomplete/not-to-be-relied-on support
for PTX symbol aliases, we're currently generally running into
"error: alias definitions not supported in this configuration"
for certain GCC/C++-front-end-generated code, for example:
"complete object destructor" aliasing "base object destructor"
(<https://itanium-cxx-abi.github.io/cxx-abi/abi.html#mangling-special-ctor-dtor>
-- per my understanding, at least).  ;-)

It'll ultimately be possible to implement more complete symbol aliasing
support for nvptx, but I found that this specific issue actually is due
to a nvptx back end mis-configuration; fixed with adequate
'TARGET_USE_LOCAL_THUNK_ALIAS_P' and/or 'TARGET_SUPPORTS_ALIASES'
definitions (details to be shared later).

However, we then still run into:

    [...]/libstdc++-v3/libsupc++/tinfo.cc:55:1: error: alias definitions not supported in this configuration
       55 | std::type_info::__equal (const std::type_info& arg) const _GLIBCXX_NOEXCEPT
          | ^~~
    make[4]: *** [Makefile:777: tinfo.lo] Error 1

That's 'libstdc++-v3/libsupc++/tinfo.cc':

     1 // Methods for type_info for -*- C++ -*- Run Time Type Identification.
    [...]
    39 // We can't rely on common symbols being shared between shared objects.
    40 bool std::type_info::
    41 operator== (const std::type_info& arg) const _GLIBCXX_NOEXCEPT
    42 {
    43 #if __GXX_MERGED_TYPEINFO_NAMES
    44   return name () == arg.name ();
    45 #else
    46   /* The name() method will strip any leading '*' prefix. Therefore
    47      take care to look at __name rather than name() when looking for
    48      the "pointer" prefix.  */
    49   return (&arg == this)
    50     || (__name[0] != '*' && (__builtin_strcmp (name (), arg.name ()) == 0));
    51 #endif
    52 }
    53
    54 bool
    55 std::type_info::__equal (const std::type_info& arg) const _GLIBCXX_NOEXCEPT
    56 __attribute__((alias("_ZNKSt9type_infoeqERKS_")));
    57 #endif
    [...]

..., so there's a manual alias from the line 55 function to the line 41
function (if I got that right).

Now we may surely address that in some different way, but I do wonder if
we need this whole file at all, given this is
"Methods for type_info for -*- C++ -*- Run Time Type Identification",
RTTI, and we're generally assuming '-fno-rtti'?  Do certain things simply
need to be '#if'-conditionalized, or similar, for example?  I've not yet
looked into the details; hoping that's maybe easy for you to answer?


Grüße
 Thomas
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