[Bug tree-optimization/66826] Unused result from dlsym in constructor results in a segfault

[tetra2005 at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66826

Yuri Gribov <tetra2005 at gmail dot com> changed:

           What    |Removed                     |Added
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                 CC|                            |tetra2005 at gmail dot com

--- Comment #3 from Yuri Gribov <tetra2005 at gmail dot com> ---
This is a very funny bug but not related to GCC per se. Firstly, let's consider
a miminal repro:
    __attribute__((constructor)) static void some_init() {
      dlsym(RTLD_DEFAULT, "anything");
    }
(segfaults just as well). Under -O0 this produces a normal call:
    call    dlsym@PLT
    ...
    ret
but with -O2 GCC is clever enough to tail-call-optimize it to a plain jump:
    jmp     dlsym@PLT

Now dlsym (and other dl-functions) secretly take shadow parameter - return
address on stack:
    void *
    __dlsym (void *handle, const char *name DL_CALLER_DECL)
    {
    ...
      struct dlsym_args args;
      args.who = DL_CALLER;
      args.handle = handle;
      args.name = name;
(from dlsym.c). As in our case return address is missing, args.who argument is
missing which causes segfault during symbol resolution (dynamic linker is lame
on checks).