[Bug 221288] lang/gcc5 links against libsupc++ when compiling

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=221288

--- Comment #9 from Mark Millard <markmi at dsl-only.net> ---
(In reply to Mark Millard from comment #6)

Returning to a g++6 based a.out back trace 
for the C++ threading to show some
infrastructure usage involved:

Thread 12 received signal SIGSEGV, Segmentation fault.
[Switching to LWP 100277 of process 18243]
uw_frame_state_for (context=context at entry=0x7fffdfbfbe20,
fs=fs at entry=0x7fffdfbfbb70) at
/usr/obj/portswork/usr/ports/lang/gcc6/work/gcc-6.4.0/libgcc/unwind-dw2.c:1249
1249          return MD_FALLBACK_FRAME_STATE_FOR (context, fs);
(gdb) disass
Dump of assembler code for function uw_frame_state_for:
. . .
   0x0000000800e14a6e <+94>:    callq  0x800e08870 <_Unwind_Find_FDE at plt>
   0x0000000800e14a73 <+99>:    test   %rax,%rax
   0x0000000800e14a76 <+102>:   mov    %rax,%r12
   0x0000000800e14a79 <+105>:   je     0x800e14bd8 <uw_frame_state_for+456>
. . .
   0x0000000800e14bd8 <+456>:   mov    0x98(%r15),%rax
=> 0x0000000800e14bdf <+463>:   cmpl   $0x247c8d48,(%rax)
   0x0000000800e14be5 <+469>:   je     0x800e14e62 <uw_frame_state_for+1106>
   0x0000000800e14beb <+475>:   nopl   0x0(%rax,%rax,1)
   0x0000000800e14bf0 <+480>:   mov    $0x5,%eax
   0x0000000800e14bf5 <+485>:   add    $0x38,%rsp
   0x0000000800e14bf9 <+489>:   pop    %rbx
   0x0000000800e14bfa <+490>:   pop    %rbp
   0x0000000800e14bfb <+491>:   pop    %r12
   0x0000000800e14bfd <+493>:   pop    %r13
   0x0000000800e14bff <+495>:   pop    %r14
   0x0000000800e14c01 <+497>:   pop    %r15
   0x0000000800e14c03 <+499>:   retq   
. . .

which matches up with the 0x247c8d48 comparison code
in:

/usr/obj/portswork/usr/ports/lang/gcc6/work/gcc-6.4.0/libgcc/config/i386/freebsd-unwind.h

in the source:
. . .
#ifdef __x86_64__
#define MD_FALLBACK_FRAME_STATE_FOR x86_64_freebsd_fallback_frame_state

static _Unwind_Reason_Code
x86_64_freebsd_fallback_frame_state
(struct _Unwind_Context *context, _Unwind_FrameState *fs)
{
  struct sigframe *sf;
  long new_cfa;

  /* Prior to FreeBSD 9, the signal trampoline was located immediately
     before the ps_strings.  To support non-executable stacks on AMD64,
     the sigtramp was moved to a shared page for FreeBSD 9.  Unfortunately
     this means looking frame patterns again (sys/amd64/amd64/sigtramp.S)
     rather than using the robust and convenient KERN_PS_STRINGS trick.

     <pc + 00>:  lea     0x10(%rsp),%rdi
     <pc + 05>:  pushq   $0x0
     <pc + 17>:  mov     $0x1a1,%rax
     <pc + 14>:  syscall

     If we can't find this pattern, we're at the end of the stack.
  */

  if (!(   *(unsigned int *)(context->ra)      == 0x247c8d48
        && *(unsigned int *)(context->ra +  4) == 0x48006a10
        && *(unsigned int *)(context->ra +  8) == 0x01a1c0c7
        && *(unsigned int *)(context->ra + 12) == 0x050f0000 ))
    return _URC_END_OF_STACK;
 . . .

where:

(gdb) print context->ra
$2 = (void *) 0x7fffdf7fb000

(gdb) print *(unsigned*)(context->ra)
Cannot access memory at address 0x7fffdf7fb000

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