SheepShaver x86_64 no jit: crash on boot

[rakslice]

While talking about #142, I did some testing on master here, and found:

i686 jit true -> boots to desktop ok
i686 jit false -> boots to desktop ok
x86_64 jit true -> boots to desktop ok
x86_64 jit false -> segfault on the boot screen before the first extension icon appears, no PPC stack trace

The x86_64 test there was on Debian 9:

Linux debcdehax 4.9.0-14-amd64 #1 SMP Debian 4.9.240-2 (2020-10-30) x86_64 GNU/Linux

Built with its:

$ gcc --version
gcc (Debian 6.3.0-18+deb9u1) 6.3.0 20170516
Copyright (C) 2016 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

The backtrace:

Thread 1 "SheepShaver" received signal SIGSEGV, Segmentation fault.
powerpc_cpu::execute_loadstore<op_template_nop<unsigned int>, input_gpr_except<bit_field<11, 15>, 0>, gpr_operand<bit_field<16, 20> >, false, 4, false, false> (this=0x79bfbfc0, opcode=<optimized out>) at ../kpx_cpu/src/cpu/ppc/ppc-execute.cpp:576
576                     memory_helper<SZ, RX>::store(ea, operand_RS::get(this, opcode));
(gdb) bt
#0  powerpc_cpu::execute_loadstore<op_template_nop<unsigned int>, input_gpr_except<bit_field<11, 15>, 0>, gpr_operand<bit_field<16, 20> >, false, 4, false, false> (this=0x79bfbfc0, opcode=<optimized out>) at ../kpx_cpu/src/cpu/ppc/ppc-execute.cpp:576
#1  0x00000000780b6518 in nv_mem_fun1_t<void, powerpc_cpu, unsigned int>::operator() (x=<optimized out>, p=0x79bfbfc0, this=0x1038f960)
    at ../kpx_cpu/include/nvmemfun.hpp:108
#2  powerpc_cpu::execute (this=0x79bfbfc0, entry=entry@entry=1085341696) at ../kpx_cpu/src/cpu/ppc/ppc-cpu.cpp:697
#3  0x00000000780cd69e in emul_ppc (entry=entry@entry=1085341696) at ../kpx_cpu/sheepshaver_glue.cpp:938
#4  0x0000000078065707 in jump_to_rom (entry=1085341696) at main_unix.cpp:1271
#5  emul_func (arg=0x0) at main_unix.cpp:1291
#6  main (argc=<optimized out>, argv=<optimized out>) at main_unix.cpp:1112

Disassembling at the point where the segfault happens:

(gdb) disas /r $pc,+30
Dump of assembler code from 0x780c3688 to 0x780c36a6:
=> 0x00000000780c3688 <powerpc_cpu::execute_loadstore<op_template_nop<unsigned int>, input_gpr_except<bit_field<11, 15>, 0>, gpr_operand<bit_field<1                                
6, 20> >, false, 4, false, false>(unsigned int)+40>:    67 89 10        mov    %edx,(%eax)
   0x00000000780c368b <powerpc_cpu::execute_loadstore<op_template_nop<unsigned int>, input_gpr_except<bit_field<11, 15>, 0>, gpr_operand<bit_field<1                                
6, 20> >, false, 4, false, false>(unsigned int)+43>:    83 87 ac 03 00 00 04    addl   $0x4,0x3ac(%rdi)
   0x00000000780c3692 <powerpc_cpu::execute_loadstore<op_template_nop<unsigned int>, input_gpr_except<bit_field<11, 15>, 0>, gpr_operand<bit_field<1                                
6, 20> >, false, 4, false, false>(unsigned int)+50>:    c3      retq
   0x00000000780c3693:  90      nop

etc...

The problem is that sigsegv.cpp's ix86_skip_instruction() doesn't know what to make of the 0x67 x86_64 32-bit address mode prefix on that instruction there.