[Vm-dev] IMPORTANT: GCC 6 generates position independent executables by default on Linux

Holger Freyther holger at freyther.de
Wed Feb 22 09:51:37 UTC 2017

> On 22 Feb 2017, at 14:58, Ronie Salgado <roniesalg at gmail.com> wrote:

Dear Ronie,

> GDB layout asm on the line 25313 shows the generated code.
> B+>│0x565aac77 <interpret+214836>   flds   -0x1d6c(%ebp)                                                                                                        │
>    │0x565aac7d <interpret+214842>   sub    $0x8,%esp                                                                                                            │
>    │0x565aac80 <interpret+214845>   lea    -0x8(%esp),%esp                                                                                                      │
>    │0x565aac84 <interpret+214849>   fstpl  (%esp)                                                                                                               │
>    │0x565aac87 <interpret+214852>   mov    -0x4008(%ebp),%ebx                                                                                                   │
>    │0x565aac8d <interpret+214858>   call   0x56570840 
> Of special importance, is the instruction: mov    -0x4008(%ebp),%ebx . this is the PLT entry for sqrt, and this is where ebx with the currentBytecode is destroyed.

I tried to reproduce it but I think I don't generate enough register pressure?

#include <stdint.h>
#include <sys/types.h>
#include <math.h>

int interpret(int *ops, const size_t num_ops)
        register int op __asm__("%ebx");
        size_t off = 0;

        while (off < num_ops) {
                op = ops[off];
                switch(op) {
                case 1:
                case 2:
                        sqrt(op + num_ops);
                off += 1;

can you think of a way to get closer to the interpreter? Is it using computed goto? If there is a reproducer I am happy to open a bug with the GCC project and try to bring it to a resolution.


More information about the Vm-dev mailing list