http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46219
Summary: Generate indirect jump instruction on x86-64
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
AssignedTo: [email protected]
ReportedBy: [email protected]
Target: x86-64
Is there a less brutal way to coax gcc into generating an indirect jump
instruction on x86-64?
typedef void (*dispatch_t)(long offset);
dispatch_t dispatch[256];
void make_indirect_jump(long offset) {
dispatch[offset](offset);
}
void force_use_of_indirect_jump_instruction(long offset) {
asm ("jmp *dispatch( ,%0, 8)\n" : : "r" (offset));
__builtin_unreachable();
}
int main() {
return 0;
}
$ gcc-snapshot.sh -std=gnu99 -O3 use-indirect-jump-instruction.c && objdump -d
-m i386:x86-64:intel a.out|less
0000000000400480 <make_indirect_jump>:
400480: 48 8b 04 fd 20 12 60 mov rax,QWORD PTR [rdi*8+0x601220]
400487: 00
400488: ff e0 jmp rax
40048a: 66 0f 1f 44 00 00 nop WORD PTR [rax+rax*1+0x0]
0000000000400490 <force_use_of_indirect_jump_instruction>:
400490: ff 24 fd 20 12 60 00 jmp QWORD PTR [rdi*8+0x601220]
400497: 66 0f 1f 84 00 00 00 nop WORD PTR [rax+rax*1+0x0]
40049e: 00 00
This combination of inline assembly and __builtin_unreachable() is not a
generally usable architecture-specific solution (there needs to be a way to
ensure the results of modified input arguments end up in the same registers for
the opaque tail call. It works in this case because offset remains unmodified,
satisfying the ABI for dispatch_t).
