------- Comment #2 from adam at consulting dot net dot nz 2010-09-11 11:15
-------
GCC snapshot has regressed compared to gcc-4.5:
#include <assert.h>
#include <stdint.h>
#define LIKELY(x) __builtin_expect(!!(x), 1)
#define UNLIKELY(x) __builtin_expect(!!(x), 0)
register uint32_t *Iptr __asm__("rbp");
typedef void (*inst_t)(uint64_t types, uint64_t a, uint64_t b);
__attribute__ ((noinline)) void dec_helper(uint64_t types, uint64_t a, uint64_t
b) {
assert("FIXME"=="");
}
void dec(uint64_t types, uint64_t a, uint64_t b) {
if (LIKELY((types & 0xFF) == 1)) {
uint32_t next = Iptr[1];
--a;
++Iptr;
((inst_t) (uint64_t) next)(types, a, b);
} else dec_helper(types, a, b);
}
int main() {
return 0;
}
$ gcc-4.5 -O3 -std=gnu99 plain-32bit-direct-dispatch.c && objdump -d -m
i386:x86-64:intel a.out|less
0000000000400520 <dec>:
400520: 40 80 ff 01 cmp dil,0x1
400524: 75 0d jne 400533 <dec+0x13>
400526: 8b 45 04 mov eax,DWORD PTR [rbp+0x4]
400529: 48 83 ee 01 sub rsi,0x1
40052d: 48 83 c5 04 add rbp,0x4
400531: ff e0 jmp rax
400533: e9 c8 ff ff ff jmp 400500 <dec_helper>
400538: eb 06 jmp 400540 <main>
40053a: 90 nop
40053b: 90 nop
40053c: 90 nop
40053d: 90 nop
40053e: 90 nop
40053f: 90 nop
The above code generation is fine. Here is what GCC snapshot {gcc (Debian
20100828-1) 4.6.0 20100828 (experimental) [trunk revision 163616]} generates:
$ gcc-snapshot.sh -O3 -std=gnu99 plain-32bit-direct-dispatch.c && objdump -d -m
i386:x86-64:intel a.out|less
0000000000400500 <dec>:
400500: 48 83 ec 08 sub rsp,0x8
400504: 40 80 ff 01 cmp dil,0x1
400508: 75 14 jne 40051e <dec+0x1e>
40050a: 48 89 e8 mov rax,rbp
40050d: 48 83 ee 01 sub rsi,0x1
400511: 48 8d 6d 04 lea rbp,[rbp+0x4]
400515: 8b 40 04 mov eax,DWORD PTR [rax+0x4]
400518: 48 83 c4 08 add rsp,0x8
40051c: ff e0 jmp rax
40051e: e8 bd ff ff ff call 4004e0 <dec_helper>
400523: eb 0b jmp 400530 <main>
400525: 90 nop
400526: 90 nop
400527: 90 nop
400528: 90 nop
400529: 90 nop
40052a: 90 nop
40052b: 90 nop
40052c: 90 nop
40052d: 90 nop
40052e: 90 nop
40052f: 90 nop
Function size has jumped from rounded up to 32 bytes to rounded up to 48 bytes.
Tail call has been missed, leading to insertion of stack alignment
instructions. Global register variable RBP is copied into RAX for no reason
whatsoever, subverting loading the next instruction before recomputing the
instruction pointer.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44281
