http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53792
Giulio Eulisse <giulio.eulisse at cern dot ch> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |giulio.eulisse at cern dot
| |ch
--- Comment #6 from Giulio Eulisse <giulio.eulisse at cern dot ch> 2012-08-08
11:17:58 UTC ---
A simpler testcase for the underlying problem is the following.
struct entry {
char const* label;
int value;
};
constexpr bool same(char const *x, char const *y) {
return !*x && !*y ? true
: /* default */ (*x == *y && same(x+1, y+1));
}
constexpr int keyToValue(char const *label, entry const *entries) {
return !entries->label ? entries->value
: same(entries->label, label) ? entries->value
: /*default*/ keyToValue(label, entries+1);
}
constexpr entry foo[] = {{"Foo", 0}, {"Bar", 1}, {"FooBar", 2}, {0, -1}};
int
bar()
{
/* constexpr */ int result = keyToValue("Foo", foo); // Uncomment constexpr
for optimized version.
return result;
}
Without the constexpr the code is fully expanded as if the arguments were
generic:
0000000000000000 <_Z3barv>:
0: 53 push %rbx
1: bf 00 00 00 00 mov $0x0,%edi
6: bb 00 00 00 00 mov $0x0,%ebx
b: eb 0f jmp 1c <_Z3barv+0x1c>
d: 0f 1f 00 nopl (%rax)
10: 48 83 c3 10 add $0x10,%rbx
14: 48 8b 3b mov (%rbx),%rdi
17: 48 85 ff test %rdi,%rdi
1a: 74 1d je 39 <_Z3barv+0x39>
1c: 80 3f 46 cmpb $0x46,(%rdi)
1f: 75 ef jne 10 <_Z3barv+0x10>
21: 80 7f 01 6f cmpb $0x6f,0x1(%rdi)
25: 75 e9 jne 10 <_Z3barv+0x10>
27: 48 83 c7 02 add $0x2,%rdi
2b: be 00 00 00 00 mov $0x0,%esi
30: e8 00 00 00 00 callq 35 <_Z3barv+0x35>
35: 84 c0 test %al,%al
37: 74 d7 je 10 <_Z3barv+0x10>
39: 8b 43 08 mov 0x8(%rbx),%eax
3c: 5b pop %rbx
3d: c3 retq
Disassembly of section .text._Z4samePKcS0_:
0000000000000000 <_Z4samePKcS0_>:
0: 0f b6 17 movzbl (%rdi),%edx
3: 84 d2 test %dl,%dl
5: 75 09 jne 10 <_Z4samePKcS0_+0x10>
7: 80 3e 00 cmpb $0x0,(%rsi)
a: 0f 94 c0 sete %al
d: c3 retq
e: 66 90 xchg %ax,%ax
10: 31 c0 xor %eax,%eax
12: 3a 16 cmp (%rsi),%dl
14: 74 0a je 20 <_Z4samePKcS0_+0x20>
16: c3 retq
17: 66 0f 1f 84 00 00 00 nopw 0x0(%rax,%rax,1)
1e: 00 00
20: 0f b6 47 01 movzbl 0x1(%rdi),%eax
24: 84 c0 test %al,%al
26: 75 10 jne 38 <_Z4samePKcS0_+0x38>
28: 80 7e 01 00 cmpb $0x0,0x1(%rsi)
2c: b8 01 00 00 00 mov $0x1,%eax
31: 75 0a jne 3d <_Z4samePKcS0_+0x3d>
33: f3 c3 repz retq
35: 0f 1f 00 nopl (%rax)
38: 3a 46 01 cmp 0x1(%rsi),%al
3b: 74 0b je 48 <_Z4samePKcS0_+0x48>
3d: 31 c0 xor %eax,%eax
3f: 90 nop
40: c3 retq
41: 0f 1f 80 00 00 00 00 nopl 0x0(%rax)
48: 48 83 ec 08 sub $0x8,%rsp
4c: 48 83 c6 02 add $0x2,%rsi
50: 48 83 c7 02 add $0x2,%rdi
54: e8 00 00 00 00 callq 59 <_Z4samePKcS0_+0x59>
59: 84 c0 test %al,%al
5b: 74 10 je 6d <_Z4samePKcS0_+0x6d>
5d: b8 01 00 00 00 mov $0x1,%eax
62: 66 0f 1f 44 00 00 nopw 0x0(%rax,%rax,1)
68: 48 83 c4 08 add $0x8,%rsp
6c: c3 retq
6d: 31 c0 xor %eax,%eax
6f: eb f7 jmp 68 <_Z4samePKcS0_+0x68>
while with constexpr on results everything is optimized out:
0000000000000000 <_Z3barv>:
0: 31 c0 xor %eax,%eax
2: c3 retq
