https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77295
Bug ID: 77295
Summary: missed optimisation when copying/moving union members
Product: gcc
Version: 6.1.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: a...@cloudius-systems.com
Target Milestone: ---
Discriminated unions of class types are becoming popular; for example
std::variant<...> or std::future<T>.
gcc doesn't optimize copies or moved of discriminated unions well:
// Will usually be a template with user-defined types
// as members of the union
struct discriminated_union {
unsigned which;
union {
int v1;
long v2;
};
discriminated_union(discriminated_union&&);
};
discriminated_union::discriminated_union(discriminated_union&& x) {
which = x.which;
switch (x.which) {
case 1:
v1 = x.v1;
break;
case 2:
v2 = x.v2;
break;
}
}
compiles into
0: 8b 06 mov (%rsi),%eax
2: 89 07 mov %eax,(%rdi)
4: 8b 06 mov (%rsi),%eax
6: 83 f8 01 cmp $0x1,%eax
9: 74 1d je 28
<discriminated_union::discriminated_union(discriminated_union&&)+0x28>
b: 83 f8 02 cmp $0x2,%eax
e: 75 10 jne 20
<discriminated_union::discriminated_union(discriminated_union&&)+0x20>
10: 48 8b 46 08 mov 0x8(%rsi),%rax
14: 48 89 47 08 mov %rax,0x8(%rdi)
18: c3 retq
19: 0f 1f 80 00 00 00 00 nopl 0x0(%rax)
20: f3 c3 repz retq
22: 66 0f 1f 44 00 00 nopw 0x0(%rax,%rax,1)
28: 8b 46 08 mov 0x8(%rsi),%eax
2b: 89 47 08 mov %eax,0x8(%rdi)
2e: c3 retq
instead of just copying the 12 bytes from (%rsi) into (rdi); unconditionally
copying the long is cheaper than the branching.
