https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109045
Bug ID: 109045
Summary: assume attribute and std::optional do not mix
Product: gcc
Version: 13.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: drepper.fsp+rhbz at gmail dot com
Target Milestone: ---
The assume attribute is meant to help expressing more complex assumptions which
involve function calls. Given that interfaces should use std::optional when
the semantics matches this should mean code like this should be optimized:
#include <optional>
std::optional<long> g(long);
long f(long a)
{
auto r = g(a);
[[assume(!r || *r > 0)]];
return r.value_or(0) / 2;
}
The generated code should use the unsigned divide by two method but it does
not. With today's gcc trunk version:
0000000000000000 <_Z1fl>:
0: 48 83 ec 18 sub $0x18,%rsp
4: e8 00 00 00 00 call 9 <_Z1fl+0x9>
9: 48 89 04 24 mov %rax,(%rsp)
d: 48 89 54 24 08 mov %rdx,0x8(%rsp)
12: 31 c0 xor %eax,%eax
14: 80 7c 24 08 00 cmpb $0x0,0x8(%rsp)
19: 74 11 je 2c <_Z1fl+0x2c>
1b: 48 8b 14 24 mov (%rsp),%rdx
1f: 48 89 d0 mov %rdx,%rax
22: 48 c1 e8 3f shr $0x3f,%rax
26: 48 01 d0 add %rdx,%rax
29: 48 d1 f8 sar %rax
2c: 48 83 c4 18 add $0x18,%rsp
30: c3 ret
The instructions from 1f to 28 including are not needed (and the initial load
at 1b adjusted).
