https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112296
Bug ID: 112296
Summary: __builtin_constant_p doesn't propagate through member
functions
Product: gcc
Version: 13.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: barry.revzin at gmail dot com
Target Milestone: ---
Here's a short example:
using size_t = decltype(sizeof(0));
struct Span {
int const* ptr;
size_t len;
inline constexpr auto size() const noexcept -> size_t { return len; }
};
inline int direct(Span span) {
return __builtin_constant_p(span.size());
}
inline int indirect(Span span) {
size_t s = span.size();
return __builtin_constant_p(s);
}
int call_direct(int const* p) {
return direct({.ptr=p, .len=8});
}
int call_indirect(int const* p) {
return indirect({.ptr=p, .len=8});
}
The functions direct() and indirect() do the same thing - try to see if span's
size is constant, with direct checking size() directly and indirect first
caching it to a local variable and checking that variable. In both cases, the
size is constant - but on -O3 call_direct() returns 0 while call_indirect()
returns 1. That is, __builtin_constant_p tells me the size is constant - but
only if I put it into a variable first.
Checking __builtin_constant_p(span.len) also returns 1, but in the real code
the member variable is private and my only access to it is via the size()
function.
