https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99091
Bug ID: 99091
Summary: constexpr variable evaluated at runtime
Product: gcc
Version: 10.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: barry.revzin at gmail dot com
Target Milestone: ---
Consider the following (link to compiler explorer:
https://godbolt.org/z/jWsxsK)
#include <array>
static constexpr auto doy = []{
std::array<int, 13> month_offset{};
for (int m=1; m<=12;++m) {
month_offset[m] = (153*(m > 2 ? m-3 : m+9) + 2)/5;
}
return month_offset;
}();
auto foo(int m) -> int {
#ifdef LOCAL
constexpr auto doy = []{
std::array<int, 13> month_offset{};
for (int m=1; m<=12;++m) {
month_offset[m] = (153*(m > 2 ? m-3 : m+9) + 2)/5;
}
return month_offset;
}();
#endif
return doy[m];
}
This is a fragment of code that does some calculation to figure out the date,
which isn't super important. If LOCAL is not defined (i.e. we declare the array
as a namespace-scope constexpr), the -O3 codegen of 'foo' is:
foo(int):
movsx rdi, edi
mov eax, DWORD PTR doy[0+rdi*4]
ret
However, if we want to move the declaration of doy to be more local to the
calculation and compile with -DLOCAL, the codegen instead is (on -O3):
foo(int):
pxor xmm0, xmm0
mov rax, QWORD PTR .LC0[rip]
movsx rdi, edi
mov DWORD PTR [rsp-24], 275
movaps XMMWORD PTR [rsp-72], xmm0
movdqa xmm0, XMMWORD PTR .LC1[rip]
mov QWORD PTR [rsp-68], rax
movaps XMMWORD PTR [rsp-56], xmm0
movdqa xmm0, XMMWORD PTR .LC2[rip]
movaps XMMWORD PTR [rsp-40], xmm0
mov eax, DWORD PTR [rsp-72+rdi*4]
ret
This can be alleviated by marked the local variable doy as being static
constexpr. Except that this prevents 'foo' from being a constexpr function
(can't have static variables).
This just seems like bad codegen, I'm not sure there's any reason that the
compiler needs to do work here. It would be nice if the codegen with a local
constexpr variable were the same as if it were a non-local constexpr variable.
