https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87987
Bug ID: 87987
Summary: Missed optimization with ranged-for loop on a
constexpr array
Product: gcc
Version: 8.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: lh_mouse at 126 dot com
Target Milestone: ---
This simple program:
```c++
static constexpr bool table[] = { 1, 0, 0, 1, 1, 0, 1, 0 };
int check()
{
int sum = 0;
for(auto value : table) {
sum += value;
}
return sum;
}
```
after being compiled by GCC 8.2 with `-std=c++11 -O2 -Wall -Wextra -Wpedantic
-Werror`, yields a loop:
```asm
check():
mov edx, OFFSET FLAT:table
xor eax, eax
.L2:
movzx ecx, BYTE PTR [rdx]
add rdx, 1
add eax, ecx
cmp rdx, OFFSET FLAT:table+8
jne .L2
ret
```
, while Clang 6.0 optimizes the body to a constant:
```asm
check(): # @check()
mov eax, 4
ret
```
( Online comparison can be viewed here: https://gcc.godbolt.org/z/oaSr6j )
Making the function `constexpr` however overcomes this obstacle:
```c++
static constexpr bool table[] = { 1, 0, 0, 1, 1, 0, 1, 0 };
// This requires only C++11.
constexpr int check_constexpr(const bool *p, int n)
{
return (n == 0) ? 0 : *p + check_constexpr(p + 1, n - 1);
}
int check()
{
return check_constexpr(table, sizeof(table));
}
```
( And here is the online comparison for this one:
https://gcc.godbolt.org/z/HZjBSh )
