https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109246
Bug ID: 109246
Summary: Missed optimization for 2-dimensional array with equal
values accessed through Enums
Product: gcc
Version: 12.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: andre.schackier at gmail dot com
Target Milestone: ---
Given the following code:
enum class E {
A = 0,
B = 1,
};
constexpr int t[2][2]{{1, 1}, {1, 1}};
int f1(E a) { return t[static_cast<int>(a)][static_cast<int>(a)]; }
int f2(E a, E b) { return t[static_cast<int>(a)][static_cast<int>(b)]; }
and compiling with '-O3 -fstrict-enums' gives:
- https://godbolt.org/z/s8jssnqq5
f1(E):
movsx rdi, edi
lea rax, [rdi+rdi*2]
mov eax, DWORD PTR t[0+rax*4]
ret
f2(E, E):
movsx rsi, esi
movsx rdi, edi
lea rax, [rsi+rdi*2]
mov eax, DWORD PTR t[0+rax*4]
ret
t:
.long 1
.long 1
.long 1
.long 1
gcc should be able to determine that in all cases both 'f1' and 'f2' can only
ever return 1.
The same is happening for
3x3 - https://godbolt.org/z/7v7hfq357
4x4 - https://godbolt.org/z/fTejK8YKb
5x5 - https://godbolt.org/z/31GcKPG6c
arrays.
Also explicitly telling the compiler about the valid values for 'a' and 'b'
using '__builtin_unreachable()' like this:
enum class E {
A = 0,
B = 1,
};
constexpr int t[2][2]{{1, 1}, {1, 1}};
int f1(E a) {
if (a != E::A && a != E::B) {
__builtin_unreachable();
}
return t[static_cast<int>(a)][static_cast<int>(a)];
}
int f2(E a, E b) {
if (a != E::A && a != E::B) {
__builtin_unreachable();
}
if (b != E::A && b != E::B) {
__builtin_unreachable();
}
return t[static_cast<int>(a)][static_cast<int>(b)];
}
does not help - https://godbolt.org/z/6x7xKo1xj
