| Issue |
161389
|
| Summary |
[Clang] Missed optimization (flatten array access)
|
| Labels |
clang
|
| Assignees |
|
| Reporter |
richtw1
|
```
extern int (*fns[8][8])(int);
int dispatch(unsigned int state, int param) {
return fns[state / 8][state % 8](param);
}
int dispatch2(unsigned int state, int param) {
// cast to 1d flattened array
return (*(int (*(*)[64])(int))&fns)[state](param);
}
```
https://godbolt.org/z/oqjP1Pvsc
In `dispatch()`. the optimizer ought to notice that the `state` index can map directly to the flattened array in memory., and emit code like `dispatch2()`. Instead it emits more complicated, redundant code:
```
dispatch:
mov eax, edi
mov ecx, edi
shr ecx, 3
and eax, 7
shl rcx, 6
add rcx, qword ptr [rip + fns@GOTPCREL]
mov edi, esi
jmp qword ptr [rcx + 8*rax]
dispatch2:
mov eax, edi
mov rcx, qword ptr [rip + fns@GOTPCREL]
mov edi, esi
jmp qword ptr [rcx + 8*rax]
```
GCC and MSVC both generate the simpler code in each case.
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