https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109950
Bug ID: 109950
Summary: can array subscripts be assumed to be non-negative?
Product: gcc
Version: 14.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: lh_mouse at 126 dot com
Target Milestone: ---
There is a lot of historical code which has been using `int` for array indexes:
```
extern int data[];
extern int next;
int
test_function(int* outptr)
{
*outptr = data[next];
return next >= 0;
}
```
In this example, the value of `next` is used as an array index. Despite the
unknown size, elements in an array can't have negative indexes, so maybe here
`next >= 0` can be optimized to a constant?
Specifically about x86_64, there is some more optimization that we can do: The
value of `next` can be loaded into a 32-bit register, zeroing the upper half
implicitly, without using a sign-extension instruction, as in machine code:
```
48 63 05 IMM32 ; movsxd rax, dword ptr [rip + IMM32]
8B 05 IMM32 ; mov eax, dword ptr [rip + IMM32]
```
The former contains an operand size override prefix, takes up one more byte,
and costs an extra cycle to decode.
