On Monday, May 11th, 2026 at 12:55, Richard Biener
<[email protected]> wrote:
> The reason is that i might overflow and thus
> the evolution of 'i' for the haystack[i] access isn't affine.
> Use unsigned long and it works again.
>
> This is a general issue with unsigned IVs smaller than
> pointer size.
Sorry, I somehow messed up my tests and misunderstood what was going on.
I was more interested in this example. This doesn't get vectorized (`-O3
-march=znver5 -ffreestanding`):
unsigned long foo(uint8_t *data) {
unsigned long i = 0;
while (1) {
if (data[i] == 0)
return i;
i++;
}
}
But this does:
long foo(uint8_t *data) {
long i = 0;
while (1) {
if (data[i] == 0)
return i;
i++;
}
}
`unsigned long` index coupled `uint16_t *` data does get vectorized,
though.
Is it because technically `uint8_t[size_t]` can span the whole address
space and therefore the `size_t` index can validly wrap around, whereas
`uint16_t[size_t]` is bounded by the address space so you can never
reach `uint16_t[2 + (SIZE_MAX/2)]`?
