[Bug middle-end/110018] Missing vectorizable_conversion(unsigned char -> double) for BB vectorizer

[crazylht at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110018

--- Comment #3 from Hongtao.liu <crazylht at gmail dot com> ---
Even we have direct optab for unsigned char to long long, BB vectorizer failer
to optimize

void
foo1 (long long* __restrict a, unsigned char* b)
{
    a[0] = b[0];
    a[1] = b[1];
    a[2] = b[2];
    a[3] = b[3];
    a[4] = b[4];
    a[5] = b[5];
    a[6] = b[6];
    a[7] = b[7];
}

since we have vectype_out as V4DImode, but vectype_in as V8QImode as *vectype =
SLP_TREE_VECTYPE (child);

If manually change the testcase to

void
foo1 (long long* __restrict a, unsigned char* b)
{
    a[0] = b[0];
    a[1] = b[1];
    a[2] = b[2];
    a[3] = b[3];
}

Then BB vectorizer can do vectorization, but still failed for

void
foo1 (double* __restrict a, unsigned char* b)
{
    a[0] = b[0];
    a[1] = b[1];
    a[2] = b[2];
    a[3] = b[3];
}

So it looks there're 2 separate issue
1. As mentioned in the PR, we can use intermediate type for vectorizable
unsigned char to double.
2. There's mismatch between vectype_out and vectype_in as SLP_TREE_VECTYPE
(child);