indexOfScalarPos

pub fn indexOfScalarPos(comptime T: type, slice: []const T, start_index: usize, value: T) ?usize {
    if (start_index >= slice.len) return null;

    var i: usize = start_index;
    if (use_vectors_for_comparison and
        !std.debug.inValgrind() and // https://github.com/ziglang/zig/issues/17717
        !@inComptime() and
        (@typeInfo(T) == .int or @typeInfo(T) == .float) and std.math.isPowerOfTwo(@bitSizeOf(T)))
    {
        if (std.simd.suggestVectorLength(T)) |block_len| {
            // For Intel Nehalem (2009) and AMD Bulldozer (2012) or later, unaligned loads on aligned data result
            // in the same execution as aligned loads. We ignore older arch's here and don't bother pre-aligning.
            //
            // Use `std.simd.suggestVectorLength(T)` to get the same alignment as used in this function
            // however this usually isn't necessary unless your arch has a performance penalty due to this.
            //
            // This may differ for other arch's. Arm for example costs a cycle when loading across a cache
            // line so explicit alignment prologues may be worth exploration.

            // Unrolling here is ~10% improvement. We can then do one bounds check every 2 blocks
            // instead of one which adds up.
            const Block = @Vector(block_len, T);
            if (i + 2 * block_len < slice.len) {
                const mask: Block = @splat(value);
                while (true) {
                    inline for (0..2) |_| {
                        const block: Block = slice[i..][0..block_len].*;
                        const matches = block == mask;
                        if (@reduce(.Or, matches)) {
                            return i + std.simd.firstTrue(matches).?;
                        }
                        i += block_len;
                    }
                    if (i + 2 * block_len >= slice.len) break;
                }
            }

            // {block_len, block_len / 2} check
            inline for (0..2) |j| {
                const block_x_len = block_len / (1 << j);
                comptime if (block_x_len < 4) break;

                const BlockX = @Vector(block_x_len, T);
                if (i + block_x_len < slice.len) {
                    const mask: BlockX = @splat(value);
                    const block: BlockX = slice[i..][0..block_x_len].*;
                    const matches = block == mask;
                    if (@reduce(.Or, matches)) {
                        return i + std.simd.firstTrue(matches).?;
                    }
                    i += block_x_len;
                }
            }
        }
    }

    for (slice[i..], i..) |c, j| {
        if (c == value) return j;
    }
    return null;
}