> It is possible to have OoO without speculative execution
That's technically correct (the best type of correct!), but without speculation, the CPU can't see beyond a branch, so the window in which the machine can reorder instructions is very small. (For x86 code, the usual rule of thumb is that branches occur once per 5 instructions.) So in practical terms, an OoO design (in the restricted-dataflow paradigm, i.e., all modern general-purpose OoO cores) always at least does branch prediction, to keep the machine adequately full of ops.
An interesting question, though, given that we're going to do speculation, is what types of speculation the core can do. The practical minimum is just branch prediction, but modern cores do all sorts of other speculation in memory disambiguation, instruction scheduling, and the like, some of which has enabled real side-channel attacks (e.g., Meltdown).
Also, FWIW, Pentium was 2-way superscalar, but not OoO in the fully general dataflow-driven sense; the Pentium Pro (aka P6), in 1995, was the first OoO. (Superscalar can technically be seen as OoO, I suppose, in that the machine checks dependencies and can execute independent instructions simultaneously, but it's not usually considered as such, as the "reordering" window is just 2 instructions.)
That's technically correct (the best type of correct!), but without speculation, the CPU can't see beyond a branch, so the window in which the machine can reorder instructions is very small. (For x86 code, the usual rule of thumb is that branches occur once per 5 instructions.) So in practical terms, an OoO design (in the restricted-dataflow paradigm, i.e., all modern general-purpose OoO cores) always at least does branch prediction, to keep the machine adequately full of ops.
An interesting question, though, given that we're going to do speculation, is what types of speculation the core can do. The practical minimum is just branch prediction, but modern cores do all sorts of other speculation in memory disambiguation, instruction scheduling, and the like, some of which has enabled real side-channel attacks (e.g., Meltdown).
Also, FWIW, Pentium was 2-way superscalar, but not OoO in the fully general dataflow-driven sense; the Pentium Pro (aka P6), in 1995, was the first OoO. (Superscalar can technically be seen as OoO, I suppose, in that the machine checks dependencies and can execute independent instructions simultaneously, but it's not usually considered as such, as the "reordering" window is just 2 instructions.)