Make GCC move instructions between a multi-cycle instruction and the next instruction that depends on its result.

[William Tambe]

On Tue, Feb 22, 2022 at 1:00 PM William Tambe <tambewilliam@gmail.com> wrote:
>
> In this CPU,
>
> A multi-cycle instruction, once decoded, runs in parallel as other
> decoded single/multi-cycle instructions.
>
> A single/multi-cycle instruction takes two operands, where the first
> operand receives the result
> of computing both operands.
>
> An example of multi-cycle instruction is "div".
> An example of single-cycle instruction is "add".
>
> GCC should be able to transform following:
> ````
> add %0 %5
> add %1 %6
> div %4 %5 #<-- Multi-cycle instruction.
> add %4 %7 #<-- Next instruction that depends on its result.
> add %3 %7
> add %2 %7
> ```
> To:
> ```
> div %4 %5 #<-- Multi-cycle instruction.
> add %0 %5
> add %1 %6
> add %3 %7
> add %2 %7
> add %4 %7 #<-- Next instruction that depends on its result.
> ```
> Without above transformation, `add %4 %7` would cause the cpu to wait
> on `div %4 %5` when it could have executed instructions that do not
> depend on the result of "div".

How to implement above transformation such that GCC moves instructions
between a multi-cycle instruction and the next instruction that
depends on its result ?