[fpc-devel] Potential whole program optimization

[Martin Frb]

On 19/07/2021 01:24, J. Gareth Moreton via fpc-devel wrote:
>
> The final assembly language of each method is identical.
>
> (Note that the trunk is not this efficient just yet... it still leaves 
> a "movq %rcx,%rax" instruction prior to "movl $1,%r8d" and then calls 
> "movq (%rax),%rax" instead of "movq (%rcx),%rax" - it's still all 
> identical though).
>
> Would it be plausible to calculate and store a form of message digest 
> (hash) of the final form of the tai entries or machine code and 
> identify collisions and potential duplicate procedures for 
> whole-program optimization? Granted I don't know anything about WPO 
> yet so I don't know how plausible this is.  This wouldn't be somethind 
> done on quick or debug builds because you'll need to be able to do 
> proper stack traces, and having identical procedures merged into one 
> might cause confusion.

Why WPO?

When you compute this hash, you could immediately check if it already 
exists. (either in this unit, or any unit used).

The only miss would be, if 2 units, that do not see each other include 
identical code. That part could then still be a WPO.