[fpc-devel] CSE and node optimisation question

[J. Gareth Moreton]

Regarding the "intolerably slow" option, I had the build script running 
for about 45 minutes as it tried to get through ppc2, and then it 
crashed with runtime error -1073741819.  Converting this to hexadecimal 
yields C0000005, which I know from experience is an access violation 
(usually in the kernel somewhere).

So, long story short, I'm still stuck for now!

Gareth aka. Kit

On 25/02/2021 03:31, J. Gareth Moreton via fpc-devel wrote:
> Hi everyone,
>
> I've been looking at some node-level optimisations as of late, and I 
> came across a couple of potential ones while doing experiments with 
> the test on i38527, namely (R is an ordinal tempref, and n is an 
> ordinal constant):
>
> add
>   |- add
>   |   |- R
>   |   |- R
>   |
>   |- R
>
> Becoming:
>
> mul
>   |- R
>   |- 3
>
> And:
>
> add
>   |- mul
>   |   |- R
>   |   |- n
>   |
>   |- R
>
> Becoming:
>
> mul
>   |- R
>   |- n+1
>
> I've been having a couple of issues though that I haven't been able to 
> solve, the main thing is that taddnode.simplify doesn't get called to 
> transmute the nodes as desired because pass 1 is already complete for 
> most of the nodes by the time CSE is executed.
>
> I've tried a few workarounds, but ran into problems:
>
> - Once CSE is complete, calling simplify via foreachnodestatic with 
> the pre-process option is unacceptably slow.
> - Attempting to selectively reset the pass 1 flags (in an attempt to 
> minimise the massive performance loss with manually calling simplify 
> as above) causes Internal Error 200405231 because it seems to upset 
> implicit finally blocks.
> - Moving "add_entry_exit_code" to after "optcse" causes compilation of 
> case blocks to malfunction.
>
> Can anyone give me advice on how to implement such a node-level 
> CSE-specific optimisation? Such a thing may be useful for pure 
> functions (as well as when writing things like "4 * n + 4 * n + 4 * n 
> + 4 * n + 4 * n"!).  Thanks in advance.
>
> Gareth aka. Kit
>
>

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