[fpc-devel] Question on updating FPC packages

J. Gareth Moreton gareth at moreton-family.com
Tue Oct 29 15:27:58 CET 2019

Oh, I just noticed you're replying to messages from a few days ago.  Oops!

There is no right way of going about optimisation.  I'm of the school 
that if you can give the compiler a helpful hint, without complicating 
the code, then do it.

In one way I compare it to the id Tech (Quake) and Unreal engines back 
in the 90s and early 2000s.  When making maps, the id Tech engines 
attempted to compile everything itself when it came to determining what 
was visible and what it should cull - as a result, the compilation 
process would take a long time and there were some situations where it 
could easily fall apart due to rounding errors or just some glitch in 
the tree.  The Unreal engine, on the other hand, had /you/, the map 
designer, decide what was visible and what wasn't, and had you decide 
where to place portals and other hints to the engine.  This was useful 
because it was much easier to subdivide areas if you were sensible about 
it and hence the Unreal engine could handle much more complex outdoor 
scenes, for example.  The cost though, especially with later versions of 
the Unreal engine that added more features, is that it was very hard for 
a novice to get started - for example, the 'terrain' feature didn't do 
any automatic visibility culling, so if you had a large hill, for 
example, you would have to insert an 'anti-portal' underneath it to give 
a hint to the engine that if it is within the viewport, any polygons 
behind it is invisible (which causes very weird artefacts if you place 
one in the middle of an open room).

I like to take a middle ground, especially as the Pascal compiler has a 
reputation of being fast.  A smart compiler is a good compiler, but 
expecting it to be able to know which procedures should be 
auto-vectorised, especially with old source code and no rules on memory 
alignment, it's either impossible or will take a disproportinately long 
time.  Other times it's an excuse for lazy programming!

As for the vectorcall tests, they should vectorise the entire argument 
on both x86_64-win64 and x86_64-linux.  If not, there's a bug 
somewhere.  I'll have a look.

Gareth aka. Kit

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