[fpc-devel] Pure function development

J. Gareth Moreton gareth at moreton-family.com
Mon May 4 22:53:53 CEST 2020

So I got my salvaged code transferred over, now I'm mostly researching 
the constant propagation and inlining code.  I may have to make 
adjustments to the constant propagation code so i can reuse it, so I can 
introduce things like a node counter (to catch infinite loops) as well 
as allow the use of TCallNode (I'll probably just modify the structure 
that the 'arg' parameter points to so it can contain flags and a counter 
and the like).

The one I'm still working out theoretically is determining if processing 
the pure function this way has actually yielded a straightforward result 
and not some complex node tree.  Ideally, it should be just a simple 
tree that writes the answer to the temporary result storage (and any 
'out' parameters that exist, although given this approach, it might work 
with 'var' parameters as well in some situations).  If it's a more 
complex tree that manipulates variables, then it should reject it and 
just call the function conventionally (and throw a compiler warning to 
indicate the function is not pure).

There is one other situation where a function might be pure but not 
inline... if the function is so large that making it inline is extremely 
wasteful, but is otherwise technically a pure function and will give a 
deterministic result for a given input.  An example might be a function 
that calcuates a MacLaurin series in order to produce a result of the 
desired precision.

Gareth aka. Kit

On 02/05/2020 20:55, J. Gareth Moreton wrote:
> Maybe I've fallen into a trap of sunken cost fallacy or being too 
> proud of my own code rather than properly looking at what's already 
> available.  Part of my fear with using the constant propagation code 
> is that it constantly copies and transforms the nodes every time the 
> pure function needs to be evaluated, which I'm concerned will incur a 
> notable speed penalty.
> I reuse the node tree that inline functions get, thereby saving 
> storage in the PPU file.
> Regarding determining if functions are pure or not, I have two flags 
> to help determine this;
> - the first is "po_pure" under tprocoptions, which is set when it sees 
> the 'pure' directive, and is cleared (with a compiler warning) if the 
> compiler spots something that makes the function ineligible (e.g. 
> raising an exception, an uninitialized variable, accessing a static 
> variable etc.)
> - the second one is "pi_pure_uncertain" under tprocinfoflags. This is 
> set when the node builder sees something that makes it uncertain if 
> the function can be pure or not, although it might still be possible 
> (e.g. calling another procedure, and currently the presence of 
> 'raise', 'goto' and any kind of loop, due to the risk of it being 
> infinite)
> The "pi_pure_uncertain" flag may be unnecessary, but if it remains 
> clear by the time 'pass_1' is finished and the procedure doesn't have 
> the 'pure' directive, the compiler is able to drop a hint to say that 
> the function is eligible, since the function has completely linear 
> flow and isn't accessing anything outside of its scope.
> For the limit on how many nodes to evaluate before it drops out, I had 
> a counter in the node emulator class that was a static variable, 
> incrementing every time a node is evaluated (it's a static var because 
> a new emulator object was instantiated if if the first one came across 
> a call to another pure function).  How should I implement a node 
> counter with the constant propagation code?
> For one final speed-up, each function that is pure can store 
> previously-calculated results for a given set of parameters; e.g. 
> after calculating Factorial(5) = 120, the compiler can recall this 
> answer (or a copy of the nodes that give the answer) for subsequent 
> calls to Factorial(5), thereby reducing compilation time and memory 
> strain.  It does subtly increase the node limit before it drops out on 
> loops or recursion that are excessively long, but not infinite (e.g. 
> the Ackermann Function), but this shouldn't incur a performance penalty.
> I'll shelve my node emulator for now because of it being entirely 
> separate to the constant propagation code, and see if I can adapt the 
> constant propagation code.
> Gareth aka. Kit
> On 02/05/2020 19:51, Jonas Maebe wrote:
>> On 02/05/2020 20:27, J. Gareth Moreton wrote:
>>> Well, as I've found, there is no straightforward method to actually
>>> determine if a function is pure or not.  For example, if it stumbles
>>> upon a procedural call, which may be itself (recursion), it doesn't
>>> immediately know if that call is to a procedure that is itself pure or
>>> not.
>> Generally, the way to deal with recursion is to start by assuming it is
>> in fact pure (or whatever property you are checking). If it is still
>> considered pure once you processed it entirely, then the property holds.
>>> There are also problems if calculating the value of a pure
>>> function may raise an exception (either by explicitly calling 
>>> 'raise' or
>>> doing an integer division by zero, for example),
>> A function that explicitly raises an exception can never be pure, since
>> an exception changes global state and there is no way to know what
>> raising this particular exception means (e.g., it could a hack to return
>> a value several levels up the stack, or to implement an interprocedural
>> goto). It might indeed not raise an exception for particular inputs, but
>> that is no different from a function that e.g. does not read from or
>> write to any global data for certain inputs.
>> Implicit exceptions, i.e. run time errors, are different. In that case
>> you will get a compile-time warning or error similar as during normal
>> constant evaluation, depending on the active switches like range 
>> checking.
>>> something that breaks
>>> things down when assigning pure function results to constant
>>> definitions.  And let's not get started if the pure function contains a
>>> deliberate infinite loop!
>> This requires limiting the number of evaluation steps/iterations.
>> Jonas
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