# [fpc-devel] Progress on pure functions

J. Gareth Moreton gareth at moreton-family.com
Sat Aug 12 19:14:17 CEST 2023

```Hi everyone,

So I'm still working on pure functions and have pushed some merge
requests that are indirectly related to it, mostly simplifying the node
tree so it can more easily be collapsed into simple assignments (what
pure functions should simplify to).  Negative testing is still limited,
but I have stumbled across one potential problem.  Using the following
code example:

program pure1a;
{\$MODE OBJFPC}
{\$COPERATORS ON}

function Factorial(N: Cardinal): Cardinal; pure;
var
X: Integer;
begin
Result := 1;
for X := N downto 2 do
Result *= X;
end;

begin
WriteLn(Factorial(32));
end.

For those not familiar with the factorial function, the result increases
in value very quickly to the point that 13! > 2^32 (LongWord), 21! >
2^64 (QWord) and 70! > 10^100 (a googol), so 32! is guaranteed to overflow.

Having not really handled this eventuality just yet, I decided to see
what happens when the compiler runs it as is:

pure1a.pp(15,24) Warning: Range check error while evaluating constants
(6227020800 must be between -2147483648 and 2147483647)
pure1a.pp(15,24) Warning: Range check error while evaluating constants
(27048749056 must be between -2147483648 and 2147483647)
pure1a.pp(15,24) Warning: Range check error while evaluating constants
(19184179200 must be between -2147483648 and 2147483647)
pure1a.pp(15,24) Warning: Range check error while evaluating constants
(32068960256 must be between -2147483648 and 2147483647)
pure1a.pp(15,24) Warning: Range check error while evaluating constants
(34071216128 must be between -2147483648 and 2147483647)
pure1a.pp(15,24) Warning: Range check error while evaluating constants
(-288522240 must be between 0 and 4294967295)
pure1a.pp(15,24) Warning: Range check error while evaluating constants
(4006445056 must be between -2147483648 and 2147483647)
pure1a.pp(15,24) Warning: Range check error while evaluating constants
(-5193400320 must be between -2147483648 and 2147483647)
pure1a.pp(15,24) Warning: Range check error while evaluating constants
(-898433024 must be between 0 and 4294967295)
pure1a.pp(15,24) Warning: Range check error while evaluating constants
(3396534272 must be between -2147483648 and 2147483647)
pure1a.pp(15,24) Warning: Range check error while evaluating constants
(-17070227456 must be between -2147483648 and 2147483647)
pure1a.pp(15,24) Warning: Range check error while evaluating constants
(-2102132736 must be between 0 and 4294967295)
pure1a.pp(15,24) Warning: Range check error while evaluating constants
(2192834560 must be between -2147483648 and 2147483647)
pure1a.pp(15,24) Warning: Range check error while evaluating constants
(-44144787456 must be between -2147483648 and 2147483647)
pure1a.pp(15,24) Warning: Range check error while evaluating constants
(-1195114496 must be between 0 and 4294967295)
pure1a.pp(15,24) Warning: Range check error while evaluating constants
(3099852800 must be between -2147483648 and 2147483647)
pure1a.pp(15,24) Warning: Range check error while evaluating constants
(-26292518912 must be between -2147483648 and 2147483647)
pure1a.pp(15,24) Warning: Range check error while evaluating constants
(-522715136 must be between 0 and 4294967295)
pure1a.pp(15,24) Warning: Range check error while evaluating constants
(3772252160 must be between -2147483648 and 2147483647)
pure1a.pp(15,24) Warning: Range check error while evaluating constants
(-12022448128 must be between -2147483648 and 2147483647)
pure1a.pp(15,24) Warning: Range check error while evaluating constants
(20698890240 must be between -2147483648 and 2147483647)
pure1a.pp(15,24) Warning: Range check error while evaluating constants
(-775946240 must be between 0 and 4294967295)
pure1a.pp(15,24) Warning: Range check error while evaluating constants
(3519021056 must be between -2147483648 and 2147483647)
pure1a.pp(15,24) Warning: Range check error while evaluating constants
(-19398656000 must be between -2147483648 and 2147483647)
pure1a.pp(15,24) Warning: Range check error while evaluating constants
(53980692480 must be between -2147483648 and 2147483647)
pure1a.pp(15,24) Warning: Range check error while evaluating constants
(-1853882368 must be between 0 and 4294967295)
pure1a.pp(15,24) Warning: Range check error while evaluating constants
(2441084928 must be between -2147483648 and 2147483647)
pure1a.pp(15,24) Warning: Range check error while evaluating constants
(-50054823936 must be between -2147483648 and 2147483647)
pure1a.pp(15,24) Warning: Range check error while evaluating constants
(41573941248 must be between -2147483648 and 2147483647)
pure1a.pp(15,24) Warning: Range check error while evaluating constants
(-1375731712 must be between 0 and 4294967295)
pure1a.pp(15,24) Warning: Range check error while evaluating constants
(2919235584 must be between -2147483648 and 2147483647)
pure1a.pp(15,24) Warning: Range check error while evaluating constants
(-39896219648 must be between -2147483648 and 2147483647)
pure1a.pp(15,24) Warning: Range check error while evaluating constants
(-1241513984 must be between 0 and 4294967295)
pure1a.pp(15,24) Warning: Range check error while evaluating constants
(3053453312 must be between -2147483648 and 2147483647)
pure1a.pp(15,24) Warning: Range check error while evaluating constants
(-37245419520 must be between -2147483648 and 2147483647)
pure1a.pp(15,24) Warning: Range check error while evaluating constants
(43687870464 must be between -2147483648 and 2147483647)
pure1a.pp(15,24) Warning: Range check error while evaluating constants
(23622320128 must be between -2147483648 and 2147483647)
pure1a.pp(15,24) Warning: Range check error while evaluating constants
(-2147483648 must be between 0 and 4294967295)

Not an ideal cascade of warnings.  I figured it might be better to throw
an error instead, so during the processing of pure functions, I enforced
range and overflow checking:

pure1a.pp(15,24) Error: Range check error while evaluating constants
(6227020800 must be between -2147483648 and 2147483647)
pure1a.pp(15,24) Error: Range check error while evaluating constants
(27048749056 must be between -2147483648 and 2147483647)
pure1a.pp(15,24) Error: Range check error while evaluating constants
(19184179200 must be between -2147483648 and 2147483647)
pure1a.pp(15,24) Error: Range check error while evaluating constants
(32068960256 must be between -2147483648 and 2147483647)
pure1a.pp(15,24) Error: Range check error while evaluating constants
(34071216128 must be between -2147483648 and 2147483647)
pure1a.pp(15,24) Error: Range check error while evaluating constants
(-288522240 must be between 0 and 4294967295)
pure1a.pp(15,24) Error: Range check error while evaluating constants
(4006445056 must be between -2147483648 and 2147483647)
pure1a.pp(15,24) Error: Range check error while evaluating constants
(-5193400320 must be between -2147483648 and 2147483647)
pure1a.pp(15,24) Error: Range check error while evaluating constants
(-898433024 must be between 0 and 4294967295)
pure1a.pp(15,24) Error: Range check error while evaluating constants
(3396534272 must be between -2147483648 and 2147483647)
pure1a.pp(15,24) Error: Range check error while evaluating constants
(-17070227456 must be between -2147483648 and 2147483647)
pure1a.pp(15,24) Error: Range check error while evaluating constants
(2192834560 must be between -2147483648 and 2147483647)
pure1a.pp(15,24) Error: Range check error while evaluating constants
(-2102132736 must be between 0 and 4294967295)
pure1a.pp(15,24) Error: Range check error while evaluating constants
(2192834560 must be between -2147483648 and 2147483647)
pure1a.pp(15,24) Error: Range check error while evaluating constants
(-44144787456 must be between -2147483648 and 2147483647)
pure1a.pp(15,24) Error: Range check error while evaluating constants
(-1195114496 must be between 0 and 4294967295)
pure1a.pp(15,24) Error: Range check error while evaluating constants
(3099852800 must be between -2147483648 and 2147483647)
pure1a.pp(15,24) Error: Range check error while evaluating constants
(-26292518912 must be between -2147483648 and 2147483647)
pure1a.pp(15,24) Error: Range check error while evaluating constants
(-522715136 must be between 0 and 4294967295)
pure1a.pp(15,24) Error: Range check error while evaluating constants
(3772252160 must be between -2147483648 and 2147483647)
pure1a.pp(15,24) Error: Range check error while evaluating constants
(-12022448128 must be between -2147483648 and 2147483647)
pure1a.pp(15,24) Error: Range check error while evaluating constants
(20698890240 must be between -2147483648 and 2147483647)
pure1a.pp(15,24) Error: Range check error while evaluating constants
(-775946240 must be between 0 and 4294967295)
pure1a.pp(15,24) Error: Range check error while evaluating constants
(3519021056 must be between -2147483648 and 2147483647)
pure1a.pp(15,24) Error: Range check error while evaluating constants
(-19398656000 must be between -2147483648 and 2147483647)
pure1a.pp(15,24) Error: Range check error while evaluating constants
(53980692480 must be between -2147483648 and 2147483647)
pure1a.pp(15,24) Error: Range check error while evaluating constants
(-1853882368 must be between 0 and 4294967295)
pure1a.pp(15,24) Error: Range check error while evaluating constants
(2441084928 must be between -2147483648 and 2147483647)
pure1a.pp(15,24) Error: Range check error while evaluating constants
(-50054823936 must be between -2147483648 and 2147483647)
pure1a.pp(15,24) Error: Range check error while evaluating constants
(41573941248 must be between -2147483648 and 2147483647)
pure1a.pp(15,24) Error: Range check error while evaluating constants
(-1375731712 must be between 0 and 4294967295)
pure1a.pp(15,24) Error: Range check error while evaluating constants
(2919235584 must be between -2147483648 and 2147483647)
pure1a.pp(15,24) Error: Range check error while evaluating constants
(-39896219648 must be between -2147483648 and 2147483647)
pure1a.pp(15,24) Error: Range check error while evaluating constants
(-1241513984 must be between 0 and 4294967295)
pure1a.pp(15,24) Error: Range check error while evaluating constants
(3053453312 must be between -2147483648 and 2147483647)
pure1a.pp(15,24) Error: Range check error while evaluating constants
(-37245419520 must be between -2147483648 and 2147483647)
pure1a.pp(15,24) Error: Range check error while evaluating constants
(43687870464 must be between -2147483648 and 2147483647)
pure1a.pp(15,24) Error: Range check error while evaluating constants
(23622320128 must be between -2147483648 and 2147483647)
pure1a.pp(15,24) Error: Range check error while evaluating constants
(-2147483648 must be between 0 and 4294967295)
pure1a.pp(17) Fatal: There were 39 errors compiling module, stopping
Fatal: Compilation aborted

This is not much better and is very unhelpful and untidy in an error
log.  Since the node tree is expanded during pure function analysis (in
this case, the for-loop is unrolled) and each node points to the
function call, each compiler message thus points here.  Ideally there
should only be one warning/error message, but since these messages are
generated elsewhere in the node compilation process, it's not that easy
to catch it and drop out the instant an error is thrown.

So to ask.  If during analysis an overflow/range check error occurs, how
do you think it should be handled? And does anyone have tips on how to
program the compiler to immediately stop processing if a single error is
found or to otherwise suppress subsequent messages that point to the
same file location?

Kit

```