[fpc-devel] [Suggestion] Enumeration range-check intrinsic

[J. Gareth Moreton]

Hi everyone,

So there have been a lot of issues going back and forth regarding case 
blocks when dealing with enumerations and what happens if the enum falls 
outside the valid range of values for some reason, because the internal 
domain check is sometimes omitted for performance reasons, since the 
value /shouldn't/ be invalid.  Thus, if it is invalid and a jump table 
is used, an access violation is raised.

Ultimately, the range check will likely not be included to safeguard 
against this, but there are still situations where an enumeration will 
take on an out-of-range value, like if it reads it from an external data 
file that is corrupted or of an incompatible version for the program.  
So I would like to suggest a new intrinsic called InRange() or 
IsDomainValid() or something to that effect that takes an ordinal-type 
parameter and returns True if it contains a valid value for the 
enumeration (always True if it's a Byte or Word etc) and False if it is 
outside the valid domain, hence fulfilling the range check if it's 
something you really need.

The reason why I ask for an intrinsic instead of something like "if 
(Value >= Low(TEnumType)) and (Value <= High(TEnumType))" is because 
such code may be optimised out due to it logically being impossible for 
Value to be less than Low(TEnumType), for example, so it's harder than 
you think to check if the variable is in range.  Additionally, by having 
a generic intrinsic, it can generate more optimised code (e.g. the 
equivalent of "if (Value - Low(TEnumType)) < (High(TEnumType) - 
Low(TEnumType))", taking advantage of unsigned overflow if Value is less 
than the lower bound) and also to check for invalid values that fall 
within the maximum range (e.g. if you have something like "type 
TEnumType = (etZero, etOne, etSpecial = $FF);" and your Value holds the 
equivalent of $FE).

What are your thoughts? Would this be a decent compromise to fix the 
case range-check issue that keeps cropping up occasionally? I know it's 
causing some people to choose not migrate away from Delphi, and that is 
never really a good thing.  It should be easy enough to document as 
well, putting in an explicit warning for case to say that if the 
enumeration is out of range, it may cause a crash, and to prevent it, 
use the intrinsic to check beforehand.

Gareth aka. Kit



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