Filter: Improve description of type system

This commit is contained in:
Ondrej Zajicek (work) 2022-03-03 15:11:05 +01:00 committed by Ondrej Zajicek
parent 93d6096c87
commit cde8094c1f

View file

@ -62,6 +62,7 @@
* m4_dnl INST(FI_NOP, in, out) { enum value, input args, output args
* m4_dnl ARG(num, type); argument, its id (in data fields) and type accessible by v1, v2, v3
* m4_dnl ARG_ANY(num); argument with no type check accessible by v1, v2, v3
* m4_dnl ARG_TYPE(num, type); just declare the type of argument
* m4_dnl VARARG; variable-length argument list; accessible by vv(i) and whati->varcount
* m4_dnl LINE(num, unused); this argument has to be converted to its own f_line
* m4_dnl SYMBOL; symbol handed from config
@ -80,7 +81,9 @@
* m4_dnl )
*
* m4_dnl RESULT(type, union-field, value); putting this on value stack
* m4_dnl RESULT_(type, union-field, value); like RESULT(), but do not declare the type
* m4_dnl RESULT_VAL(value-struct); pass the struct f_val directly
* m4_dnl RESULT_TYPE(type); just declare the type of result value
* m4_dnl RESULT_VOID; return undef
* m4_dnl }
*
@ -91,6 +94,24 @@
*
* Other code is just copied into the interpreter part.
*
* The filter language uses a simple type system, where values have types
* (constants T_*) and also terms (instructions) are statically typed. Our
* static typing is partial (some terms do not declare types of arguments
* or results), therefore it can detect most but not all type errors and
* therefore we still have runtime type checks.
*
* m4_dnl Types of arguments are declared by macros ARG() and ARG_TYPE(),
* m4_dnl types of results are declared by RESULT() and RESULT_TYPE().
* m4_dnl Macros ARG_ANY(), RESULT_() and RESULT_VAL() do not declare types
* m4_dnl themselves, but can be combined with ARG_TYPE() / RESULT_TYPE().
*
* m4_dnl Note that types should be declared only once. If there are
* m4_dnl multiple RESULT() macros in an instruction definition, they must
* m4_dnl use the exact same expression for type, or they should be replaced
* m4_dnl by multiple RESULT_() macros and a common RESULT_TYPE() macro.
* m4_dnl See e.g. FI_EA_GET or FI_MIN instructions.
*
*
* If you are satisfied with this, you don't need to read the following
* detailed description of what is really done with the instruction definitions.
*
@ -216,6 +237,37 @@
*
* m4_dnl If you are stymied, see FI_CALL or FI_CONSTANT or just search for
* m4_dnl the mentioned macros in this file to see what is happening there in wild.
*
*
* A note about soundness of the type system:
*
* A type system is sound when types of expressions are consistent with
* types of values resulting from evaluation of such expressions. Untyped
* expressions are ok, but badly typed expressions are not sound. So is
* the type system of BIRD filtering code sound? There are some points:
*
* All cases of (one) m4_dnl RESULT() macro are obviously ok, as the macro
* both declares a type and returns a value. One have to check instructions
* that use m4_dnl RESULT_TYPE() macro. There are two issues:
*
* FI_AND, FI_OR - second argument is statically checked to be T_BOOL and
* passed as result without dynamic typecheck, declared to be T_BOOL. If
* an untyped non-bool expression is used as a second argument, then
* the mismatched type is returned.
*
* FI_VAR_GET - soundness depends on consistency of declared symbol types
* and stored values. This is maintained when values are stored by
* FI_VAR_SET, but when they are stored by FI_CALL, only static checking is
* used, so when an untyped expression returning mismatched value is used
* as a function argument, then inconsistent value is stored and subsequent
* FI_VAR_GET would be unsound.
*
* Both of these issues are inconsequential, as mismatched values from
* unsound expressions will be caught by dynamic typechecks like mismatched
* values from untyped expressions.
*
* Also note that FI_CALL is the only expression without properly declared
* result type.
*/
/* Binary operators */