diff options
| -rw-r--r-- | filter/f-inst.c | 52 |
1 files changed, 52 insertions, 0 deletions
diff --git a/filter/f-inst.c b/filter/f-inst.c index e9bae684..2120440d 100644 --- a/filter/f-inst.c +++ b/filter/f-inst.c @@ -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 */ |