![]() The “ret” instruction either consumes nothing or a single type, dependent on the signature of the method it is used in in this case it consume an “int” which we know because the method is a “Func”. We know “ldc.i4” consumes nothing, and produces an int, “add” actually consumes and produces a wide range of types but one of them is “int int -> int”. NET examples, though the basic idea applies to all similar verifiers.įor example, assume the following is an implementation of a Func: You can think of it as executing instructions as if they consumed and produced types instead of values. If you ignore branches, verifying a bytecode sequence is pretty simple. Similar complications exist when verification is successful, as things like eliding trivial casts and replacing branches with their short forms (which are deferred until an instruction stream is finalized) requires a lot of information about the instruction stream be retained. Sigil also has to explain itself when it fails, so it has to be able to deliver where and why a given sequence became invalid (which can be far removed from the last emitted instruction because of branches). It’s quite common to branch to an instruction you haven’t actually emitted yet or emit instructions that aren’t yet provably reachable, both cases a traditional verifier can never encounter. It verifies as you build a sequence, failing as soon as it can be sure the sequence is invalid.īecause Sigil deals with incomplete instruction sequences it also has to do with a lot of unknowns, especially around branches. Where Sigil differs from other bytecode verifiers is that it doesn’t operate on “finished” instruction sequences. One way to think about bytecode verifiers is that they guarantee that every operation receives the correct types as inputs and every operation leaves the runtime in a well formed state. For example, a bytecode verifier would guarantee that invoking an instance method is passed the proper number and types of parameters and that it is invoked against an instance of the appropriate type. The definition of “safe” is very particular, a bytecode verifier doesn’t prevent errors from occurring at execution time but rather prevents invariants of the runtime from being violated. NET development you should be familiar with the concept, the bytecode verifiers on those platforms make sure that the class files or assemblies you load contain bytecode that can safely be executed. In essence Sigil is a “bytecode verifier”. I figure now’s as good a time as any to write up the “how” of Sigil, given that I covered the “what” and “why” in an earlier post and that recent releases have refactored Sigil’s internals into a state I’m happier with. ![]() ![]() ![]() NET delegate.īut that’s not what I’m writing about. The big new feature this release is a disassembler, one which allows for some inspection of the flow of values through a. Version 3.0.0 of Sigil was just released ( grab it on Nuget and check out the source on Github). ![]()
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