Subject: Re: [bitcoin-dev] Beyond Jets: Microcode: Consensus-Critical Jets Without Softforks

From: "Russell O'Connor" <[email-masked]>
Date: Tue, 22 Mar 2022 11:08:33 -0400

Setting aside my thoughts that something like Simplicity would make a
better platform than Bitcoin Script (due to expression operating on a more
narrow interface than the entire stack (I'm looking at you OP_DEPTH)) there
is an issue with namespace management.

If I understand correctly, your implication was that once opcodes are
redefined by an OP_RETURN transaction, subsequent transactions of that
opcode refer to the new microtransaction.  But then we have a race
condition between people submitting transactions expecting the outputs to
refer to the old code and having their code redefined by the time they do
get confirmed  (or worse having them reorged).

I've partially addressed this issue in my Simplicity design where the
commitment of a Simplicity program in a scriptpubkey covers the hash of the
specification of the jets used, which makes commits unambiguously to the
semantics (rightly or wrongly).  But the issue resurfaces at redemption
time where I (currently) have a consensus critical map of codes to jets
that is used to decode the witness data into a Simplicity program.  If one
were to allow this map of codes to jets to be replaced (rather than just
extended) then it would cause redemption to fail, because the hash of the
new jets would no longer match the hash of the jets appearing the the
input's scriptpubkey commitment.  While this is still not good and I don't
recommend it, it is probably better than letting the semantics of your
programs be changed out from under you.

This comment is not meant as an endorsement of ths idea, which is a little
bit out there, at least as far as Bitcoin is concerned. :)

My long term plans are to move this consensus critical map of codes out of
the consensus layer and into the p2p layer where peers can negotiate their
own encodings between each other.  But that plan is also a little bit out
there, and it still doesn't solve the issue of how to weight reused jets,
where weight is still consensus critical.

On Tue, Mar 22, 2022 at 1:37 AM ZmnSCPxj via bitcoin-dev <
[email-masked]> wrote:

> Good morning list,
>
> It is entirely possible that I have gotten into the deep end and am now
> drowning in insanity, but here goes....
>
> Subject: Beyond Jets: Microcode: Consensus-Critical Jets Without Softforks
>
> Introduction
> ============
>
> Recent (Early 2022) discussions on the bitcoin-dev mailing
> list have largely focused on new constructs that enable new
> functionality.
>
> One general idea can be summarized this way:
>
> * We should provide a very general language.
>   * Then later, once we have learned how to use this language,
>     we can softfork in new opcodes that compress sections of
>     programs written in this general language.
>
> There are two arguments against this style:
>
> 1.  One of the most powerful arguments the "general" side of
>     the "general v specific" debate is that softforks are
>     painful because people are going to keep reiterating the
>     activation parameters debate in a memoryless process, so
>     we want to keep the number of softforks low.
>     * So, we should just provide a very general language and
>       never softfork in any other change ever again.
> 2.  One of the most powerful arguments the "general" side of
>     the "general v specific" debate is that softforks are
>     painful because people are going to keep reiterating the
>     activation parameters debate in a memoryless process, so
>     we want to keep the number of softforks low.
>     * So, we should just skip over the initial very general
>       language and individually activate small, specific
>       constructs, reducing the needed softforks by one.
>
> By taking a page from microprocessor design, it seems to me
> that we can use the same above general idea (a general base
> language where we later "bless" some sequence of operations)
> while avoiding some of the arguments against it.
>
> Digression: Microcodes In CISC Microprocessors
> ----------------------------------------------
>
> In the 1980s and 1990s, two competing microprocessor design
> paradigms arose:
>
> * Complex Instruction Set Computing (CISC)
>   - Few registers, many addressing/indexing modes, variable
>     instruction length, many obscure instructions.
> * Reduced Instruction Set Computing (RISC)
>   - Many registers, usually only immediate and indexed
>     addressing modes, fixed instruction length, few
>     instructions.
>
> In CISC, the microprocessor provides very application-specific
> instructions, often with a small number of registers with
> specific uses.
> The instruction set was complicated, and often required
> multiple specific circuits for each application-specific
> instruction.
> Instructions had varying sizes and varying number of cycles.
>
> In RISC, the micrprocessor provides fewer instructions, and
> programmers (or compilers) are supposed to generate the code
> for all application-specific needs.
> The processor provided large register banks which could be
> used very generically and interchangeably.
> Instructions had the same size and every instruction took a
> fixed number of cycles.
>
> In CISC you usually had shorter code which could be written
> by human programmers in assembly language or machine language.
> In RISC, you generally had longer code, often difficult for
> human programmers to write, and you *needed* a compiler to
> generate it (unless you were very careful, or insane enough
> you could scroll over multiple pages of instructions without
> becoming more insane), or else you might forget about stuff
> like jump slots.
>
> For the most part, RISC lost, since most modern processors
> today are x86 or x86-64, an instruction set with varying
> instruction sizes, varying number of cycles per instruction,
> and complex instructions with application-specific uses.
>
> Or at least, it *looks like* RISC lost.
> In the 90s, Intel was struggling since their big beefy CISC
> designs were becoming too complicated.
> Bugs got past testing and into mass-produced silicon.
> RISC processors were beating the pants off 386s in terms of
> raw number of computations per second.
>
> RISC processors had the major advantage that they were
> inherently simpler, due to having fewer specific circuits
> and filling up their silicon with general-purpose registers
> (which are large but very simple circuits) to compensate.
> This meant that processor designers could fit more of the
> design in their merely human meat brains, and were less
> likely to make mistakes.
> The fixed number of cycles per instruction made it trivial
> to create a fixed-length pipeline for instruction processing,
> and practical RISC processors could deliver one instruction
> per clock cycle.
> Worse, the simplicity of RISC meant that smaller and less
> experienced teams could produce viable competitors to the
> Intel x86s.
>
> So what Intel did was to use a RISC processor, and add a
> special Instruction Decoder unit.
> The Instruction Decoder would take the CISC instruction
> stream accepted by classic Intel x86 processors, and emit
> RISC instructions for the internal RISC processor.
> CISC instructions might be variable length and have variable
> number of cycles, but the emitted RISC instructions were
> individually fixed length and fixed number of cycles.
> A CISC instruction might be equivalent to a single RISC
> instruction, or several.
>
> With this technique, Intel could deliver performance
> approaching their RISC-only competition, while retaining
> back-compatibility with existing software written for their
> classic CISC processors.
>
> At its core, the Instruction Decoder was a table-driven
> parser.
> This lookup table could be stored into on-chip flash memory.
> This had the advantage that the on-chip flash memory could be
> updated in case of bugs in the implementation of CISC
> instructions.
> This on-chip flash memory was then termed "microcode".
>
> Important advantages of this "microcode" technique were:
>
> * Back-compatibility with existing instruction sets.
> * Easier and more scalable underlying design due to ability
>   to use RISC techniques while still supporting CISC instruction
>   sets.
> * Possible to fix bugs in implementations of complex CISC
>   instructions by uploading new microcode.
>
> (Obviously I have elided a bunch of stuff, but the above
> rough sketch should be sufficient as introduction.)
>
> Bitcoin Consensus Layer As Hardware
> -----------------------------------
>
> While Bitcoin fullnode implementations are software, because
> of the need for consensus, this software is not actually very
> "soft".
> One can consider that, just as it would take a long time for
> new hardware to be designed with a changed instruction set,
> it is similarly taking a long time to change Bitcoin to
> support changed feature sets.
>
> Thus, we should really consider the Bitcoin consensus layer,
> and its SCRIPT, as hardware that other Bitcoin software and
> layers run on top of.
>
> This thus opens up the thought of using techniques that were
> useful in hardware design.
> Such as microcode: a translation layer from "old" instruction
> sets to "new" instruction sets, with the ability to modify this
> mapping.
>
> Microcode For Bitcoin SCRIPT
> ============================
>
> I propose:
>
> * Define a generic, low-level language (the "RISC language").
> * Define a mapping from a specific, high-level language to
>   the above language (the microcode).
> * Allow users to sacrifice Bitcoins to define a new microcode.
> * Have users indicate the microcode they wish to use to
>   interpret their Tapscripts.
>
> As a concrete example, let us consider the current Bitcoin
> SCRIPT as the "CISC" language.
>
> We can then support a "RISC" language that is composed of
> general instructions, such as arithmetic, SECP256K1 scalar
> and point math, bytevector concatenation, sha256 midstates,
> bytevector bit manipulation, transaction introspection, and
> so on.
> This "RISC" language would also be stack-based.
> As the "RISC" language would have more possible opcodes,
> we may need to use 2-byte opcodes for the "RISC" language
> instead of 1-byte opcodes.
> Let us call this "RISC" language the micro-opcode language.
>
> Then, the "microcode" simply maps the existing Bitcoin
> SCRIPT `OP_` codes to one or more `UOP_` micro-opcodes.
>
> An interesting fact is that stack-based languages have
> automatic referential transparency; that is, if I define
> some new word in a stack-based language and use that word,
> I can replace verbatim the text of the new word in that
> place without issue.
> Compare this to a language like C, where macro authors
> have to be very careful about inadvertent variable
> capture, wrapping `do { ... } while(0)` to avoid problems
> with `if` and multiple statements, multiple execution, and
> so on.
>
> Thus, a sequence of `OP_` opcodes can be mapped to a
> sequence of equivalent `UOP_` micro-opcodes without
> changing the interpretation of the source language, an
> important property when considering such a "compiled"
> language.
>
> We start with a default microcode which is equivalent
> to the current Bitcoin language.
> When users want to define a new microcode to implement
> new `OP_` codes or change existing `OP_` codes, they
> can refer to a "base" microcode, and only have to
> provide the new mappings.
>
> A microcode is fundamentally just a mapping from an
> `OP_` code to a variable-length sequence of `UOP_`
> micro-opcodes.
>
> ```Haskell
> import Data.Map
> -- type Opcode
> -- type UOpcode
> newtype Microcode = Microcode (Map.Map Opcode [UOpcode])
> ```
>
> Semantically, the SCRIPT interpreter processes `UOP_`
> micro-opcodes.
>
> ```Haskell
> -- instance Monad Interpreter -- can `fail`.
> interpreter :: Transaction -> TxInput -> [UOpcode] -> Interpreter ()
> ```
>
> Example
> -------
>
> Suppose a user wants to re-enable `OP_CAT`, and nothing
> else.
>
> That user creates a microcode, referring to the current
> default Bitcoin SCRIPT microcode as the "base".
> The base microcode defines `OP_CAT` as equal to the
> sequence `UOP_FAIL` i.e. a micro-opcode that always fails.
> However, the new microcode will instead redefine the
> `OP_CAT` as the micro-opcode sequence `UOP_CAT`.
>
> Microcodes then have a standard way of being represented
> as a byte sequence.
> The user serializes their new microcode as a byte
> sequence.
>
> Then, the user creates a new transaction where one of
> the outputs contains, say, 1.0 Bitcoins (exact required
> value TBD), and has the `scriptPubKey` of
> `OP_TRUE OP_RETURN <serialized_microcode>`.
> This output is a "microcode introduction output", which
> is provably unspendable, thus burning the Bitcoins.
>
> (It need not be a single user, multiple users can
> coordinate by signing a single transaction that commits
> their funds to the microcode introduction.)
>
> Once the above transaction has been deeply confirmed,
> the user can then take the hash of the microcode
> serialization.
> Then the user can use a SCRIPT with `OP_CAT` enabled,
> by using a Tapscript with, say, version `0xce`, and
> with the SCRIPT having the microcode hash as its first
> bytes, followed by the `OP_` codes.
>
> Fullnodes will then process recognized microcode
> introduction outputs and store mappings from their
> hashes to the microcodes in a new microcodes index.
> Fullnodes can then process version-`0xce` Tapscripts
> by checking if the microcodes index has the indicated
> microcode hash.
>
> Semantically, fullnodes take the SCRIPT, and for each
> `OP_` code in it, expands it to a sequence of `UOP_`
> micro-opcodes, then concatenates each such sequence.
> Then, the SCRIPT interpreter operates over a sequence
> of `UOP_` micro-opcodes.
>
> Optimizing Microcodes
> ---------------------
>
> Suppose there is some new microcode that users have
> published onchain.
>
> We want to be able to execute the defined microcode
> faster than expanding an `OP_`-code SCRIPT to a
> `UOP_`-code SCRIPT and having an interpreter loop
> over the `UOP_`-code SCRIPT.
>
> We can use LLVM.
>
> WARNING: LLVM might not be appropriate for
> network-facing security-sensitive applications.
> In particular, LLVM bugs. especially nondeterminism
> bugs, can lead to consensus divergence and disastrous
> chainsplits!
> On the other hand, LLVM bugs are compiler bugs and
> the same bugs can hit the static compiler `cc`, too,
> since the same LLVM code runs in both JIT and static
> compilation, so this risk already exists for Bitcoin.
> (i.e. we already rely on LLVM not being buggy enough
> to trigger Bitcoin consensus divergence, else we would
> have written Bitcoin Core SCRIPT interpreter in
> assembly.)
>
> Each `UOP_`-code has an equivalent tree of LLVM code.
> For each `Opcode` in the microcode, we take its
> sequence of `UOpcode`s and expand them to this tree,
> concatenating the equivalent trees for each `UOpcode`
> in the sequence.
> Then we ask LLVM to JIT-compile this code to a new
> function, running LLVM-provided optimizers.
> Then we put a pointer to this compiled function to a
> 256-long array of functions, where the array index is
> the `OP_` code.
>
> The SCRIPT interpreter then simply iterates over the
> `OP_` code SCRIPT and calls each of the JIT-compiled
> functions.
> This reduces much of the overhead of the `UOP_` layer
> and makes it approach the current performance of the
> existing `OP_` interpreter.
>
> For the default Bitcoin SCRIPT, the opcodes array
> contains pointers to statically-compiled functions.
> A microcode that is based on the default Bitcoin
> SCRIPT copies this opcodes array, then overwrites
> the entries.
>
> Future versions of Bitcoin Core can "bless"
> particular microcodes by providing statically-compiled
> functions for those microcodes.
> This leads to even better performance (there is
> no need to recompile ancient onchain microcodes each
> time Bitcoin Core starts) without any consensus
> divergence.
> It is a pure optimization and does not imply a
> tightening of rules, and is thus not a softfork.
>
> (To reduce the chance of network faults being used
> to poke into `W|X` memory (since `W|X` memory is
> needed in order to actually JIT compile) we can
> isolate the SCRIPT interpreter into its own process
> separate from the network-facing code.
> This does imply additional overhead in serializing
> transactions we want to ask the SCRIPT interpreter
> to validate.)
>
> Comparison To Jets
> ------------------
>
> This technique allows users to define "jets", i.e.
> sequences of low-level general operations that users
> have determined are common enough they should just
> be implemented as faster code that is executed
> directly by the underlying hardware processor rather
> than via a software interpreter.
> Basically, each redefined `OP_` code is a jet of a
> sequence of `UOP_` micro-opcodes.
>
> We implement this by dynamically JIT-compiling the
> proposed jets, as described above.
> SCRIPTs using jetted code remain smaller, as the
> jet definition is done in a previous transaction and
> does not require copy-pasta (Do Not Repeat Yourself!).
> At the same time, jettification is not tied to
> developers, thus removing the need to keep softforking
> new features --- we only need define a sufficiently
> general language and then we can implement pretty much
> anything worth implementing (and a bunch of other things
> that should not be implemented, but hey, users gonna
> use...).
>
> Bugs in existing microcodes can be fixed by basing a
> new microcode from the existing microcode, and
> redefining the buggy implementation.
> Existing Tapscripts need to be re-spent to point to
> the new bugfixed microcode, but if you used the
> point-spend branch as an N-of-N of all participants
> you have an upgrade mechanism for free.
>
> In order to ensure that the JIT-compilation of new
> microcodes is not triggered trivially, we require
> that users petitioning for the jettification of some
> operations (i.e. introducing a new microcode) must
> sacrifice Bitcoins.
>
> Burning Bitcoins is better than increasing the weight
> of microcode introduction outputs; all fullnodes are
> affected by the need to JIT-compile the new microcode,
> so they benefit from the reduction in supply, thus
> getting compensated for the work of JIT-compiling the
> new microcode.
> Ohter mechanisms for making microcode introduction
> outputs expensive are also possible.
>
> Nothing really requires that we use a stack-based
> language for this; any sufficiently FP language
> should allow referential transparency.
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