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docs[venom]: expand venom docs (vyperlang#4314)
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expand venom docs. document venom instructions and structure of a venom
program.

---------

Co-authored-by: Charles Cooper <[email protected]>
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Expand Up @@ -160,3 +160,316 @@ A number of passes that are planned to be implemented, or are implemented for im
### Function inlining

### Load-store elimination

---

## Structure of a venom program

### IRContext
An `IRContext` consists of multiple `IRFunctions`, with one designated as the main entry point of the program.
Additionally, the `IRContext` maintains its own representation of the data segment.

### IRFunction
An `IRFunction` is composed of a name and multiple `IRBasicBlocks`, with one marked as the entry point to the function.

### IRBasicBlock
An `IRBasicBlock` contains a label and a sequence of `IRInstructions`.
Each `IRBasicBlock` has a single entry point and exit point.
The exit point must be one of the following terminator instructions:
- `jmp`
- `djmp`
- `jnz`
- `ret`
- `return`
- `stop`
- `exit`

Normalized basic blocks cannot have multiple predecessors and successors. It has either one (or zero) predecessors and potentially multiple successors or vice versa.

### IRInstruction
An `IRInstruction` consists of an opcode, a list of operands, and an optional return value.
An operand can be a label, a variable, or a literal.

By convention, variables have a `%-` prefix, e.g. `%1` is a valid variable. However, the prefix is not required.

## Instructions
To enable Venom IR in Vyper, use the `--experimental-codegen` flag. To view the Venom IR output, use `-f bb_runtime` for the runtime code, or `-f bb` to see the deploy code. To get a dot file (for use e.g. with `xdot -`), use `-f cfg` or `-f cfg_runtime`.

Assembly can be inspected with `-f asm`, whereas an opcode view of the final bytecode can be seen with `-f opcodes` or `-f opcodes_runtime`, respectively.

### Special instructions

- `invoke`
- ```
invoke offset, label
```
- Causes control flow to jump to a function denoted by the `label`.
- Return values are passed in the return buffer at the `offset` address.
- Used for internal functions.
- Effectively translates to `JUMP`, and marks the call site as a valid return destination (for callee to jump back to) by `JUMPDEST`.
- `alloca`
- ```
out = alloca size, offset
```
- Allocates memory of a given `size` at a given `offset` in memory.
- The output is the offset value itself.
- Because the SSA form does not allow changing values of registers, handling mutable variables can be tricky. The `alloca` instruction is meant to simplify that.

- `palloca`
- ```
out = palloca size, offset
```
- Like the `alloca` instruction but only used for parameters of internal functions which are passed by memory.
- `iload`
- ```
out = iload offset
```
- Loads value at an immutable section of memory denoted by `offset` into `out` variable.
- The operand can be either a literal, which is a statically computed offset, or a variable.
- Essentially translates to `MLOAD` on an immutable section of memory. So, for example
```
%op = 12
%out = iload %op
```
could compile into `PUSH1 12 _mem_deploy_end ADD MLOAD`.
- When `offset` is a literal the location is computed statically during compilation from assembly to bytecode.
- `istore`
- ```
istore offset value
```
- Represents a store into immutable section of memory.
- Like in `iload`, the offset operand can be a literal.
- Essentially translates to `MSTORE` on an immutable section of memory. For example,
```
%op = 12
istore 24 %op
```
could compile to
`PUSH1 12 PUSH1 24 _mem_deploy_end ADD MSTORE`.
- `phi`
- ```
out = phi %var_a, label_a, %var_b, label_b
```
- Because in SSA form each variable is assigned just once, it is tricky to handle that variables may be assigned to something different based on which program path was taken.
- Therefore, we use `phi` instructions. They are are magic instructions, used in basic blocks where the control flow path merges.
- In this example, essentially the `out` variable is set to `%var_a` if the program entered the current block from `label_a` or to `%var_b` when it went through `label_b`.
- `offset`
- ```
ret = offset label, op
```
- Statically compute offset before compiling into bytecode. Useful for `mstore`, `mload` and such.
- Basically `label` + `op`.
- The `asm` output could show something like `_OFST _sym_<op> label`.
- `param`
- ```
out = param
```
- The `param` instruction is used to represent function arguments passed by the stack.
- We assume the argument is on the stack and the `param` instruction is used to ensure we represent the argument by the `out` variable.
- `store`
- ```
out = op
```
- Store variable value or literal into `out` variable.
- `dbname`
- ```
dbname label
```
- Mark memory with a `label` in the data segment so it can be referenced.
- `db`
- ```
db data
```
- Store `data` into data segment.
- `dloadbytes`
- Alias for `codecopy` for legacy reasons. May be removed in future versions.
- Translates to `CODECOPY`.
- `ret`
- ```
ret op
```
- Represents return from an internal call.
- Jumps to a location given by `op`.
- If `op` is a label it can effectively translate into `op JUMP`.
- `exit`
- ```
exit
```
- Similar to `stop`, but used for constructor exit. The assembler is expected to jump to a special initcode sequence which returns the runtime code.
- Might translate to something like `_sym__ctor_exit JUMP`.
- `sha3_64`
- ```
out = sha3_64 x y
```
- Shortcut to access the `SHA3` EVM opcode where `out` is the result.
- Essentially translates to
```
PUSH y PUSH FREE_VAR_SPACE MSTORE
PUSH x PUSH FREE_VAR_SPACE2 MSTORE
PUSH 64 PUSH FREE_VAR_SPACE SHA3
```
where `FREE_VAR_SPACE` and `FREE_VAR_SPACE2` are locations reserved by the compiler, set to 0 and 32 respectively.

- `assert`
- ```
assert op
```
- Assert that `op` is zero. If it is not, revert.
- Calls that terminate this way receive a gas refund.
- For example
```
%op = 13
assert %op
```
could compile to
`PUSH1 13 ISZERO _sym___revert JUMPI`.
- `assert_unreachable`
- ```
assert_unreachable op
```
- Check that `op` is zero. If it is not, terminate with `0xFE` ("INVALID" opcode).
- Calls that end this way do not receive a gas refund.
- Could translate to `op reachable JUMPI INVALID reachable JUMPDEST`.
- For example
```
%op = 13
assert_unreachable %op
```
could compile to
```
PUSH1 13 _sym_reachable1 JUMPI
INVALID
_sym_reachable1 JUMPDEST
```
- `log`
- ```
log offset, size, [topic] * topic_count , topic_count
```
- Corresponds to the `LOGX` instruction in EVM.
- Depending on the `topic_count` value (which can be only from 0 to 4) translates to `LOG0` ... `LOG4`.
- The rest of the operands correspond to the `LOGX` instructions.
- For example
```
log %53, 32, 64, %56, 2
```
could translate to:
```
%56, 64, 32, %53 LOG2
```
- `nop`
- ```
nop
```
- No operation, does nothing.
- `offset`
- ```
%2 = offset %1 label1
- Similar to `add`, but takes a label as the second argument. If the first argument is a literal, the addition will get optimized at assembly time.
### Jump instructions
- `jmp`
- ```
jmp label
```
- Unconditional jump to code denoted by given `label`.
- Translates to `label JUMP`.
- `jnz`
- ```
jnz label1, label2, op
```
- A conditional jump depending on the value of `op`.
- Jumps to `label2` when `op` is not zero, otherwise jumps to `label1`.
- For example
```
%op = 15
jnz label1, label2, %op
```
could translate to: `PUSH1 15 label2 JUMPI label1 JUMP`.
- `djmp`
- ```
djmp %var, label1, label2, label3, ...
```
- Dynamic jump to an address specified by the variable operand, constrained to the provided labels.
- Accepts a variable number of labels.
- The target is not a fixed label but rather a value stored in a variable, making the jump dynamic.
- The jump target can be any of the provided labels.
- Translates to `JUMP`.
### EVM instructions
The following instructions map one-to-one with [EVM instructions](https://www.evm.codes/).
Operands correspond to stack inputs in the same order. Stack outputs are the instruction's output.
Instructions have the same effects.
- `return`
- `revert`
- `coinbase`
- `calldatasize`
- `calldatacopy`
- `mcopy`
- `calldataload`
- `gas`
- `gasprice`
- `gaslimit`
- `chainid`
- `address`
- `origin`
- `number`
- `extcodesize`
- `extcodehash`
- `extcodecopy`
- `returndatasize`
- `returndatacopy`
- `callvalue`
- `selfbalance`
- `sload`
- `sstore`
- `mload`
- `mstore`
- `tload`
- `tstore`
- `timestamp`
- `caller`
- `blockhash`
- `selfdestruct`
- `signextend`
- `stop`
- `shr`
- `shl`
- `sar`
- `and`
- `xor`
- `or`
- `add`
- `sub`
- `mul`
- `div`
- `smul`
- `sdiv`
- `mod`
- `smod`
- `exp`
- `addmod`
- `mulmod`
- `eq`
- `iszero`
- `not`
- `lt`
- `gt`
- `slt`
- `sgt`
- `create`
- `create2`
- `msize`
- `balance`
- `call`
- `staticcall`
- `delegatecall`
- `codesize`
- `basefee`
- `blobhash`
- `blobbasefee`
- `prevrandao`
- `difficulty`
- `invalid`
- `sha3`

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