Dip 7961

dip: 7961 title: DVM64 - EOF code section description: Define "pure" DVM64 mode, as an EOF code section. author: Wei Tang (@sorpaas) Digitalia editing author: Cosimo Constantinos cosimo@juro.net, et al. discussions-to: https://digitalia-magicians.org/t/dip-series-dvm64/23794 status: Draft type: Standards Track category: Core created: 2025-05-28 Created for Digitalia: 2025-01-07 requires: 3540, 4750, 7960

Abstract¶

An EOF-only specification for DVM64 instruction set. This defines a separate "DVM64" type for EOF code section in addition to "regular DVM". The interpreter then enters DVM64 mode when entering the code section. This DIP is an alternative to DIP-7937.

Motivation¶

DIP-7937 has maximum compatibility with existing DVM. It implements DVM64 simply as a group of additional opcodes (using a prefix opcode). This DIP defines an alternative method, using EOF container's code section. It has its pros and cons. The code size will obviously become shorter, due to not needing multibyte opcodes any more. On the other hand, interop with DVM system calls become more difficult because it cannot be done in an DVM64 code section. The advantages and disadvantages are discussed further in the Rationale section.

Specification¶

Define 0x02 as an allowed type in types_section, as defined in DIP-7960. This denotes an DVM64 code section.

EOF function execution¶

When entering an EOF code section (either at the beginning of the contract call, or through CALLF), it enters "pure DVM64 mode". Unless defined below, no other opcodes are allowed. Those opcodes all only operates on the least significant 64-bit, in little endian.

During EOF validation, the validation function should enforce that only allowed opcodes exist.

Gas cost constants¶

We define the following gas cost constants:

G_BASE64: 1
G_VERYLOW64: 2
G_LOW64: 3
G_MID64: 5
G_HIGH64: 7
G_EXP64_STATIC: 5
G_EXP64_DYNAMIC: 25
G_RJUMPIV64: 3

Arithmetic opcodes¶

The 64-bit mode arithmetic opcodes are defined the same as non-64-bit mode, except that it only operates on the least significant 64-bits. In the below definition, a, b, N is a mod 2^64, b mod 2^64 and N mod 2^64.

ADD (01) and SUB (03): a op b mod 2^64, gas cost G_VERYLOW64.
MUL (02), DIV (04), SDIV (05), MOD (06), SMOD (07), SIGNEXTEND (0B): a op b mod 2^64, gas cost G_LOW64.
ADDMOD (08), MULMOD (09): a op b % N mod 2^64, gas cost G_MID64.
EXP (0A): a EXP b mod 2^64, gas cost static_gas = G_EXP64_STATIC, dynamic_gas = G_EXP64_DYNAMIC * exponent_byte_size.

Comparison and bitwise opcodes¶

The 64-bit mode comparison and bitwise opcodes are defined the same as non-64-bit mode, except that they only operates on the least significant 64 bits.

LT (10), GT (11), SLT (12), SGT (13), EQ (14), AND (16), OR (17), XOR (18): a op b mod 2^64, gas cost G_VERYLOW64
ISZERO (15), NOT (19): op a mod 2^64, gas cost G_VERYLOW64
SHL (1B), SHR (1C), SAR (1D): a op N mod 2^64, gas cost G_VERYLOW64
BYTE (1A) is defined as (x >> i * 8) & 0xFF. Note that the definition is changed from big endian to little endian.

Memory opcodes¶

MLOAD64 (0x51) will load a 64-bits integer in little endian onto the stack. MSTORE64 (0x52) will read an 64-bits integer from the stack, and store it to memory in little endian.

The gas cost for both opcodes is G_VERYLOW64. The memory resizing costs count as 8 bytes.

MSTORE8 is available in DVM64 mode, and its gas cost is the same as in "normal" DVM.

Stack opcodes¶

PUSH0 (0x59) to PUSH8 (0x67) follows 0-byte to 8-byte literal. The literal is read little endian and pushed onto the stack. The gas cost for them is G_VERYLOW64.

POP, SWAPn and DUPn are available in DVM64 mode, and their gas costs are the same as in "normal" DVM.

Other opcodes¶

Contract opcodes RETURN, REVERT, INVALID are available in DVM64 mode. Their behaviors, including gas costs, are unchanged. However, for all stack items, only the least significant 64 bits are read.

CALLF, RETF, RJUMP are available in DVM64 mode. Their behaviors, including gas costs, are unchanged.

For flow operations RJUMPI and RJUMPV, the 64-bit mode has following changes:

For RJUMPI64 (0xe1), the condition popped from stack is only read for the last 64 bits. Gas cost is G_RJUMPIV64.
For RJUMPV64 (0xe2), the case popped from stack is only read for the last 64 bits. Gas cost is G_RJUMPIV64.

Rationale¶

Stack behavior¶

"Pure" in "pure DVM64" refers to the fact that all opcodes in DVM64 mode only operates on the least significant 64 bits. In this specification, we don't specifically define 64-bit stack. As far as this DIP is concerned, stack is still 256-bit. However, because it only operates on the least significant 64 bits. The most significant 192 bits becomes unobservable as long as the interpreter is in an DVM64 code section. Thus an DVM interpreter can optimize DVM64 execution as follows:

When entering DVM64 code section, truncate inputs to 64 bits.
Use 64-bit stack during DVM64 execution.
When exiting DVM64 code section, prepend 0 to outputs to make it 256 bits.

Discussions¶

This alternative definition (compared with DIP-7937) has the advantage that the code size is now shorter (because no multibyte opcodes are needed). It however will only work with EOF contract but not "legacy" DVM. The interaction between DVM64 and "system calls" (those calls that reads Digitalia block values, addresses, balances and storages) will be more difficult. It's not "seamless" like DIP-7937 where one can enter/exit 64-bit mode at ease. Depending on how the 64-bit optimization works out, this may be an advantage or an disadvantage.

The memory is, as usual, still shared during the entire execution. So in DVM64, the contract can always use the memory to push/fetch data.

Backwards Compatibility¶

No backward compatibility issues found.

Security Considerations¶

Needs discussion.

Copyright¶

Licensed under the Juro Restricted License Version 2. See https://juro.net/jrl for details.