forked from DecentralizedClimateFoundation/DCIPs
105 lines
4.6 KiB
Markdown
105 lines
4.6 KiB
Markdown
---
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eip: 5656
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title: Memory copying instruction
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description: An efficient EVM instruction for copying memory areas
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author: Alex Beregszaszi (@axic), Paul Dworzanski (@poemm), Jared Wasinger (@jwasinger), Casey Detrio (@cdetrio), Pawel Bylica (@chfast), Charles Cooper (@charles-cooper)
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discussions-to: https://ethereum-magicians.org/t/eip-5656-mcopy-instruction/10890
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status: Draft
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type: Standards Track
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category: Core
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created: 2021-02-01
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---
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## Abstract
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Provide an efficient EVM instruction for copying memory areas.
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## Motivation
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Memory copying is a basic operation, yet implementing it on the EVM comes with overhead.
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This was recognised and alleviated early on with the introduction of the "identity" precompile, which accomplishes
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memory copying by the use of `CALL`'s input and output memory offsets. Its cost is `15 + 3 * (length / 32)` gas, plus
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the call overhead. The identity precompile was rendered ineffective by the raise of the cost of `CALL` to 700, but subsequently
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the reduction by [EIP-2929](./eip-2929.md) made it slightly more economical.
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Copying exact words can be accomplished with `<offset> MLOAD <offset> MSTORE` or `<offset> DUP1 MLOAD DUP2 MSTORE`,
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at a cost of at least 12 gas per word. This is fairly efficient if the offsets are known upfront and the copying can be unrolled.
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In case copying is implemented at runtime with arbitrary starting offsets, besides the control flow overhead, the offset
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will need to be incremented using `32 ADD`, adding at least 6 gas per word.
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Copying non-exact words is more tricky, as for the last partial word, both the source and destination needs to be loaded,
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masked, or'd, and stored again. This overhead is significant. One edge case is if the last "partial word" is a single byte,
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it can be efficiently stored using `MSTORE8`.
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As example use case, copying 256 bytes costs:
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- at least 757 gas pre-EIP-2929 using the identity precompile
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- at least 157 gas post-EIP-2929 using the identity precompile
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- at least 96 gas using unrolled `MLOAD`/`MSTORE` instructions
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- 27 gas using this EIP
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According to an analysis of blocks 10537502 to 10538702, roughly 10.5% of memory copies would have had improved performance with the
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availability of an `MCOPY` instruction.
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Memory copying is used by languages like Solidity and Vyper, where we expect this improvement to provide efficient means of building
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data structures, including efficient sliced access and copies of memory objects. Having a dedicated `MCOPY` instruction would also add
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forward protection against future gas cost changes to `CALL` instructions in general.
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Having a special `MCOPY` instruction makes the job of static analyzers and optimizers easier, since the effects of a `CALL` in general
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have to be fenced, whereas an `MCOPY` instruction would be known to only have memory effects. Even if special cases are added
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for precompiles, a future hard fork could change `CALL` effects, and so any analysis of code using the identity precompile would only
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be valid for a certain range of blocks.
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Finally, we expect memory copying to be immensely useful for various computationally heavy operations, such as EVM384,
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where it is identified as a significant overhead.
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## Specification
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The instruction `MCOPY` is introduced at `0x5c`.
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### Input stack
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| Stack | Value |
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|-------|-------|
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| top - 0 | `dst` |
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| top - 1 | `src` |
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| top - 2 | `length` |
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This ordering matches the other copying instructions, i.e. `CALLDATACOPY`, `RETURNDATACOPY`.
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### Output stack
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This instructions returns no stack items.
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### Semantics
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It copies `length` bytes from the offset pointed at `src` to the offset pointed at `dst` in memory.
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Copying takes place as if an intermediate buffer were used, allowing the destination and source to overlap.
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If `length > 0` and (`src + length` or `dst + length`) is beyond the current memory length, the memory is extended with respective gas cost applied.
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The gas cost of this instruction mirrors that of other `Wcopy` instructions and is `Gverylow + Gcopy * ceil(length / 32)`.
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## Rationale
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Production implementation of exact-word memory copying and partial-word memory copying can be found in the Solidity, Vyper and Fe compilers.
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With [EIP-2929](./eip-2929.md) the call overhead using the identity precompile was reduced from 700 to 100 gas.
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This is still prohibitive for making the precompile a reasonable alternative again.
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## Backwards Compatibility
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This EIP introduces a new instruction which did not exists previously. Already deployed contracts using this instruction could change their behaviour after this EIP.
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## Test Cases
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TBA
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## Security Considerations
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TBA
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## Copyright
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Copyright and related rights waived via [CC0](../LICENSE.md).
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