mirror of https://github.com/status-im/EIPs.git
95 lines
3.3 KiB
Markdown
95 lines
3.3 KiB
Markdown
## Preamble
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EIP: 145
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Title: Bitwise shifting instructions in EVM
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Author: Alex Beregszaszi, Paweł Bylica
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Type: Standard Track
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Category: Core
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Status: Final
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Created: 2017-02-13
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## Simple Summary
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To provide native bitwise shifting with cost on par with other arithmetic operations.
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## Abstract
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Native bitwise shifting instructions are introduced, which are more efficient processing wise on the host and are cheaper to use by a contract.
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## Motivation
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EVM is lacking bitwise shifting operators, but supports other logical and arithmetic operators. Shift operations can be implemented via arithmetic operators, but that has a higher cost and requires more processing time from the host. Implementing `SHL` and `SHR` using arithmetics cost each 35 gas, while the proposed instructions take 3 gas.
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## Specification
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The following instructions are introduced:
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### `0x1b`: `SHL` (shift left)
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The `SHL` instruction (shift left) pops 2 values from the stack, first `arg1` and then `arg2`, and pushes on the stack `arg2` shifted to the left by `arg1` number of bits. The result is equal to
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```
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(arg2 * 2^arg1) mod 2^256
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```
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Notes:
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- The value (`arg2`) is interpreted as an unsigned number.
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- The shift amount (`arg1`) is interpreted as an unsigned number.
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- If the shift amount (`arg1`) is greater or equal 256 the result is 0.
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- This is equivalent to `PUSH1 2 EXP MUL`.
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### `0x1c`: `SHR` (logical shift right)
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The `SHR` instruction (logical shift right) pops 2 values from the stack, first `arg1` and then `arg2`, and pushes on the stack `arg2` shifted to the right by `arg1` number of bits with zero fill. The result is equal to
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```
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floor(arg2 / 2^arg1)
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```
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Notes:
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- The value (`arg2`) is interpreted as an unsigned number.
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- The shift amount (`arg1`) is interpreted as an unsigned number.
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- If the shift amount (`arg1`) is greater or equal 256 the result is 0.
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- This is equivalent to `PUSH1 2 EXP DIV`.
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### `0x1d`: `SAR` (arithmetic shift right)
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The `SAR` instruction (arithmetic shift right) pops 2 values from the stack, first `arg1` and then `arg2`, and pushes on the stack `arg2` shifted to the right by `arg1` number of bits with sign extension. The result is equal to
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```
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floor(arg2 / 2^arg1)
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```
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Notes:
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- The value (`arg2`) is interpreted as a signed number.
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- The shift amount (`arg1`) is interpreted as an unsigned number.
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- If the shift amount (`arg1`) is greater or equal 256 the result is 0 if `arg2` is non-negative or -1 if `arg2` is negative.
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- This is **not** equivalent to `PUSH1 2 EXP SDIV`, since it rounds differently. See `SDIV(-1, 2) == 0`, while `SAR(-1, 1) == -1`.
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The cost of the shift instructions is set at `verylow` tier (3 gas).
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## Rationale
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Instruction operands were chosen to fit the more natural use case of shifting a value already on the stack. This means the operand order is swapped compared to most arithmetic insturctions.
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## Backwards Compatibility
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The newly introduced instructions have no effect on bytecode created in the past.
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## Test Cases
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TBA
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## Implementation
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Client support:
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- cpp-ethereum: https://github.com/ethereum/cpp-ethereum/pull/4054
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Compiler support:
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- Solidity/LLL: https://github.com/ethereum/solidity/pull/2541
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## Copyright
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Copyright and related rights waived via [CC0](https://creativecommons.org/publicdomain/zero/1.0/).
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