diff --git a/EIPS/eip-draft_bitwise_shifts.md b/EIPS/eip-draft_bitwise_shifts.md new file mode 100644 index 00000000000000..0f42f9fc0a2c8f --- /dev/null +++ b/EIPS/eip-draft_bitwise_shifts.md @@ -0,0 +1,108 @@ +## Preamble + + EIP: + Title: Bitwise shifting instructions in EVM + Author: Alex Beregszaszi, Paweł Bylica + Type: Standard Track + Category Core + Status: Draft + Created: 2017-02-13 + + +## Simple Summary + +To provide native bitwise shifting with cost on par with other arithmetic operations. + +## Abstract + +Native bitwise shifting instructions are introduced, which are more efficient processing wise on the host and are cheaper to user by a contract. + +## Motivation + +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. + +## Specification + +The following instructions are introduced: + +### `0x1b`: `SHL` (shift left) + +The `SHL` instruction (shift left) pops 2 values from the stack, `arg1` and `arg2`, and pushes on the stack the second popped value `arg2` shifted to the left by the number of bits in the first popped value `arg1`. The result is equal to + +``` +(arg2 * 2^arg1) mod 2^256 +``` + +Notes: +- If the shift amount is greater or equal 256 the result is 0. + +### `0x1c`: `SHR` (logical shift right) + +The `SHR` instruction (logical shift right) pops 2 values from the stack, `arg1` and `arg2`, and pushes on the stack the second popped value `arg2` shifted to the right by the number of bits in the first popped value `arg1` with zero fill. The result is equal to + +``` +arg2 udiv 2^arg1 +``` + +Notes: +- If the shift amount is greater or equal 256 the result is 0. + +### `0x1d`: `SAR` (arithmetic shift right) + +The `SAR` instruction (arithmetic shift right) pops 2 values from the stack, `arg1` and `arg2`, and pushes on the stack the second popped value `arg2` shifted to the right by the number of bits in the first popped value `arg1` with sign extension. The result is equal to + +``` +arg2 sdiv 2^arg1 +``` + +Notes: +- `arg1` is interpreted as unsigned number. +- If the shift amount is greater or equal 256 the result is 0 if `arg2` is non-negative or -1 if `arg2` is negative. + +### `0x1e`: `ROL` (rotate left) + +The `ROL` instruction (rotate left) pops 2 values from the stack, `arg1` and `arg2`, and pushes on the stack the second popped value `arg2` circular shifted to the left by the number of bits in the first popped value `arg1`. + +``` +(arg1 shl arg2) or (arg1 shr (256 - arg2) +``` + +Notes: +- `arg2 rol arg1` is equivalent of `arg2 rol (arg1 mod 2^256)` + +### `0x1f`: `ROR` (rotate right) + +The `ROL` instruction (rotate right) pops 2 values from the stack, `arg1` and `arg2`, and pushes on the stack the second popped value `arg2` circular shifted to the right by the number of bits in the first popped value `arg1`. + +``` +(arg1 shr arg2) or (arg1 shl (256 - arg2) +``` + +Notes: +- `arg2 ror arg1` is equivalent of `arg2 ror (arg1 mod 2^256)` + +The cost of the shift instructions is set at `verylow` tier (3 gas), while the rotations are 12 gas each. + +## Rationale + +Instruction operands were chosen to match the other logical and arithmetic instructions. + +## Backwards Compatibility + +The newly introduced instructions have no effect on bytecode created in the past. + +## Test Cases + +TBA + +## Implementation + +Client support: +TBA + +Compiler support: +- Solidity: https://github.com/ethereum/solidity/tree/asm-bitshift + +## Copyright + +Copyright and related rights waived via [CC0](https://creativecommons.org/publicdomain/zero/1.0/).