EIPs/EIPS/eip-draft_bitwise_shifts.md

Preamble

EIP: <to be assigned>
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.

The cost of the shift instructions is set at verylow tier (3 gas).

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.