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Preamble

EIP: 145
Title: Bitwise shifting instructions in EVM
Author: Alex Beregszaszi, Paweł Bylica
Type: Standard Track
Category: Core
Status: Final
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 use 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, 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

(arg2 * 2^arg1) mod 2^256

Notes:

  • The value (arg2) is interpreted as an unsigned number.
  • The shift amount (arg1) is interpreted as an unsigned number.
  • If the shift amount (arg1) is greater or equal 256 the result is 0.
  • This is equivalent to PUSH1 2 EXP MUL.

0x1c: SHR (logical shift right)

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

floor(arg2 / 2^arg1)

Notes:

  • The value (arg2) is interpreted as an unsigned number.
  • The shift amount (arg1) is interpreted as an unsigned number.
  • If the shift amount (arg1) is greater or equal 256 the result is 0.
  • This is equivalent to PUSH1 2 EXP DIV.

0x1d: SAR (arithmetic shift right)

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

floor(arg2 / 2^arg1)

Notes:

  • The value (arg2) is interpreted as a signed number.
  • The shift amount (arg1) is interpreted as an unsigned number.
  • 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.
  • This is not equivalent to PUSH1 2 EXP SDIV, since it rounds differently. See SDIV(-1, 2) == 0, while SAR(-1, 1) == -1.

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

Rationale

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.

Backwards Compatibility

The newly introduced instructions have no effect on bytecode created in the past.

Test Cases

TBA

Implementation

Client support:

Compiler support:

Copyright

Copyright and related rights waived via CC0.