EIP-145: Bitwise shifting instructions in EVM
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 arithmetic 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
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
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
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 ifarg2
is non-negative or -1 ifarg2
is negative. - This is not equivalent to
PUSH1 2 EXP SDIV
, since it rounds differently. SeeSDIV(-1, 2) == 0
, whileSAR(-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 instructions.
Backwards Compatibility
The newly introduced instructions have no effect on bytecode created in the past.
Test Cases
SHL
(shift left)
SHR
(logical shift right)
SAR
(arithmetic shift right)
Implementation
Client support:
- cpp-ethereum: https://github.com/ethereum/cpp-ethereum/pull/4054
Compiler support:
- Solidity/LLL: https://github.com/ethereum/solidity/pull/2541
Tests
Sources:
Filled Tests:
- https://github.com/ethereum/tests/tree/develop/GeneralStateTests/stShift
- https://github.com/ethereum/tests/tree/develop/BlockchainTests/GeneralStateTests/stShift
Copyright
Copyright and related rights waived via CC0.