RSKIPs
Shift Operations
| RSKIP | 05 |
|---|---|
| Title | Shift Operations |
| Created | 22-JUN-2016 |
| Author | SDL |
| Purpose | Sca |
| Layer | Core |
| Complexity | 1 |
| Status | Accepted |
Abstract
The EVM lack shift operations. Although shift operations can be carried on using MUL / DIV, shift operations should be much less expensive. An example of use of the shift operation is in ABI parsing, where the method id must be extracted. Currently this is performed by DIV.
Motivation
Allow optimization of contracts that do bit-manipulation.
There are two related opcodes (rotate) that are often present in modern CPU. However there is no clear use case for RROTATE and LROTATE for 256-bit arithmetic. There could be specific opcodes for 32-bit or 64-bit rotation, as some cryptographic primitives requires it.
This must be further researched.
Important Update
As of december 2016, the Ethereum team created their own specification for the same requirement.
It can be seen here. The solidity compiler has also been modified to support these opcodes.
https://github.com/ethereum/EIPs/issues/145
The full EIP can be seen here:
Specification
This section has been replaced by a literal copy of the EIP.
Preamble
EIP:
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:
Copyright
Copyright and related rights waived via CC0.