Draft of bitwise shifting opcodes

Author: axic

EIPS/eip-draft_bitwise_shifts.md

@@ -0,0 +1,108 @@
+## 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.
+
+### `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
+
+Test cases for an implementation are mandatory for EIPs that are affecting consensus changes. Other EIPs can choose to include links to test cases if applicable.
+
+## 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/).