use "reduce to", add footnotes

Author: cdetrio

evm_vs_ewasm_opcode_table.md

@@ -1,15 +1,15 @@
 ## EVM opcodes vs ewasm methods
 
-These tables compares EVM opcodes with ewasm methods and wasm instructions. The EVM defines 134 opcodes (as of the Byzantium hard fork). Of these 134 opcodes, some are generic machine instructions (ADD, MUL, ISZERO, XOR, etc.) and the rest are Ethereum-specific opcodes (SLOAD, SSTORE, CALLVALUE, BLOCKHASH, etc.).
+These tables compares EVM opcodes with ewasm methods and wasm instructions. The EVM defines 134 opcodes (as of the Byzantium hard fork). Of these 134 opcodes, some represent generic machine instructions (ADD, MUL, ISZERO, XOR, etc.) and the rest are Ethereum-specific operations (SLOAD, SSTORE, GASPRICE, BLOCKHASH, etc.).
 
-When EVM bytecode is translated into ewasm bytecode, the Ethereum-specific opcodes translate to ewasm interface methods (Ethereum Environment Interface (EEI) methods). These interface methods are provided as [host functions](https://webassembly.github.io/threads/exec/runtime.html#syntax-hostfunc) to the wasm VM. All other EVM opcodes are not Ethereum-specific and do not access data from the Ethereum environment, so they can be translated to plain wasm instructions.
+When EVM bytecode is transpiled to ewasm bytecode, the Ethereum-specific opcodes translate to corresponding ewasm interface methods (Ethereum Environment Interface (EEI) methods). These interface methods are provided as [host functions](https://webassembly.github.io/threads/exec/runtime.html#syntax-hostfunc) to the wasm VM. All other EVM opcodes are not Ethereum-specific and do not access data from the Ethereum environment, so they reduce to pure wasm instructions.
 
 To reference EVM opcodes, try the [yellow paper](https://ethereum.github.io/yellowpaper/paper.pdf), the [readable yellow paper](https://github.com/chronaeon/beigepaper/blob/master/beigepaper.pdf), or the [pyethereum implementation](https://github.com/ethereum/pyethereum/blob/develop/ethereum/opcodes.py).
 
 To reference ewasm interface methods, see the [EEI spec](https://github.com/ewasm/design/blob/master/eth_interface.md).
 
 
-### EVM opcodes that translate to ewasm methods (Ethereum Environment Interface (EEI) methods)
+### EVM opcodes with corresponding ewasm methods (Ethereum Environment Interface (EEI) methods)
 
 EVM opcode            |     ewasm interface method
 ----------------------|-----------------------------
@@ -26,8 +26,8 @@ EVM opcode            |     ewasm interface method
 0x3a: GASPRICE        |      getTxGasPrice
 0x3b: EXTCODESIZE     |      getExternalCodeSize
 0x3c: EXTCODECOPY     |      externalCodeCopy
-0x3d: RETURNDATASIZ   |      getReturnDataSize
-0x3e: RETURNDATACOP   |      returnDataCopy
+0x3d: RETURNDATASIZE  |      getReturnDataSize
+0x3e: RETURNDATACOPY  |      returnDataCopy
 0x40: BLOCKHASH       |      getBlockHash
 0x41: COINBASE        |      getBlockCoinbase
 0x42: TIMESTAMP       |      getBlockTimestamp
@@ -36,7 +36,7 @@ EVM opcode            |     ewasm interface method
 0x45: GASLIMIT        |      getBlockGasLimit
 0x54: SLOAD           |      storageLoad
 0x55: SSTORE          |      storageStore
-0x58: PC              |      -
+0x58: PC              |      -<sup>[†](#f1)</sup>
 0x5a: GAS             |      getGasLeft
 0xa0: LOG0            |      log(0,..)
 ...                   |
@@ -51,6 +51,8 @@ EVM opcode            |     ewasm interface method
 0xff: SELFDESTRUCT    |      selfDestruct
 
 
+†. <small id="f1">In EVM, the Program Counter (PC) opcode returns the currently executing bytecode position. When EVM is transpiled to ewasm with evm2wasm, this bytecode position is calculated by the transpiler and [inserted as a wasm constant](https://github.com/ewasm/evm2wasm/blob/80136977553c9b22e385f919fff808ef8a54be95/index.js#L247-L248). For ewasm code that is not transpiled from EVM, there is no interface method corresponding to the `PC` opcode.</small>
+
 
 ## EVM opcodes vs WebAssembly instructions
 
@@ -72,9 +74,9 @@ ewasm excludes the wasm floating point instructions:
 To reference wasm instructions, see the [wasm spec](https://github.com/WebAssembly/design/blob/master/Semantics.md) or [reference manual](https://github.com/sunfishcode/wasm-reference-manual/blob/master/WebAssembly.md#instructions).
 
 
-### EVM opcodes that translate to wasm instructions
+### EVM opcodes that reduce to wasm instructions
 
-EVM opcode            |    wasm instruction (i32/i64)
+EVM opcode (256-bit)  |    wasm instruction i32/i64 (32-bit or 64-bit)
 ----------------------|----------------------
 0x00: STOP            |       end
 0x01: ADD             |       i32.add
@@ -84,10 +86,10 @@ EVM opcode            |    wasm instruction (i32/i64)
 0x05: SDIV            |       i32.div_s
 0x06: MOD             |       i32.rem_u
 0x07: SMOD            |       i32.rem_s
-0x08: ADDMOD          |       -
-0x09: MULMOD          |       -
-0x0a: EXP             |       -
-0x0b: SIGNEXTEND      |       -
+0x08: ADDMOD          |       -<sup>[‡](#f2)</sup>
+0x09: MULMOD          |       -<sup>[‡](#f2)</sup>
+0x0a: EXP             |       -<sup>[‡](#f2)</sup>
+0x0b: SIGNEXTEND      |       -<sup>[‡](#f2)</sup>
 0x10: LT              |       i32.lt_u
 0x11: GT              |       i32.gt_u
 0x12: SLT             |       i32.lt_s
@@ -97,9 +99,9 @@ EVM opcode            |    wasm instruction (i32/i64)
 0x16: AND             |       i32.and
 0x17: OR              |       i32.or
 0x18: XOR             |       i32.xor
-0x19: NOT             |       -
-0x1a: BYTE            |       - [i32.load8]
-0x20: SHA3            |       -
+0x19: NOT             |       i32.sub (i32.const 0xffffffff)
+0x1a: BYTE            |       i32.load8
+0x20: SHA3            |       -<sup>[‡](#f2)</sup>
 0x50: POP             |       drop
 0x51: MLOAD           |       i32.load
 0x52: MSTORE          |       i32.store
@@ -115,6 +117,9 @@ EVM opcode            |    wasm instruction (i32/i64)
 0x80: DUP1            |       get_global, get_local
 ...                   |
 0x8f: DUP16           |       get_global, get_local
-0x90: SWAP1           |       -
+0x90: SWAP1           |       get_global, get_local
 ...                   |
-0x9f: SWAP16          |       -
+0x9f: SWAP16          |       get_global, get_local
+
+
+‡. <small id="f2">Some EVM opcodes, such as `ADDMOD` (modular addition), `EXP` (exponentiation), or `SHA3` (Keccak-256 hash), do not have simple reductions to single wasm instructions (or to only a few wasm instructions). The functionality of these opcodes is replicated by a relatively large wasm module (for instance, `SHA3` reduces to about [900 lines of wast code](https://github.com/ewasm/evm2wasm/blob/80136977553c9b22e385f919fff808ef8a54be95/wasm/keccak.wast)).</small>