core/vm: implement eip-7939 CLZ instruction

Co-authored-by: Giulio <giulio.rebuffo@gmail.com>
Co-authored-by: spencer-tb <spencer@spencertaylorbrown.uk>

core/vm/eips.go

@@ -42,6 +42,7 @@ var activators = map[int]func(*JumpTable){
 	4762: enable4762,
 	7702: enable7702,
 	7907: enable7907,
+	7939: enable7939,
 }
 
 // EnableEIP enables the given EIP on the config.
@@ -294,6 +295,13 @@ func opBlobBaseFee(pc *uint64, interpreter *EVMInterpreter, scope *ScopeContext)
 	return nil, nil
 }
 
+func opCLZ(pc *uint64, interpreter *EVMInterpreter, scope *ScopeContext) ([]byte, error) {
+	x := scope.Stack.pop()
+	// count leading zero bits in x
+	scope.Stack.push(new(uint256.Int).SetUint64(256 - uint64(x.BitLen())))
+	return nil, nil
+}
+
 // enable4844 applies EIP-4844 (BLOBHASH opcode)
 func enable4844(jt *JumpTable) {
 	jt[BLOBHASH] = &operation{
@@ -304,6 +312,15 @@ func enable4844(jt *JumpTable) {
 	}
 }
 
+func enable7939(jt *JumpTable) {
+	jt[CLZ] = &operation{
+		execute:     opCLZ,
+		constantGas: GasFastestStep,
+		minStack:    minStack(1, 1),
+		maxStack:    maxStack(1, 1),
+	}
+}
+
 // enable7516 applies EIP-7516 (BLOBBASEFEE opcode)
 func enable7516(jt *JumpTable) {
 	jt[BLOBBASEFEE] = &operation{

core/vm/instructions_test.go

@@ -972,3 +972,47 @@ func TestPush(t *testing.T) {
 		}
 	}
 }
+
+func TestOpCLZ(t *testing.T) {
+	// set up once
+	evm := NewEVM(BlockContext{}, nil, params.TestChainConfig, Config{})
+
+	tests := []struct {
+		name     string
+		inputHex string // hexadecimal input for clarity
+		want     uint64 // expected CLZ result
+	}{
+		{"zero", "0x0", 256},
+		{"one", "0x1", 255},
+		{"all-ones (256 bits)", "0xffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff", 0},
+		{"low-10-bytes ones", "0xffffffffff", 216}, // 10 bytes = 80 bits, so 256-80=176? Actually input is 0xffffffffff = 40 bits so 256-40=216
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+
+			// prepare a fresh stack and PC
+			stack := newstack()
+			pc := uint64(0)
+
+			// parse input
+			val := new(uint256.Int)
+			if _, err := fmt.Sscan(tc.inputHex, val); err != nil {
+				// fallback: try hex
+				val.SetFromHex(tc.inputHex)
+			}
+
+			stack.push(val)
+			opCLZ(&pc, evm.interpreter, &ScopeContext{Stack: stack})
+
+			if gotLen := stack.len(); gotLen != 1 {
+				t.Fatalf("stack length = %d; want 1", gotLen)
+			}
+			result := stack.pop()
+
+			if got := result.Uint64(); got != tc.want {
+				t.Fatalf("clz(%q) = %d; want %d", tc.inputHex, got, tc.want)
+			}
+		})
+	}
+}

core/vm/interpreter.go

@@ -106,6 +106,8 @@ func NewEVMInterpreter(evm *EVM) *EVMInterpreter {
 	// If jump table was not initialised we set the default one.
 	var table *JumpTable
 	switch {
+	case evm.chainRules.IsOsaka:
+		table = &osakaInstructionSet
 	case evm.chainRules.IsVerkle:
 		// TODO replace with proper instruction set when fork is specified
 		table = &verkleInstructionSet

core/vm/jump_table.go

@@ -95,6 +95,7 @@ func newVerkleInstructionSet() JumpTable {
 func newOsakaInstructionSet() JumpTable {
 	instructionSet := newPragueInstructionSet()
 	enable7907(&instructionSet)
+	enable7939(&instructionSet) // EIP-7939 (CLZ opcode)
 	return validate(instructionSet)
 }
 

core/vm/jump_table_export.go

@@ -29,7 +29,7 @@ func LookupInstructionSet(rules params.Rules) (JumpTable, error) {
 	case rules.IsVerkle:
 		return newCancunInstructionSet(), errors.New("verkle-fork not defined yet")
 	case rules.IsOsaka:
-		return newPragueInstructionSet(), errors.New("osaka-fork not defined yet")
+		return newOsakaInstructionSet(), nil
 	case rules.IsPrague:
 		return newPragueInstructionSet(), nil
 	case rules.IsCancun:

core/vm/opcodes.go

@@ -62,6 +62,7 @@ const (
 	SHL    OpCode = 0x1b
 	SHR    OpCode = 0x1c
 	SAR    OpCode = 0x1d
+	CLZ    OpCode = 0x1e
 )
 
 // 0x20 range - crypto.
@@ -282,6 +283,7 @@ var opCodeToString = [256]string{
 	SHL:    "SHL",
 	SHR:    "SHR",
 	SAR:    "SAR",
+	CLZ:    "CLZ",
 	ADDMOD: "ADDMOD",
 	MULMOD: "MULMOD",
 
@@ -484,6 +486,7 @@ var stringToOp = map[string]OpCode{
 	"SHL":             SHL,
 	"SHR":             SHR,
 	"SAR":             SAR,
+	"CLZ":             CLZ,
 	"ADDMOD":          ADDMOD,
 	"MULMOD":          MULMOD,
 	"KECCAK256":       KECCAK256,