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+

pub mod prelude;
+pub mod rt;
+
+mod imm;
+mod insn;
+mod label;
+mod reg;
+
+use imm::Imm;
+use label::Label;
+use reg::Reg;
+use reg::{Reg16, Reg32, Reg64, Reg8};
+
+pub enum MemOp {
+    Indirect(Reg64),
+    IndirectDisp(Reg64, i32),
+}
+
+impl MemOp {
+    const fn base(&self) -> Reg64 {
+        match self {
+            MemOp::Indirect(base) => *base,
+            MemOp::IndirectDisp(base, ..) => *base,
+        }
+    }
+}
+
+/// Encode the `REX` byte.
+const fn rex(w: bool, r: u8, x: u8, b: u8) -> u8 {
+    let w = if w { 1 } else { 0 };
+    let r = (r >> 3) & 1;
+    let x = (x >> 3) & 1;
+    let b = (b >> 3) & 1;
+    0b0100_0000 | ((w & 1) << 3) | (r << 2) | (x << 1) | b
+}
+
+/// Encode the `ModR/M` byte.
+const fn modrm(mod_: u8, reg: u8, rm: u8) -> u8 {
+    ((mod_ & 0b11) << 6) | ((reg & 0b111) << 3) | (rm & 0b111)
+}
+
+pub struct Asm {
+    buf: Vec<u8>,
+}
+
+impl Asm {
+    pub fn new() -> Asm {
+        let buf = Vec::with_capacity(1024);
+        Asm { buf }
+    }
+
+    pub fn into_code(self) -> Vec<u8> {
+        self.buf
+    }
+
+    fn emit(&mut self, bytes: &[u8]) {
+        self.buf.extend_from_slice(bytes);
+    }
+
+    fn emit_optional(&mut self, bytes: &[Option<u8>]) {
+        for byte in bytes.iter().filter_map(|&b| b) {
+            self.buf.push(byte);
+        }
+    }
+
+    fn emit_at(&mut self, pos: usize, bytes: &[u8]) {
+        if let Some(buf) = self.buf.get_mut(pos..pos + bytes.len()) {
+            buf.copy_from_slice(bytes);
+        } else {
+            unimplemented!();
+        }
+    }
+
+    /// Bind the [Label] to the current location.
+    pub fn bind(&mut self, label: &mut Label) {
+        // Bind the label to the current offset.
+        label.bind(self.buf.len());
+
+        // Resolve any pending relocations for the label.
+        self.resolve(label);
+    }
+
+    /// If the [Label] is bound, patch any pending relocation.
+    pub fn resolve(&mut self, label: &mut Label) {
+        if let Some(loc) = label.location() {
+            let loc = i32::try_from(loc).expect("Label location did not fit into i32.");
+
+            // Resolve any pending relocations for the label.
+            for off in label.offsets_mut().drain() {
+                // Displacement is relative to the next instruction following the jump.
+                // We record the offset to patch at the first byte of the disp32 therefore we need
+                // to account for that in the disp computation.
+                let disp32 = loc - i32::try_from(off).expect("Label offset did not fit into i32") - 4 /* account for the disp32 */;
+
+                // Patch the relocation with the disp32.
+                self.emit_at(off, &disp32.to_ne_bytes());
+            }
+        }
+    }
+
+    // -- Encode utilities.
+
+    fn encode_rr<T: Reg>(&mut self, opc: u8, op1: T, op2: T)
+    where
+        Self: EncodeRR<T>,
+    {
+        // MR operand encoding.
+        //   op1 -> modrm.rm
+        //   op2 -> modrm.reg
+        let modrm = modrm(
+            0b11,      /* mod */
+            op2.idx(), /* reg */
+            op1.idx(), /* rm */
+        );
+
+        let prefix = <Self as EncodeRR<T>>::legacy_prefix();
+        let rex = <Self as EncodeRR<T>>::rex(op1, op2);
+
+        self.emit_optional(&[prefix, rex]);
+        self.emit(&[opc, modrm]);
+    }
+
+    fn encode_oi<T: Reg, U: Imm>(&mut self, opc: u8, op1: T, op2: U)
+    where
+        Self: EncodeRI<T>,
+    {
+        let opc = opc + (op1.idx() & 0b111);
+        let prefix = <Self as EncodeRI<T>>::legacy_prefix();
+        let rex = <Self as EncodeRI<T>>::rex(op1);
+
+        self.emit_optional(&[prefix, rex]);
+        self.emit(&[opc]);
+        self.emit(op2.bytes());
+    }
+
+    fn encode_ri<T: Reg, U: Imm>(&mut self, opc: u8, opc_ext: u8, op1: T, op2: U)
+    where
+        Self: EncodeRI<T>,
+    {
+        // MI operand encoding.
+        //   op1           -> modrm.rm
+        //   opc extension -> modrm.reg
+        let modrm = modrm(
+            0b11,      /* mod */
+            opc_ext,   /* reg */
+            op1.idx(), /* rm */
+        );
+
+        let prefix = <Self as EncodeRI<T>>::legacy_prefix();
+        let rex = <Self as EncodeRI<T>>::rex(op1);
+
+        self.emit_optional(&[prefix, rex]);
+        self.emit(&[opc, modrm]);
+        self.emit(op2.bytes());
+    }
+
+    fn encode_r<T: Reg>(&mut self, opc: u8, opc_ext: u8, op1: T)
+    where
+        Self: EncodeRI<T>,
+    {
+        // M operand encoding.
+        //   op1           -> modrm.rm
+        //   opc extension -> modrm.reg
+        let modrm = modrm(
+            0b11,      /* mod */
+            opc_ext,   /* reg */
+            op1.idx(), /* rm */
+        );
+
+        let prefix = <Self as EncodeRI<T>>::legacy_prefix();
+        let rex = <Self as EncodeRI<T>>::rex(op1);
+
+        self.emit_optional(&[prefix, rex]);
+        self.emit(&[opc, modrm]);
+    }
+
+    fn encode_mr<T: Reg>(&mut self, opc: u8, op1: MemOp, op2: T)
+    where
+        Self: EncodeMR<T>,
+    {
+        // MR operand encoding.
+        //   op1 -> modrm.rm
+        //   op2 -> modrm.reg
+        let mode = match op1 {
+            MemOp::Indirect(..) => {
+                assert!(!op1.base().need_sib() && !op1.base().is_pc_rel());
+                0b00
+            }
+            MemOp::IndirectDisp(..) => {
+                assert!(!op1.base().need_sib());
+                0b10
+            }
+        };
+
+        let modrm = modrm(
+            mode,             /* mode */
+            op2.idx(),        /* reg */
+            op1.base().idx(), /* rm */
+        );
+        let prefix = <Self as EncodeMR<T>>::legacy_prefix();
+        let rex = <Self as EncodeMR<T>>::rex(&op1, op2);
+
+        self.emit_optional(&[prefix, rex]);
+        self.emit(&[opc, modrm]);
+        if let MemOp::IndirectDisp(_, disp) = op1 {
+            self.emit(&disp.to_ne_bytes());
+        }
+    }
+
+    fn encode_rm<T: Reg>(&mut self, opc: u8, op1: T, op2: MemOp)
+    where
+        Self: EncodeMR<T>,
+    {
+        // RM operand encoding.
+        //   op1 -> modrm.reg
+        //   op2 -> modrm.rm
+        self.encode_mr(opc, op2, op1);
+    }
+
+    fn encode_jmp_label(&mut self, opc: &[u8], op1: &mut Label) {
+        // Emit the opcode.
+        self.emit(opc);
+
+        // Record relocation offset starting at the first byte of the disp32.
+        op1.record_offset(self.buf.len());
+
+        // Emit a zeroed disp32, which serves as placeholder for the relocation.
+        self.emit(&[0u8; 4]);
+
+        // Resolve any pending relocations for the label.
+        self.resolve(op1);
+    }
+}
+
+// -- Encoder helper.
+
+trait EncodeRR<T: Reg> {
+    fn legacy_prefix() -> Option<u8> {
+        None
+    }
+
+    fn rex(op1: T, op2: T) -> Option<u8> {
+        if op1.need_rex() || op2.need_rex() {
+            Some(rex(op1.rexw(), op2.idx(), 0, op1.idx()))
+        } else {
+            None
+        }
+    }
+}
+
+impl EncodeRR<Reg8> for Asm {}
+impl EncodeRR<Reg32> for Asm {}
+impl EncodeRR<Reg16> for Asm {
+    fn legacy_prefix() -> Option<u8> {
+        Some(0x66)
+    }
+}
+impl EncodeRR<Reg64> for Asm {}
+
+trait EncodeRI<T: Reg> {
+    fn legacy_prefix() -> Option<u8> {
+        None
+    }
+
+    fn rex(op1: T) -> Option<u8> {
+        if op1.need_rex() {
+            Some(rex(op1.rexw(), 0, 0, op1.idx()))
+        } else {
+            None
+        }
+    }
+}
+
+impl EncodeRI<Reg8> for Asm {}
+impl EncodeRI<Reg32> for Asm {}
+impl EncodeRI<Reg16> for Asm {
+    fn legacy_prefix() -> Option<u8> {
+        Some(0x66)
+    }
+}
+impl EncodeRI<Reg64> for Asm {}
+
+trait EncodeMR<T: Reg> {
+    fn legacy_prefix() -> Option<u8> {
+        None
+    }
+
+    fn rex(op1: &MemOp, op2: T) -> Option<u8> {
+        if op2.need_rex() || (op1.base().is_ext()) {
+            Some(rex(op2.rexw(), op2.idx(), 0, op1.base().idx()))
+        } else {
+            None
+        }
+    }
+}
+
+impl EncodeMR<Reg8> for Asm {}
+impl EncodeMR<Reg16> for Asm {
+    fn legacy_prefix() -> Option<u8> {
+        Some(0x66)
+    }
+}
+impl EncodeMR<Reg32> for Asm {}
+impl EncodeMR<Reg64> for Asm {}
+

+