diff --git a/include/circt/Support/FVInt.h b/include/circt/Support/FVInt.h
new file mode 100644
index 000000000000..13cb5198d487
--- /dev/null
+++ b/include/circt/Support/FVInt.h
@@ -0,0 +1,591 @@
+//===- FVInt.h - Four-valued integer ----------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements a class to represent arbitrary precision integers where
+// each bit can be one of four values. This corresponds to SystemVerilog's
+// four-valued `logic` type (originally defined in IEEE 1364, later merged into
+// IEEE 1800).
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef CIRCT_SUPPORT_FVINT_H
+#define CIRCT_SUPPORT_FVINT_H
+
+#include "circt/Support/LLVM.h"
+#include "llvm/ADT/APInt.h"
+#include "llvm/ADT/SmallString.h"
+#include "llvm/ADT/StringRef.h"
+#include "llvm/Support/raw_ostream.h"
+
+namespace circt {
+
+/// Four-valued arbitrary precision integers.
+///
+/// Each bit of the integer can be 0, 1, X, or Z. Internally the bits are stored
+/// in a pair of `APInt`s, one of which specifies the value of each bit (0/X or
+/// 1/Z), and the other whether the bit is unknown (X or Z).
+class FVInt {
+public:
+  /// Construct an `FVInt` from a 64-bit value. The result has no X or Z bits.
+  FVInt(unsigned numBits, uint64_t value, bool isSigned = false)
+      : FVInt(APInt(numBits, value, isSigned)) {}
+
+  /// Construct an `FVInt` from an `APInt`. The result has no X or Z bits.
+  FVInt(const APInt &value)
+      : value(value), unknown(APInt::getZero(value.getBitWidth())) {}
+
+  /// Construct an `FVInt` from two `APInt`s used internally to store the bit
+  /// data. The first argument specifies whether each bit is 0/X or 1/Z. The
+  /// second argument specifies whether each bit is 0/1 or X/Z. Both `APInt`s
+  /// must have the same bitwidth. The two arguments correspond to the results
+  /// of `getRawValue()` and `getRawUnknown()`.
+  FVInt(APInt &&rawValue, APInt &&rawUnknown)
+      : value(rawValue), unknown(rawUnknown) {
+    assert(rawValue.getBitWidth() == rawUnknown.getBitWidth());
+  }
+
+  /// Construct an `FVInt` with all bits set to 0.
+  static FVInt getZero(unsigned numBits) {
+    return FVInt(APInt::getZero(numBits));
+  }
+
+  /// Construct an `FVInt` with all bits set to 1.
+  static FVInt getAllOnes(unsigned numBits) {
+    return FVInt(APInt::getAllOnes(numBits));
+  }
+
+  /// Construct an `FVInt` with all bits set to X.
+  static FVInt getAllX(unsigned numBits) {
+    return FVInt(APInt::getZero(numBits), APInt::getAllOnes(numBits));
+  }
+
+  /// Construct an `FVInt` with all bits set to Z.
+  static FVInt getAllZ(unsigned numBits) {
+    return FVInt(APInt::getAllOnes(numBits), APInt::getAllOnes(numBits));
+  }
+
+  /// Return the number of bits this integer has.
+  unsigned getBitWidth() const { return value.getBitWidth(); }
+
+  /// Return the underlying `APInt` used to store whether a bit is 0/X or 1/Z.
+  const APInt &getRawValue() const { return value; }
+
+  /// Return the underlying `APInt` used to store whether a bit is unknown (X or
+  /// Z).
+  const APInt &getRawUnknown() const { return unknown; }
+
+  /// Convert the four-valued `FVInt` to a two-valued `APInt` by mapping X and Z
+  /// bits to either 0 or 1.
+  APInt toAPInt(bool unknownBitMapping) const {
+    auto v = value;
+    if (unknownBitMapping)
+      v |= unknown; // set unknown bits to 1
+    else
+      v &= ~unknown; // set unknown bits to 0
+    return v;
+  }
+
+  //===--------------------------------------------------------------------===//
+  // Resizing
+  //===--------------------------------------------------------------------===//
+
+  /// Zero-extend the integer to a new bit width. The additional high-order bits
+  /// are filled in with zero.
+  FVInt zext(unsigned bitWidth) const {
+    return FVInt(value.zext(bitWidth), unknown.zext(bitWidth));
+  }
+
+  /// Sign-extend the integer to a new bit width. The additional high-order bits
+  /// are filled in with the sign bit (top-most bit) of the original number. If
+  /// the sign bit is X or Z, fills in X or Z, respectively.
+  FVInt sext(unsigned bitWidth) const {
+    return FVInt(value.sext(bitWidth), unknown.sext(bitWidth));
+  }
+
+  //===--------------------------------------------------------------------===//
+  // Value Tests
+  //===--------------------------------------------------------------------===//
+
+  /// Determine if any bits are X or Z.
+  bool hasUnknown() const { return !unknown.isZero(); }
+
+  /// Determine if all bits are 0. This is true for zero-width values.
+  bool isZero() const { return value.isZero() && unknown.isZero(); }
+
+  /// Determine if all bits are 1. This is true for zero-width values.
+  bool isAllOnes() const { return value.isAllOnes() && unknown.isZero(); }
+
+  /// Determine if all bits are X. This is true for zero-width values.
+  bool isAllX() const { return value.isZero() && unknown.isAllOnes(); }
+
+  /// Determine if all bits are Z. This is true for zero-width values.
+  bool isAllZ() const { return value.isAllOnes() && unknown.isAllOnes(); }
+
+  //===--------------------------------------------------------------------===//
+  // Bit Manipulation
+  //===--------------------------------------------------------------------===//
+
+  /// The value of an individual bit. Can be 0, 1, X, or Z.
+  enum Bit { V0 = 0b00, V1 = 0b01, X = 0b10, Z = 0b11 };
+
+  /// Get the value of an individual bit.
+  Bit getBit(unsigned index) const {
+    return static_cast<Bit>(value[index] | unknown[index] << 1);
+  }
+
+  /// Set the value of an individual bit.
+  void setBit(unsigned index, Bit bit) {
+    value.setBitVal(index, (bit >> 0) & 1);
+    unknown.setBitVal(index, (bit >> 1) & 1);
+  }
+
+  /// Compute a mask of all the 0 bits in this integer.
+  APInt getZeroBits() const { return ~value & ~unknown; }
+
+  /// Compute a mask of all the 1 bits in this integer.
+  APInt getOneBits() const { return value & ~unknown; }
+
+  /// Compute a mask of all the X bits in this integer.
+  APInt getXBits() const { return ~value & unknown; }
+
+  /// Compute a mask of all the Z bits in this integer.
+  APInt getZBits() const { return value & unknown; }
+
+  /// Set the value of all bits in the mask to 0.
+  template <typename T>
+  void setZeroBits(const T &mask) {
+    value &= ~mask;
+    unknown &= ~mask;
+  }
+
+  /// Set the value of all bits in the mask to 1.
+  template <typename T>
+  void setOneBits(const T &mask) {
+    value |= mask;
+    unknown &= ~mask;
+  }
+
+  /// Set the value of all bits in the mask to X.
+  template <typename T>
+  void setXBits(const T &mask) {
+    value &= ~mask;
+    unknown |= mask;
+  }
+
+  /// Set the value of all bits in the mask to Z.
+  template <typename T>
+  void setZBits(const T &mask) {
+    value |= mask;
+    unknown |= mask;
+  }
+
+  /// Set all bits to 0.
+  void setAllZero() {
+    value.clearAllBits();
+    unknown.clearAllBits();
+  }
+
+  /// Set all bits to 1.
+  void setAllOne() {
+    value.setAllBits();
+    unknown.clearAllBits();
+  }
+
+  /// Set all bits to X.
+  void setAllX() {
+    value.clearAllBits();
+    unknown.setAllBits();
+  }
+
+  /// Set all bits to Z.
+  void setAllZ() {
+    value.setAllBits();
+    unknown.setAllBits();
+  }
+
+  /// Replace all Z bits with X. This is useful since most logic operations will
+  /// treat X and Z bits the same way and produce an X bit in the output. By
+  /// mapping Z bits to X, these operations can then just handle 0, 1, and X
+  /// bits.
+  void replaceZWithX() {
+    // Z bits have value and unknown set to 1. X bits have value set to 0 and
+    // unknown set to 1. To convert between the two, make sure that value is 0
+    // wherever unknown is 1.
+    value &= ~unknown;
+  }
+
+  /// If any bits are X or Z, set the entire integer to X.
+  void setAllXIfAnyUnknown() {
+    if (hasUnknown())
+      setAllX();
+  }
+
+  /// If any bits in this integer or another integer are X or Z, set the entire
+  /// integer to X. This is useful for binary operators which want to set their
+  /// result to X if either of the two inputs contained an X or Z bit.
+  void setAllXIfAnyUnknown(const FVInt &other) {
+    if (hasUnknown() || other.hasUnknown())
+      setAllX();
+  }
+
+  //===--------------------------------------------------------------------===//
+  // Shift Operators
+  //===--------------------------------------------------------------------===//
+
+  /// Perform a logical left-shift. If any bits in the shift amount are unknown,
+  /// the entire result is X.
+  FVInt &operator<<=(const FVInt &amount) {
+    if (amount.hasUnknown()) {
+      setAllX();
+    } else {
+      value <<= amount.value;
+      unknown <<= amount.value;
+    }
+    return *this;
+  }
+
+  /// Perform a logical left-shift by a two-valued amount.
+  template <typename T>
+  FVInt &operator<<=(const T &amount) {
+    value <<= amount;
+    unknown <<= amount;
+    return *this;
+  }
+
+  //===--------------------------------------------------------------------===//
+  // Logic Operators
+  //===--------------------------------------------------------------------===//
+
+  /// Compute the logical NOT of this integer. This implements the following
+  /// bit-wise truth table:
+  /// ```
+  /// 0 | 1
+  /// 1 | 0
+  /// X | X
+  /// Z | X
+  /// ```
+  void flipAllBits() {
+    value = ~value;
+    replaceZWithX();
+  }
+
+  /// Compute the logical NOT.
+  FVInt operator~() const {
+    auto v = *this;
+    v.flipAllBits();
+    return v;
+  }
+
+  /// Compute the logical AND of this integer and another. This implements the
+  /// following bit-wise truth table:
+  /// ```
+  ///     0 1 X Z
+  ///   +--------
+  /// 0 | 0 0 0 0
+  /// 1 | 0 1 X X
+  /// X | 0 X X X
+  /// Z | 0 X X X
+  /// ```
+  FVInt &operator&=(const FVInt &other) {
+    auto zeros = getZeroBits() | other.getZeroBits();
+    value &= other.value;
+    unknown |= other.unknown;
+    unknown &= ~zeros;
+    replaceZWithX();
+    return *this;
+  }
+
+  /// Compute the logical AND of this integer and a two-valued integer.
+  template <typename T>
+  FVInt &operator&=(T other) {
+    value &= other;
+    unknown &= other; // make 0 bits known
+    replaceZWithX();
+    return *this;
+  }
+
+  /// Compute the logical AND.
+  template <typename T>
+  FVInt operator&(const T &other) const {
+    auto v = *this;
+    v &= other;
+    return v;
+  }
+
+  /// Compute the logical OR of this integer and another. This implements the
+  /// following bit-wise truth table:
+  /// ```
+  ///     0 1 X Z
+  ///   +--------
+  /// 0 | 0 1 X X
+  /// 1 | 1 1 1 1
+  /// X | X 1 X X
+  /// Z | X 1 X X
+  /// ```
+  FVInt &operator|=(const FVInt &other) {
+    auto ones = getOneBits() | other.getOneBits();
+    value |= other.value;
+    unknown |= other.unknown;
+    unknown &= ~ones;
+    replaceZWithX();
+    return *this;
+  }
+
+  /// Compute the logical OR of this integer and a two-valued integer.
+  template <typename T>
+  FVInt &operator|=(T other) {
+    value |= other;
+    unknown &= ~other; // make 1 bits known
+    replaceZWithX();
+    return *this;
+  }
+
+  /// Compute the logical OR.
+  template <typename T>
+  FVInt operator|(const T &other) const {
+    auto v = *this;
+    v |= other;
+    return v;
+  }
+
+  /// Compute the logical XOR of this integer and another. This implements the
+  /// following bit-wise truth table:
+  /// ```
+  ///     0 1 X Z
+  ///   +--------
+  /// 0 | 0 1 X X
+  /// 1 | 1 0 X X
+  /// X | X X X X
+  /// Z | X X X X
+  /// ```
+  FVInt &operator^=(const FVInt &other) {
+    value ^= other.value;
+    unknown |= other.unknown;
+    replaceZWithX();
+    return *this;
+  }
+
+  /// Compute the logical XOR of this integer and a two-valued integer.
+  template <typename T>
+  FVInt &operator^=(const T &other) {
+    value ^= other;
+    replaceZWithX();
+    return *this;
+  }
+
+  /// Compute the logical XOR.
+  template <typename T>
+  FVInt operator^(const T &other) const {
+    auto v = *this;
+    v ^= other;
+    return v;
+  }
+
+  //===--------------------------------------------------------------------===//
+  // Arithmetic Operators
+  //===--------------------------------------------------------------------===//
+
+  /// Compute the negation of this integer. If any bits are unknown, the entire
+  /// result is X.
+  void negate() {
+    value.negate();
+    setAllXIfAnyUnknown();
+  }
+
+  /// Compute the negation of this integer.
+  FVInt operator-() const {
+    auto v = *this;
+    v.negate();
+    return v;
+  }
+
+  /// Compute the addition of this integer and another. If any bits in either
+  /// integer are unknown, the entire result is X.
+  FVInt &operator+=(const FVInt &other) {
+    value += other.value;
+    setAllXIfAnyUnknown(other);
+    return *this;
+  }
+
+  /// Compute the addition of this integer and a two-valued integer. If any bit
+  /// in the integer is unknown, the entire result is X.
+  template <typename T>
+  FVInt &operator+=(const T &other) {
+    value += other;
+    setAllXIfAnyUnknown();
+    return *this;
+  }
+
+  /// Compute an addition.
+  template <typename T>
+  FVInt operator+(const T &other) const {
+    auto v = *this;
+    v += other;
+    return v;
+  }
+
+  /// Compute the subtraction of this integer and another. If any bits in either
+  /// integer are unknown, the entire result is X.
+  FVInt &operator-=(const FVInt &other) {
+    value -= other.value;
+    setAllXIfAnyUnknown(other);
+    return *this;
+  }
+
+  /// Compute the subtraction of this integer and a two-valued integer. If any
+  /// bit in the integer is unknown, the entire result is X.
+  template <typename T>
+  FVInt &operator-=(const T &other) {
+    value -= other;
+    setAllXIfAnyUnknown();
+    return *this;
+  }
+
+  /// Compute an subtraction.
+  template <typename T>
+  FVInt operator-(const T &other) const {
+    auto v = *this;
+    v -= other;
+    return v;
+  }
+
+  /// Compute the multiplication of this integer and another. If any bits in
+  /// either integer are unknown, the entire result is X.
+  FVInt &operator*=(const FVInt &other) {
+    value *= other.value;
+    setAllXIfAnyUnknown(other);
+    return *this;
+  }
+
+  /// Compute the multiplication of this integer and a two-valued integer. If
+  /// any bit in the integer is unknown, the entire result is X.
+  template <typename T>
+  FVInt &operator*=(const T &other) {
+    value *= other;
+    setAllXIfAnyUnknown();
+    return *this;
+  }
+
+  /// Compute a multiplication.
+  template <typename T>
+  FVInt operator*(const T &other) const {
+    auto v = *this;
+    v *= other;
+    return v;
+  }
+
+  //===--------------------------------------------------------------------===//
+  // Comparison
+  //===--------------------------------------------------------------------===//
+
+  /// Determine whether this integer is equal to another. Note that this
+  /// corresponds to SystemVerilog's `===` operator.
+  bool operator==(const FVInt &other) const {
+    return value == other.value && unknown == other.unknown;
+  }
+
+  /// Determine whether this integer is equal to a two-valued integer. Note that
+  /// this corresponds to SystemVerilog's `===` operator.
+  template <typename T>
+  bool operator==(const T &other) const {
+    return value == other && !hasUnknown();
+  }
+
+  /// Determine whether this integer is not equal to another.
+  bool operator!=(const FVInt &other) const { return !((*this) == other); }
+
+  /// Determine whether this integer is not equal to a two-valued integer.
+  template <typename T>
+  bool operator!=(const T &other) const {
+    return !((*this) == other);
+  }
+
+  //===--------------------------------------------------------------------===//
+  // String Conversion
+  //===--------------------------------------------------------------------===//
+
+  /// Convert a string into an `FVInt`.
+  ///
+  /// The radix can be 2, 8, 10, or 16. For radix 2, the input string may
+  /// contain the characters `x` or `X` to indicate an unknown X bit, and `z` or
+  /// `Z` to indicate an unknown Z bit. For radix 8, each X or Z counts as 3
+  /// bits. For radix 16, each X and Z counts as 4 bits. When radix is 10 the
+  /// input cannot contain any X or Z.
+  ///
+  /// Returns the parsed integer if the string is non-empty and a well-formed
+  /// number, otherwise returns none.
+  static std::optional<FVInt> tryFromString(StringRef str, unsigned radix = 10);
+
+  /// Convert a string into an `FVInt`. Same as `tryFromString`, but aborts if
+  /// the string is malformed.
+  static FVInt fromString(StringRef str, unsigned radix = 10) {
+    auto v = tryFromString(str, radix);
+    assert(v.has_value() && "string is not a well-formed FVInt");
+    return *v;
+  }
+
+  /// Convert an `FVInt` to a string.
+  ///
+  /// The radix can be 2, 8, 10, or 16. For radix 8 or 16, the integer can only
+  /// contain unknown bits in groups of 3 or 4, respectively, such that a `X` or
+  /// `Z` can be printed for the entire group of bits. For radix 10, the integer
+  /// cannot contain any unknown bits. In case the output contains letters,
+  /// `uppercase` specifies whether they are printed as uppercase letters.
+  ///
+  /// Appends the output characters to `str` and returns true if the integer
+  /// could be printed with the given configuration. Otherwise returns false and
+  /// leaves `str` in its original state. Always succeeds for radix 2.
+  bool tryToString(SmallVectorImpl<char> &str, unsigned radix = 10,
+                   bool uppercase = true) const;
+
+  /// Convert an `FVInt` to a string. Same as `tryToString`, but directly
+  /// returns the string and aborts if the conversion is unsuccessful.
+  SmallString<16> toString(unsigned radix = 10, bool uppercase = true) const {
+    SmallString<16> str;
+    bool success = tryToString(str, radix, uppercase);
+    assert(success && "radix cannot represent FVInt");
+    return str;
+  }
+
+  /// Print an `FVInt` to an output stream.
+  void print(raw_ostream &os) const;
+
+private:
+  APInt value;
+  APInt unknown;
+};
+
+inline FVInt operator&(uint64_t a, const FVInt &b) { return b & a; }
+inline FVInt operator|(uint64_t a, const FVInt &b) { return b | a; }
+inline FVInt operator^(uint64_t a, const FVInt &b) { return b ^ a; }
+inline FVInt operator+(uint64_t a, const FVInt &b) { return b + a; }
+inline FVInt operator*(uint64_t a, const FVInt &b) { return b * a; }
+
+inline FVInt operator&(const APInt &a, const FVInt &b) { return b & a; }
+inline FVInt operator|(const APInt &a, const FVInt &b) { return b | a; }
+inline FVInt operator^(const APInt &a, const FVInt &b) { return b ^ a; }
+inline FVInt operator+(const APInt &a, const FVInt &b) { return b + a; }
+inline FVInt operator*(const APInt &a, const FVInt &b) { return b * a; }
+
+inline FVInt operator-(uint64_t a, const FVInt &b) {
+  return FVInt(b.getBitWidth(), a) - b;
+}
+
+inline FVInt operator-(const APInt &a, const FVInt &b) { return FVInt(a) - b; }
+
+inline bool operator==(uint64_t a, const FVInt &b) { return b == a; }
+inline bool operator!=(uint64_t a, const FVInt &b) { return b != a; }
+
+inline raw_ostream &operator<<(raw_ostream &os, const FVInt &value) {
+  value.print(os);
+  return os;
+}
+
+} // namespace circt
+
+#endif // CIRCT_SUPPORT_FVINT_H
diff --git a/lib/Support/CMakeLists.txt b/lib/Support/CMakeLists.txt
index 08b9d250ad35..1a38545425ab 100644
--- a/lib/Support/CMakeLists.txt
+++ b/lib/Support/CMakeLists.txt
@@ -11,6 +11,7 @@ add_circt_library(CIRCTSupport
   CustomDirectiveImpl.cpp
   Debug.cpp
   FieldRef.cpp
+  FVInt.cpp
   JSON.cpp
   LoweringOptions.cpp
   Naming.cpp
@@ -33,13 +34,15 @@ add_circt_library(CIRCTSupport
 #-------------------------------------------------------------------------------
 # Generate Version.cpp
 #-------------------------------------------------------------------------------
+
 find_first_existing_vc_file("${CIRCT_SOURCE_DIR}" CIRCT_GIT_LOGS_HEAD)
 set(GEN_VERSION_SCRIPT "${CIRCT_SOURCE_DIR}/cmake/modules/GenVersionFile.cmake")
 
 if (CIRCT_RELEASE_TAG_ENABLED)
   add_custom_command(OUTPUT "${VERSION_CPP}"
     DEPENDS "${CIRCT_GIT_LOGS_HEAD}" "${GEN_VERSION_SCRIPT}"
-    COMMAND ${CMAKE_COMMAND} -DIN_FILE="${CMAKE_CURRENT_SOURCE_DIR}/Version.cpp.in"
+    COMMAND ${CMAKE_COMMAND}
+    -DIN_FILE="${CMAKE_CURRENT_SOURCE_DIR}/Version.cpp.in"
     -DOUT_FILE="${VERSION_CPP}" -DRELEASE_PATTERN=${CIRCT_RELEASE_TAG}*
     -DDRY_RUN=OFF -DSOURCE_ROOT="${CIRCT_SOURCE_DIR}"
     -P "${GEN_VERSION_SCRIPT}")
@@ -48,7 +51,8 @@ else ()
   # cmake configuration.
   add_custom_command(OUTPUT "${VERSION_CPP}"
     DEPENDS "${GEN_VERSION_SCRIPT}"
-    COMMAND ${CMAKE_COMMAND} -DIN_FILE="${CMAKE_CURRENT_SOURCE_DIR}/Version.cpp.in"
+    COMMAND ${CMAKE_COMMAND}
+    -DIN_FILE="${CMAKE_CURRENT_SOURCE_DIR}/Version.cpp.in"
     -DOUT_FILE="${VERSION_CPP}" -DDRY_RUN=ON -DSOURCE_ROOT="${CIRCT_SOURCE_DIR}"
     -P "${GEN_VERSION_SCRIPT}")
 endif()
diff --git a/lib/Support/FVInt.cpp b/lib/Support/FVInt.cpp
new file mode 100644
index 000000000000..af68bc8b23c0
--- /dev/null
+++ b/lib/Support/FVInt.cpp
@@ -0,0 +1,139 @@
+//===- FVInt.cpp - Four-valued integer --------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "circt/Support/FVInt.h"
+#include "llvm/ADT/StringExtras.h"
+
+#define DEBUG_TYPE "fvint"
+
+using namespace circt;
+
+std::optional<FVInt> FVInt::tryFromString(StringRef str, unsigned radix) {
+  assert(radix == 2 || radix == 8 || radix == 10 || radix == 16);
+  if (str.empty())
+    return {};
+
+  // Overestimate the number of bits that will be needed to hold all digits.
+  unsigned radixLog2 = 0;
+  for (unsigned r = radix - 1; r > 0; r >>= 1)
+    ++radixLog2;
+  bool radixIsPow2 = (radix == (1 << radixLog2));
+
+  // Parse the string.
+  auto result = FVInt::getZero(str.size() * radixLog2);
+  while (!str.empty()) {
+    unsigned digit = llvm::toLower(str[0]);
+    str = str.drop_front();
+
+    // Handle X and Z digits.
+    if (digit == 'x' || digit == 'z') {
+      if (!radixIsPow2)
+        return {};
+      result <<= radixLog2;
+      result.unknown.setLowBits(radixLog2);
+      if (digit == 'z')
+        result.value.setLowBits(radixLog2);
+      continue;
+    }
+
+    // Determine the value of the current digit.
+    if (digit >= '0' && digit <= '9')
+      digit = digit - '0';
+    else if (digit >= 'a' && digit <= 'z')
+      digit = digit - 'a' + 10;
+    else
+      return {};
+    if (digit >= radix)
+      return {};
+
+    // Add the digit to the result.
+    if (radixIsPow2) {
+      result <<= radixLog2;
+      result.value |= digit;
+    } else {
+      result.value *= radix;
+      result.value += digit;
+    }
+  }
+
+  return result;
+}
+
+bool FVInt::tryToString(SmallVectorImpl<char> &str, unsigned radix,
+                        bool uppercase) const {
+  size_t strBaseLen = str.size();
+  assert(radix == 2 || radix == 8 || radix == 10 || radix == 16);
+
+  // Determine if the radix is a power of two.
+  unsigned radixLog2 = 0;
+  for (unsigned r = radix - 1; r > 0; r >>= 1)
+    ++radixLog2;
+  bool radixIsPow2 = (radix == (1 << radixLog2));
+  unsigned radixMask = (1 << radixLog2) - 1;
+
+  // If the number has no X or Z bits, take the easy route and print the `APInt`
+  // directly.
+  if (!hasUnknown()) {
+    value.toString(str, radix, /*Signed=*/false, /*formatAsCLiteral=*/false,
+                   uppercase);
+    return true;
+  }
+
+  // We can only print with non-power-of-two radices if there are no X or Z
+  // bits. So at this point we require radix be a power of two.
+  if (!radixIsPow2)
+    return false;
+
+  // Otherwise chop off digits at the bottom and print them to the string. This
+  // prints the digits in reverse order, with the least significant digit as the
+  // first character.
+  APInt value = this->value;
+  APInt unknown = this->unknown;
+
+  char chrA = uppercase ? 'A' : 'a';
+  char chrX = uppercase ? 'X' : 'x';
+  char chrZ = uppercase ? 'Z' : 'z';
+
+  while (!value.isZero() || !unknown.isZero()) {
+    unsigned digitValue = value.getRawData()[0] & radixMask;
+    unsigned digitUnknown = unknown.getRawData()[0] & radixMask;
+    value.lshrInPlace(radixLog2);
+    unknown.lshrInPlace(radixLog2);
+
+    // Handle unknown bits. Since we only get to print a single X or Z character
+    // to the string, either all bits in the digit have to be X, or all have to
+    // be Z. But we cannot represent the case where X, Z and 0/1 bits are mixed.
+    if (digitUnknown != 0) {
+      if (digitUnknown != radixMask ||
+          (digitValue != 0 && digitValue != radixMask)) {
+        str.resize(strBaseLen);
+        return false;
+      }
+      str.push_back(digitValue == 0 ? chrX : chrZ);
+      continue;
+    }
+
+    // Handle known bits.
+    if (digitValue < 10)
+      str.push_back(digitValue + '0');
+    else
+      str.push_back(digitValue - 10 + chrA);
+  }
+
+  // Reverse the digits.
+  std::reverse(str.begin() + strBaseLen, str.end());
+  return true;
+}
+
+void FVInt::print(raw_ostream &os) const {
+  SmallString<32> buffer;
+  if (!tryToString(buffer))
+    if (!tryToString(buffer, 16))
+      tryToString(buffer, 2);
+  os << buffer;
+}
diff --git a/unittests/Support/CMakeLists.txt b/unittests/Support/CMakeLists.txt
index 48431537442c..005e2389e396 100644
--- a/unittests/Support/CMakeLists.txt
+++ b/unittests/Support/CMakeLists.txt
@@ -1,4 +1,5 @@
 add_circt_unittest(CIRCTSupportTests
+  FVIntTest.cpp
   JSONTest.cpp
   PrettyPrinterTest.cpp
 )
diff --git a/unittests/Support/FVIntTest.cpp b/unittests/Support/FVIntTest.cpp
new file mode 100644
index 000000000000..8f78c09bbdbf
--- /dev/null
+++ b/unittests/Support/FVIntTest.cpp
@@ -0,0 +1,129 @@
+//===- FVIntTest.cpp - Four-valued integer unit tests ===------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Unit tests for the `FVInt` class.
+//
+//===----------------------------------------------------------------------===//
+
+#include "circt/Support/FVInt.h"
+#include "gtest/gtest.h"
+
+using namespace circt;
+
+namespace {
+
+TEST(FVIntTest, Resizing) {
+  ASSERT_EQ(FVInt::fromString("01", 2).zext(5), FVInt::fromString("00001", 2));
+  ASSERT_EQ(FVInt::fromString("01", 2).sext(5), FVInt::fromString("00001", 2));
+  ASSERT_EQ(FVInt::fromString("10", 2).zext(5), FVInt::fromString("00010", 2));
+  ASSERT_EQ(FVInt::fromString("10", 2).sext(5), FVInt::fromString("11110", 2));
+  ASSERT_EQ(FVInt::fromString("X1", 2).zext(5), FVInt::fromString("000X1", 2));
+  ASSERT_EQ(FVInt::fromString("X1", 2).sext(5), FVInt::fromString("XXXX1", 2));
+  ASSERT_EQ(FVInt::fromString("Z1", 2).zext(5), FVInt::fromString("000Z1", 2));
+  ASSERT_EQ(FVInt::fromString("Z1", 2).sext(5), FVInt::fromString("ZZZZ1", 2));
+}
+
+TEST(FVIntTest, Basics) {
+  ASSERT_TRUE(FVInt::getZero(42).isZero());
+  ASSERT_TRUE(FVInt::getAllOnes(42).isAllOnes());
+  ASSERT_TRUE(FVInt::getAllX(42).isAllX());
+  ASSERT_TRUE(FVInt::getAllZ(42).isAllZ());
+
+  ASSERT_FALSE(FVInt::getZero(42).hasUnknown());
+  ASSERT_FALSE(FVInt::getAllOnes(42).hasUnknown());
+  ASSERT_TRUE(FVInt::getAllX(42).hasUnknown());
+  ASSERT_TRUE(FVInt::getAllZ(42).hasUnknown());
+
+  auto x = FVInt::fromString("01XZ", 2);
+  ASSERT_EQ(x.toAPInt(false), 0b0100);
+  ASSERT_EQ(x.toAPInt(true), 0b0111);
+  ASSERT_EQ(x.getBit(0), FVInt::Z);
+  ASSERT_EQ(x.getBit(1), FVInt::X);
+  ASSERT_EQ(x.getBit(2), FVInt::V1);
+  ASSERT_EQ(x.getBit(3), FVInt::V0);
+  ASSERT_EQ(FVInt::V1, 1);
+  ASSERT_EQ(FVInt::V0, 0);
+
+  ASSERT_EQ(FVInt(32, 9001), FVInt(32, 9001));
+  ASSERT_EQ(FVInt(32, 9001), 9001);
+  ASSERT_EQ(9001, FVInt(32, 9001));
+
+  ASSERT_NE(FVInt(32, 9001), FVInt(32, 1337));
+  ASSERT_NE(FVInt(32, 9001), 1337);
+  ASSERT_NE(9001, FVInt(32, 1337));
+}
+
+TEST(FVIntTest, StringConversion) {
+  auto v = FVInt::fromString("ZX1001XZ", 2);
+  ASSERT_EQ(v.getZeroBits(), 0b00011000);
+  ASSERT_EQ(v.getOneBits(), 0b00100100);
+  ASSERT_EQ(v.getXBits(), 0b01000010);
+  ASSERT_EQ(v.getZBits(), 0b10000001);
+
+  // Parsing/printing without unknown values.
+  ASSERT_EQ(FVInt::fromString("10101100", 2).toString(2),
+            StringRef("10101100"));
+  ASSERT_EQ(FVInt::fromString("1234567", 8).toString(8), StringRef("1234567"));
+  ASSERT_EQ(FVInt::fromString("1234567890", 10).toString(10),
+            StringRef("1234567890"));
+  ASSERT_EQ(FVInt::fromString("1234567890ABCDEF", 16).toString(16),
+            "1234567890ABCDEF");
+  ASSERT_EQ(FVInt::fromString("1234567890abcdef", 16).toString(16, false),
+            "1234567890abcdef");
+
+  // Parsing/printing with unknown values.
+  ASSERT_EQ(FVInt::fromString("10XZ1XZ0", 2).toString(2),
+            StringRef("10XZ1XZ0"));
+  ASSERT_EQ(FVInt::fromString("10xz1xz0", 2).toString(2, false),
+            StringRef("10xz1xz0"));
+  ASSERT_EQ(FVInt::fromString("1234XZ567", 8).toString(8),
+            StringRef("1234XZ567"));
+  ASSERT_EQ(FVInt::fromString("1234xz567", 8).toString(8, false),
+            StringRef("1234xz567"));
+  ASSERT_EQ(FVInt::fromString("12345XZ67890ABCDEF", 16).toString(16),
+            StringRef("12345XZ67890ABCDEF"));
+  ASSERT_EQ(FVInt::fromString("12345xz67890abcdef", 16).toString(16, false),
+            StringRef("12345xz67890abcdef"));
+}
+
+TEST(FVIntTest, LogicOps) {
+  auto a = FVInt::fromString("01XZ01XZ01XZ01XZ", 2);
+  auto b = FVInt::fromString("00001111XXXXZZZZ", 2);
+  auto c = FVInt::fromString("01XZ", 2);
+
+  ASSERT_EQ(~c, FVInt::fromString("10XX", 2));
+  ASSERT_EQ(a & b, FVInt::fromString("000001XX0XXX0XXX", 2));
+  ASSERT_EQ(a | b, FVInt::fromString("01XX1111X1XXX1XX", 2));
+  ASSERT_EQ(a ^ b, FVInt::fromString("01XX10XXXXXXXXXX", 2));
+}
+
+TEST(FVIntTest, ArithmeticOps) {
+  auto a = FVInt::fromString("123").zext(32);
+  auto b = FVInt::fromString("234").zext(32);
+  auto c = FVInt::fromString("1XZ", 16).zext(32);
+
+  ASSERT_EQ(-a, uint32_t(-123));
+  ASSERT_TRUE((-c).isAllX());
+
+  ASSERT_EQ(a + 1, FVInt::fromString("124").zext(32));
+  ASSERT_EQ(1 + b, FVInt::fromString("235").zext(32));
+  ASSERT_EQ(a + b, FVInt::fromString("357").zext(32));
+  ASSERT_TRUE((a + c).isAllX());
+
+  ASSERT_EQ(a - 1, FVInt::fromString("122").zext(32));
+  ASSERT_EQ(234 - a, FVInt::fromString("111").zext(32));
+  ASSERT_EQ(b - a, FVInt::fromString("111").zext(32));
+  ASSERT_TRUE((a - c).isAllX());
+
+  ASSERT_EQ(a * 2, FVInt::fromString("246").zext(32));
+  ASSERT_EQ(2 * b, FVInt::fromString("468").zext(32));
+  ASSERT_EQ(a * b, FVInt::fromString("28782").zext(32));
+  ASSERT_TRUE((a * c).isAllX());
+}
+
+} // namespace