diff --git a/src/hotspot/cpu/aarch64/register_aarch64.hpp b/src/hotspot/cpu/aarch64/register_aarch64.hpp
index 3c2571eb4a3..8e340fefbb4 100644
--- a/src/hotspot/cpu/aarch64/register_aarch64.hpp
+++ b/src/hotspot/cpu/aarch64/register_aarch64.hpp
@@ -389,14 +389,14 @@ typedef AbstractRegSet<PRegister> PRegSet;
 
 template <>
 inline Register AbstractRegSet<Register>::first() {
-  uint32_t first = _bitset & -_bitset;
-  return first ? as_Register(exact_log2(first)) : noreg;
+  if (_bitset == 0) { return noreg; }
+  return as_Register(count_trailing_zeros(_bitset));
 }
 
 template <>
 inline FloatRegister AbstractRegSet<FloatRegister>::first() {
-  uint32_t first = _bitset & -_bitset;
-  return first ? as_FloatRegister(exact_log2(first)) : fnoreg;
+  if (_bitset == 0) { return fnoreg; }
+  return as_FloatRegister(count_trailing_zeros(_bitset));
 }
 
 inline Register as_Register(FloatRegister reg) {
diff --git a/src/hotspot/cpu/x86/register_x86.hpp b/src/hotspot/cpu/x86/register_x86.hpp
index 2cfc3ec58ed..5449f0d9724 100644
--- a/src/hotspot/cpu/x86/register_x86.hpp
+++ b/src/hotspot/cpu/x86/register_x86.hpp
@@ -391,27 +391,27 @@ class ConcreteRegisterImpl : public AbstractRegisterImpl {
 
 template <>
 inline Register AbstractRegSet<Register>::first() {
-  uint32_t first = _bitset & -_bitset;
-  return first ? as_Register(exact_log2(first)) : noreg;
+  if (_bitset == 0) { return noreg; }
+  return as_Register(count_trailing_zeros(_bitset));
 }
 
 template <>
 inline Register AbstractRegSet<Register>::last() {
   if (_bitset == 0) { return noreg; }
-  uint32_t last = 31 - count_leading_zeros(_bitset);
+  int last = max_size() - 1 - count_leading_zeros(_bitset);
   return as_Register(last);
 }
 
 template <>
 inline XMMRegister AbstractRegSet<XMMRegister>::first() {
-  uint32_t first = _bitset & -_bitset;
-  return first ? as_XMMRegister(exact_log2(first)) : xnoreg;
+  if (_bitset == 0) { return xnoreg; }
+  return as_XMMRegister(count_trailing_zeros(_bitset));
 }
 
 template <>
 inline XMMRegister AbstractRegSet<XMMRegister>::last() {
   if (_bitset == 0) { return xnoreg; }
-  uint32_t last = 31 - count_leading_zeros(_bitset);
+  int last = max_size() - 1 - count_leading_zeros(_bitset);
   return as_XMMRegister(last);
 }
 
diff --git a/src/hotspot/share/asm/register.hpp b/src/hotspot/share/asm/register.hpp
index fab548ed36d..7dfc9b03f87 100644
--- a/src/hotspot/share/asm/register.hpp
+++ b/src/hotspot/share/asm/register.hpp
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2000, 2022, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2000, 2023, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -93,67 +93,79 @@ template <class RegImpl> class ReverseRegSetIterator;
 // A set of registers
 template <class RegImpl>
 class AbstractRegSet {
-  uint32_t _bitset;
+#ifndef ARM
+  STATIC_ASSERT(RegImpl::number_of_registers <= 64);
+#endif
+  uint64_t _bitset;
 
-  AbstractRegSet(uint32_t bitset) : _bitset(bitset) { }
+  constexpr AbstractRegSet(uint64_t bitset) : _bitset(bitset) { }
+
+  static constexpr int max_size() {
+    return (int)(sizeof(_bitset) * BitsPerByte);
+  }
 
 public:
 
-  AbstractRegSet() : _bitset(0) { }
+  constexpr AbstractRegSet() : _bitset(0) { }
 
-  AbstractRegSet(RegImpl r1) : _bitset(1 << r1->encoding()) { }
+  constexpr AbstractRegSet(RegImpl r1)
+    : _bitset(r1->is_valid() ? size_t(1) << r1->encoding() : 0) {
+  }
 
-  AbstractRegSet operator+(const AbstractRegSet aSet) const {
+  constexpr AbstractRegSet operator+(const AbstractRegSet aSet) const {
     AbstractRegSet result(_bitset | aSet._bitset);
     return result;
   }
 
-  AbstractRegSet operator-(const AbstractRegSet aSet) const {
+  constexpr AbstractRegSet operator-(const AbstractRegSet aSet) const {
     AbstractRegSet result(_bitset & ~aSet._bitset);
     return result;
   }
 
-  AbstractRegSet &operator+=(const AbstractRegSet aSet) {
+  constexpr AbstractRegSet &operator+=(const AbstractRegSet aSet) {
     *this = *this + aSet;
     return *this;
   }
 
-  AbstractRegSet &operator-=(const AbstractRegSet aSet) {
+  constexpr AbstractRegSet &operator-=(const AbstractRegSet aSet) {
     *this = *this - aSet;
     return *this;
   }
 
-  static AbstractRegSet of(RegImpl r1) {
+  constexpr static AbstractRegSet of(RegImpl r1) {
     return AbstractRegSet(r1);
   }
 
-  static AbstractRegSet of(RegImpl r1, RegImpl r2) {
+  constexpr static AbstractRegSet of(RegImpl r1, RegImpl r2) {
     return of(r1) + r2;
   }
 
-  static AbstractRegSet of(RegImpl r1, RegImpl r2, RegImpl r3) {
+  constexpr static AbstractRegSet of(RegImpl r1, RegImpl r2, RegImpl r3) {
     return of(r1, r2) + r3;
   }
 
-  static AbstractRegSet of(RegImpl r1, RegImpl r2, RegImpl r3, RegImpl r4) {
+  constexpr static AbstractRegSet of(RegImpl r1, RegImpl r2, RegImpl r3, RegImpl r4) {
     return of(r1, r2, r3) + r4;
   }
 
-  static AbstractRegSet range(RegImpl start, RegImpl end) {
+  constexpr static AbstractRegSet range(RegImpl start, RegImpl end) {
     int start_enc = start->encoding();
     int   end_enc = end->encoding();
     assert(start_enc <= end_enc, "must be");
-    uint32_t bits = ~0;
+    size_t bits = ~(size_t)0;
     bits <<= start_enc;
-    bits <<= 31 - end_enc;
-    bits >>= 31 - end_enc;
+    bits <<= max_size() - 1 - end_enc;
+    bits >>= max_size() - 1 - end_enc;
 
     return AbstractRegSet(bits);
   }
 
-  uint size() const { return population_count(_bitset); }
+  constexpr bool contains(RegImpl reg) {
+    return (AbstractRegSet(reg).bits() & bits()) != 0;
+  }
 
-  uint32_t bits() const { return _bitset; }
+  constexpr uint size() const { return population_count(_bitset); }
+  constexpr uint64_t bits() const { return _bitset; }
 
 private:
 
@@ -243,16 +255,37 @@ inline ReverseRegSetIterator<RegImpl> AbstractRegSet<RegImpl>::rbegin() {
 
 #include CPU_HEADER(register)
 
-// Debugging support
+// Debugging and assertion support
+
+template<typename R>
+constexpr bool different_registers(AbstractRegSet<R> allocated_regs, R first_register) {
+  return !allocated_regs.contains(first_register);
+}
+
+template<typename R, typename... Rx>
+constexpr bool different_registers(AbstractRegSet<R> allocated_regs, R first_register, Rx... more_registers) {
+  if (allocated_regs.contains(first_register)) {
+    return false;
+  }
+  return different_registers(allocated_regs + first_register, more_registers...);
+}
+
+template<typename R, typename... Rx>
+inline constexpr bool different_registers(R first_register, Rx... more_registers) {
+  return different_registers(AbstractRegSet<R>(first_register), more_registers...);
+}
 
 template<typename R, typename... Rx>
 inline void assert_different_registers(R first_register, Rx... more_registers) {
 #ifdef ASSERT
-  const R regs[] = { first_register, more_registers... };
-  // Verify there are no equal entries.
-  for (size_t i = 0; i < ARRAY_SIZE(regs) - 1; ++i) {
-    for (size_t j = i + 1; j < ARRAY_SIZE(regs); ++j) {
-      assert(regs[i] != regs[j], "Multiple uses of register: %s", regs[i]->name());
+  if (!different_registers(first_register, more_registers...)) {
+    const R regs[] = { first_register, more_registers... };
+    // Find a duplicate entry.
+    for (size_t i = 0; i < ARRAY_SIZE(regs) - 1; ++i) {
+      for (size_t j = i + 1; j < ARRAY_SIZE(regs); ++j) {
+        assert(!regs[i]->is_valid() || regs[i] != regs[j],
+               "Multiple uses of register: %s", regs[i]->name());
+      }
     }
   }
 #endif
diff --git a/src/hotspot/share/utilities/population_count.hpp b/src/hotspot/share/utilities/population_count.hpp
index 97c5ce2a368..0cf35d325dc 100644
--- a/src/hotspot/share/utilities/population_count.hpp
+++ b/src/hotspot/share/utilities/population_count.hpp
@@ -44,7 +44,7 @@
 // generate a call to a similar but slower 64-bit version when calling with
 // a 32-bit integer type.
 template <typename T>
-inline unsigned population_count(T x) {
+constexpr unsigned population_count(T x) {
   STATIC_ASSERT(BitsPerWord <= 128);
   STATIC_ASSERT(BitsPerByte == 8);
   STATIC_ASSERT(std::is_integral<T>::value);