diff --git a/projects/rccl/test/host/CMakeLists.txt b/projects/rccl/test/host/CMakeLists.txt
index 26a486a70cd9..e1d32bb1ec2a 100644
--- a/projects/rccl/test/host/CMakeLists.txt
+++ b/projects/rccl/test/host/CMakeLists.txt
@@ -97,6 +97,7 @@ function(rccl_define_micro_source_lists)
     devcomm-test.cc
     wrap-test.cc
     dev-runtime-test.cc
+    nccl-utility-test.cc                 # src/include/nccl_device/utility.h (header-only, no fakes)
     # sym-kernels-index-test.cc: added conditionally below, only when GENERATE_SYM_KERNELS is ON.
     gdr-probe-test.cc                    # src/misc/gdr_probe.cc runtime GDR registration probe
     process-isolated-runner-test.cc
diff --git a/projects/rccl/test/host/MICROTEST_README.md b/projects/rccl/test/host/MICROTEST_README.md
index a1df556582cc..88215181402e 100644
--- a/projects/rccl/test/host/MICROTEST_README.md
+++ b/projects/rccl/test/host/MICROTEST_README.md
@@ -80,6 +80,8 @@ export colliding non-`static` symbols; otherwise a unit needs its own binary:
   - `dev_runtime.cc` (`DEV_RUNTIME_CC_PATH`, from `dev-runtime-test.cc`);
     suites `Alloc*`, `Comm*`, `Compute*`, `DeepCopy*`, `Dev*`, `Gin*`, `Nccl*`,
     `Sym*`, and `Win*`.
+  - `include/nccl_device/utility.h` (header-only, from `nccl-utility-test.cc`);
+    suite `NcclUtilityMicrotest.*`.
   - the generated `sym_kernels_host.cc` (`SYM_KERNELS_HOST_CC_PATH`, from
     `sym-kernels-index-test.cc`); suites `SymKernelIndex*`,
     `SymAllEmittedCombinations/*`, `SymUnhandledCombinations/*` (covered by the
diff --git a/projects/rccl/test/host/nccl-utility-test.cc b/projects/rccl/test/host/nccl-utility-test.cc
new file mode 100644
index 000000000000..716c707db603
--- /dev/null
+++ b/projects/rccl/test/host/nccl-utility-test.cc
@@ -0,0 +1,355 @@
+/*************************************************************************
+ * Copyright (c) 2026 Advanced Micro Devices, Inc. All rights reserved.
+ *
+ * See LICENSE.txt for license information
+ ************************************************************************/
+
+// Host-only microtests for the host/device math helpers in src/include/nccl_device/utility.h.
+
+#include <gtest/gtest.h>
+
+#include <atomic>
+#include <cstdint>
+
+#include "nccl_device/utility.h"
+
+// bitops.h declares the same ::divUp, ::isPow2, ... so these using-declarations break if it is
+// ever included here; qualify the calls instead.
+using nccl::utility::acquireOrderOf;
+using nccl::utility::add4G;
+using nccl::utility::alignDown;
+using nccl::utility::alignUp;
+using nccl::utility::divUp;
+using nccl::utility::idivFast32;
+using nccl::utility::idivFast64;
+using nccl::utility::idivmodFast32;
+using nccl::utility::idivmodFast64;
+using nccl::utility::idivRcp32;
+using nccl::utility::idivRcp64;
+using nccl::utility::imodFast32;
+using nccl::utility::imodFast64;
+using nccl::utility::imulRcp32;
+using nccl::utility::imulRcp64;
+using nccl::utility::isPow2;
+using nccl::utility::mul32hi;
+using nccl::utility::mul64hi;
+using nccl::utility::Optional;
+using nccl::utility::present;
+using nccl::utility::Present;
+using nccl::utility::releaseOrderOf;
+using nccl::utility::roundDown;
+using nccl::utility::roundUp;
+using nccl::utility::rollingLessEq;
+using nccl::utility::rollingLessThan;
+using nccl::utility::toAtomicBuiltinOrder;
+using nccl::utility::toCudaOrder;
+
+namespace {
+
+// Plain division, so the fast path can't share a bug with its golden.
+uint64_t refDiv64(uint64_t x, uint64_t y) { return x / y; }
+
+} // namespace
+
+TEST(NcclUtilityMicrotest, DivUp_RoundsAwayFromZeroOnlyWhenRemainderIsNonzero) {
+  EXPECT_EQ(divUp(0u, 4u), 0u);
+  EXPECT_EQ(divUp(1u, 4u), 1u);
+  EXPECT_EQ(divUp(4u, 4u), 1u);
+  EXPECT_EQ(divUp(5u, 4u), 2u);
+  EXPECT_EQ(divUp(7u, 3u), 3u);
+}
+
+TEST(NcclUtilityMicrotest, RoundUpDown_AreInverseDirectionsAroundAMultiple) {
+  EXPECT_EQ(roundUp(0u, 8u), 0u);
+  EXPECT_EQ(roundUp(1u, 8u), 8u);
+  EXPECT_EQ(roundUp(8u, 8u), 8u);
+  EXPECT_EQ(roundUp(9u, 8u), 16u);
+
+  EXPECT_EQ(roundDown(0u, 8u), 0u);
+  EXPECT_EQ(roundDown(7u, 8u), 0u);
+  EXPECT_EQ(roundDown(8u, 8u), 8u);
+  EXPECT_EQ(roundDown(15u, 8u), 8u);
+
+  // Non-power-of-two modulus, so `% y` can't pass as `& (y - 1)`.
+  EXPECT_EQ(roundUp(5u, 3u), 6u);
+  EXPECT_EQ(roundDown(7u, 3u), 6u);
+}
+
+TEST(NcclUtilityMicrotest, AlignUpDown_ScalarOverload_AssumesPowerOfTwoAlignment) {
+  EXPECT_EQ(alignUp(0u, 16u), 0u);
+  EXPECT_EQ(alignUp(1u, 16u), 16u);
+  EXPECT_EQ(alignUp(16u, 16u), 16u);
+  EXPECT_EQ(alignUp(17u, 16u), 32u);
+
+  EXPECT_EQ(alignDown(0u, 16u), 0u);
+  EXPECT_EQ(alignDown(15u, 16u), 0u);
+  EXPECT_EQ(alignDown(16u, 16u), 16u);
+  EXPECT_EQ(alignDown(31u, 16u), 16u);
+  // Above 2*a, where `x & -Z(a)` differs from `x & Z(a)`.
+  EXPECT_EQ(alignDown(48u, 16u), 48u);
+  EXPECT_EQ(alignDown(33u, 16u), 32u);
+}
+
+TEST(NcclUtilityMicrotest, AlignUpDown_PointerOverload_RoundsTheAddressNotTheByte) {
+  alignas(32) char buf[64];
+  // An already-aligned pointer pins alignUp's `- 1`: buf + 3 rounds to buf + 16 either way.
+  char* base = buf + 3;
+  char* up = alignUp(base, size_t(16));
+  char* down = alignDown(base, size_t(16));
+  EXPECT_EQ(up, buf + 16);
+  EXPECT_EQ(down, buf);
+  EXPECT_EQ(alignUp(buf, size_t(16)), buf);
+}
+
+TEST(NcclUtilityMicrotest, IsPow2_TrueOnlyForPowersOfTwoAndZero) {
+  EXPECT_TRUE(isPow2(0u));
+  EXPECT_TRUE(isPow2(1u));
+  EXPECT_TRUE(isPow2(2u));
+  EXPECT_TRUE(isPow2(1024u));
+  EXPECT_FALSE(isPow2(3u));
+  EXPECT_FALSE(isPow2(6u));
+  EXPECT_FALSE(isPow2(1023u));
+}
+
+TEST(NcclUtilityMicrotest, RollingLessEqAndLessThan_WrapAroundHalfTheRangeCorrectly) {
+  // 8-bit rolling counter: 250 is "less than" 4 (wrapped), but not vice versa.
+  EXPECT_TRUE(rollingLessEq(uint8_t(250), uint8_t(4), 8));
+  EXPECT_FALSE(rollingLessEq(uint8_t(4), uint8_t(250), 8));
+  EXPECT_TRUE(rollingLessThan(uint8_t(250), uint8_t(4), 8));
+  EXPECT_FALSE(rollingLessThan(uint8_t(4), uint8_t(250), 8));
+
+  // Equal values: LessEq true, LessThan false (strict).
+  EXPECT_TRUE(rollingLessEq(uint8_t(7), uint8_t(7), 8));
+  EXPECT_FALSE(rollingLessThan(uint8_t(7), uint8_t(7), 8));
+
+  // The half-range boundary: 127 is still "less or equal", 128 is not.
+  EXPECT_TRUE(rollingLessEq(uint8_t(0), uint8_t(127), 8));
+  EXPECT_FALSE(rollingLessEq(uint8_t(0), uint8_t(128), 8));
+
+  // A partial nBits on a wider type, as production uses it (gin_barrier__funcs.h passes 32 for a
+  // uint64_t counter): true only because the high 32 bits are masked out.
+  EXPECT_TRUE(rollingLessEq(uint64_t(0xFFFFFFF0ull), uint64_t(0x10ull), 32));
+  EXPECT_FALSE(rollingLessEq(uint64_t(0x10ull), uint64_t(0xFFFFFFF0ull), 32));
+
+  // rollingLessThan must forward nBits: dropping it flips both.
+  EXPECT_TRUE(rollingLessThan(uint64_t(0xFFFFFFF0ull), uint64_t(0x10ull), 32));
+  EXPECT_FALSE(rollingLessThan(uint64_t(0x10ull), uint64_t(0xFFFFFFF0ull), 32));
+}
+
+TEST(NcclUtilityMicrotest, IdivRcp32_MatchesExactFormulaIncludingPow2Correction) {
+  // uint32_t(-1)/x + isPow2(x), pinned exactly: idivFast32's correction would hide a wrong +1.
+  EXPECT_EQ(idivRcp32(1u), uint32_t(-1) / 1u + 1u);
+  EXPECT_EQ(idivRcp32(2u), uint32_t(-1) / 2u + 1u);
+  EXPECT_EQ(idivRcp32(3u), uint32_t(-1) / 3u + 0u);
+  EXPECT_EQ(idivRcp32(100u), uint32_t(-1) / 100u + 0u);
+}
+
+TEST(NcclUtilityMicrotest, IdivRcp64_MatchesExactFormulaIncludingPow2Correction) {
+  EXPECT_EQ(idivRcp64(1ull), uint64_t(-1) / 1ull + 1ull);
+  EXPECT_EQ(idivRcp64(2ull), uint64_t(-1) / 2ull + 1ull);
+  EXPECT_EQ(idivRcp64(3ull), uint64_t(-1) / 3ull + 0ull);
+  EXPECT_EQ(idivRcp64(1ull << 40), uint64_t(-1) / (1ull << 40) + 1ull);
+}
+
+TEST(NcclUtilityMicrotest, Mul32Hi_ReturnsTheHighHalfOfTheFull64BitProduct) {
+  EXPECT_EQ(mul32hi(0u, 0xFFFFFFFFu), 0u);
+  EXPECT_EQ(mul32hi(0xFFFFFFFFu, 0xFFFFFFFFu), 0xFFFFFFFEu);
+  EXPECT_EQ(mul32hi(0x10000u, 0x10000u), 1u); // 2^16 * 2^16 == 2^32, high half == 1
+}
+
+TEST(NcclUtilityMicrotest, Mul64Hi_ReturnsTheHighHalfOfTheFull128BitProduct) {
+  EXPECT_EQ(mul64hi(0ull, ~0ull), 0ull);
+  uint64_t big = 0x1'0000'0000ull; // 2^32
+  EXPECT_EQ(mul64hi(big, big), 1ull); // 2^32 * 2^32 == 2^64, high half == 1
+  EXPECT_EQ(mul64hi(~0ull, ~0ull), 0xFFFFFFFFFFFFFFFEull);
+}
+
+TEST(NcclUtilityMicrotest, ImulRcp32_HandlesZeroReciprocalShortCircuitsAndMatchesIdivRcpOfProduct) {
+  // A zero reciprocal means "this factor is 1", so the other one is returned.
+  EXPECT_EQ(imulRcp32(5u, 0u, 7u, idivRcp32(7u)), idivRcp32(7u));
+  EXPECT_EQ(imulRcp32(5u, idivRcp32(5u), 7u, 0u), idivRcp32(5u));
+
+  // (3,5) takes the `x*y <= rem` rounding correction.
+  uint32_t x = 3, y = 5;
+  uint32_t got = imulRcp32(x, idivRcp32(x), y, idivRcp32(y));
+  EXPECT_EQ(got, idivRcp32(x * y));
+
+  // (2,2) doesn't.
+  EXPECT_EQ(imulRcp32(2u, idivRcp32(2u), 2u, idivRcp32(2u)), idivRcp32(4u));
+
+  // (3,3) pins the `0u -` in rem: without it the correction fires and returns one too many.
+  EXPECT_EQ(imulRcp32(3u, idivRcp32(3u), 3u, idivRcp32(3u)), idivRcp32(9u));
+}
+
+TEST(NcclUtilityMicrotest, ImulRcp64_HandlesZeroReciprocalShortCircuitsAndMatchesIdivRcpOfProduct) {
+  EXPECT_EQ(imulRcp64(5ull, 0ull, 7ull, idivRcp64(7ull)), idivRcp64(7ull));
+  EXPECT_EQ(imulRcp64(5ull, idivRcp64(5ull), 7ull, 0ull), idivRcp64(5ull));
+
+  // Same three cases as the 32-bit version.
+  uint64_t x = 3, y = 5;
+  uint64_t got = imulRcp64(x, idivRcp64(x), y, idivRcp64(y));
+  EXPECT_EQ(got, idivRcp64(x * y));
+
+  EXPECT_EQ(imulRcp64(2ull, idivRcp64(2ull), 2ull, idivRcp64(2ull)), idivRcp64(4ull));
+
+  EXPECT_EQ(imulRcp64(3ull, idivRcp64(3ull), 3ull, idivRcp64(3ull)), idivRcp64(9ull));
+}
+
+TEST(NcclUtilityMicrotest, IdivmodFast32_MatchesOrdinaryDivisionAcrossDivisorsAndYrcpZeroShortcut) {
+  for (uint32_t y : {1u, 2u, 3u, 7u, 16u, 100u, 4096u}) {
+    uint32_t yrcp = idivRcp32(y);
+    // The top of the range catches an over-estimated reciprocal; smaller values don't.
+    for (uint32_t x : {0u, 1u, 5u, 41u, 4095u, 4096u, 1000000u, 0xFFFFFFFFu}) {
+      uint32_t q, r;
+      idivmodFast32(&q, &r, x, y, yrcp);
+      EXPECT_EQ(q, x / y) << "x=" << x << " y=" << y;
+      EXPECT_EQ(r, x % y) << "x=" << x << " y=" << y;
+    }
+  }
+  // yrcp == 0: q = x ("divisor is 1").
+  uint32_t q, r;
+  idivmodFast32(&q, &r, 42u, 1u, 0u);
+  EXPECT_EQ(q, 42u);
+  EXPECT_EQ(r, 0u);
+}
+
+TEST(NcclUtilityMicrotest, IdivmodFast64_MatchesOrdinaryDivisionAcrossDivisorsAndYrcpZeroShortcut) {
+  for (uint64_t y : {1ull, 2ull, 3ull, 7ull, 16ull, 100ull, 1ull << 20}) {
+    uint64_t yrcp = idivRcp64(y);
+    for (uint64_t x : {0ull, 1ull, 5ull, 41ull, (1ull << 20) - 1, 1ull << 20, 1ull << 40, ~0ull}) {
+      uint64_t q, r;
+      idivmodFast64(&q, &r, x, y, yrcp);
+      EXPECT_EQ(q, x / y) << "x=" << x << " y=" << y;
+      EXPECT_EQ(r, x % y) << "x=" << x << " y=" << y;
+    }
+  }
+  uint64_t q, r;
+  idivmodFast64(&q, &r, 42ull, 1ull, 0ull);
+  EXPECT_EQ(q, 42ull);
+  EXPECT_EQ(r, 0ull);
+}
+
+TEST(NcclUtilityMicrotest, IdivFast32_ReturnsTheQuotientAndIdivFast64TruncatesToLow32Bits) {
+  // Wrapper over idivmodFast32: returns q (13), not r (2).
+  EXPECT_EQ(idivFast32(41u, 3u, idivRcp32(3u)), 13u);
+  // The second case's quotient is far above 2^32, so the narrowing to uint32_t matters.
+  uint64_t y = 1000, yrcp64 = idivRcp64(y);
+  uint64_t x = 123456789ull;
+  EXPECT_EQ(idivFast64(x, y, yrcp64), uint32_t(refDiv64(x, y)));
+  EXPECT_EQ(idivFast64(1ull << 40, 1ull, idivRcp64(1ull)), 0u);
+}
+
+TEST(NcclUtilityMicrotest, ImodFast32And64_ReturnTheRemainder) {
+  // Wrappers over idivmodFast32/64: return r (2), not q (13).
+  EXPECT_EQ(imodFast32(41u, 3u, idivRcp32(3u)), 2u);
+  EXPECT_EQ(imodFast64(41ull, 3ull, idivRcp64(3ull)), 2ull);
+}
+
+TEST(NcclUtilityMicrotest, Optional_DefaultAndAbsentConstructLeavePresentFalse) {
+  Optional<int> a;
+  EXPECT_FALSE(a.present);
+  Optional<int> b{nccl::utility::Absent{}};
+  EXPECT_FALSE(b.present);
+}
+
+TEST(NcclUtilityMicrotest, Optional_PresentConstructorForwardsArgsAndSetsPresentTrue) {
+  // Both values must reach T's constructor, in order.
+  struct Pair {
+    int a, b;
+    NCCL_HOST_DEVICE_INLINE Pair(int a_, int b_) : a(a_), b(b_) {}
+  };
+  Optional<Pair> opt(present(3, 4));
+  ASSERT_TRUE(opt.present);
+  EXPECT_EQ(opt.thing.a, 3);
+  EXPECT_EQ(opt.thing.b, 4);
+}
+
+TEST(NcclUtilityMicrotest, Optional_DestructorOnlyRunsWhenPresentIsTrue) {
+  static int destructorCalls = 0;
+  struct Counted {
+    NCCL_HOST_DEVICE_INLINE Counted() {}
+    NCCL_HOST_DEVICE_INLINE ~Counted() { destructorCalls++; }
+  };
+
+  destructorCalls = 0;
+  {
+    Optional<Counted> absent; // present == false: ~Counted() must NOT run.
+  }
+  EXPECT_EQ(destructorCalls, 0);
+
+  destructorCalls = 0;
+  {
+    Optional<Counted> presentOpt(present());
+  }
+  EXPECT_EQ(destructorCalls, 1);
+}
+
+TEST(NcclUtilityMicrotest, Add4G_AddsDeltaIntoTheHighHalfOfA64BitValueViaUnion) {
+  // add4G adds delta4G into the high 32 bits; check against plain 64-bit arithmetic.
+  uint64_t base = 0x0000'0000'1234'5678ull;
+  EXPECT_EQ(add4G(base, 1), base + (uint64_t(1) << 32));
+  EXPECT_EQ(add4G(base, 5), base + (uint64_t(5) << 32));
+  // Negative delta: subtracts a 4G chunk from the high half.
+  uint64_t highBase = 0x0000'0003'0000'0000ull;
+  EXPECT_EQ(add4G(highBase, -1), highBase - (uint64_t(1) << 32));
+}
+
+TEST(NcclUtilityMicrotest, AcquireOrderOfStd_DowngradesReleaseAndAcqRelPassesEverythingElseThrough) {
+  EXPECT_EQ(acquireOrderOf(std::memory_order_release), std::memory_order_relaxed);
+  EXPECT_EQ(acquireOrderOf(std::memory_order_acq_rel), std::memory_order_acquire);
+  EXPECT_EQ(acquireOrderOf(std::memory_order_relaxed), std::memory_order_relaxed);
+  EXPECT_EQ(acquireOrderOf(std::memory_order_acquire), std::memory_order_acquire);
+  EXPECT_EQ(acquireOrderOf(std::memory_order_seq_cst), std::memory_order_seq_cst);
+}
+
+TEST(NcclUtilityMicrotest, ReleaseOrderOfStd_DowngradesAcquireAndAcqRelPassesEverythingElseThrough) {
+  EXPECT_EQ(releaseOrderOf(std::memory_order_acquire), std::memory_order_relaxed);
+  EXPECT_EQ(releaseOrderOf(std::memory_order_acq_rel), std::memory_order_release);
+  EXPECT_EQ(releaseOrderOf(std::memory_order_relaxed), std::memory_order_relaxed);
+  EXPECT_EQ(releaseOrderOf(std::memory_order_release), std::memory_order_release);
+  EXPECT_EQ(releaseOrderOf(std::memory_order_seq_cst), std::memory_order_seq_cst);
+}
+
+TEST(NcclUtilityMicrotest, ToAtomicBuiltinOrder_MapsEveryStdOrderAndFallsBackToSeqCstForConsume) {
+  EXPECT_EQ(toAtomicBuiltinOrder(std::memory_order_relaxed), __ATOMIC_RELAXED);
+  EXPECT_EQ(toAtomicBuiltinOrder(std::memory_order_acquire), __ATOMIC_ACQUIRE);
+  EXPECT_EQ(toAtomicBuiltinOrder(std::memory_order_release), __ATOMIC_RELEASE);
+  EXPECT_EQ(toAtomicBuiltinOrder(std::memory_order_acq_rel), __ATOMIC_ACQ_REL);
+  EXPECT_EQ(toAtomicBuiltinOrder(std::memory_order_seq_cst), __ATOMIC_SEQ_CST);
+  // memory_order_consume has no case label; must fall through to default.
+  EXPECT_EQ(toAtomicBuiltinOrder(std::memory_order_consume), __ATOMIC_SEQ_CST);
+}
+
+TEST(NcclUtilityMicrotest, AcquireOrderOfCuda_DowngradesReleaseAndAcqRelPassesEverythingElseThrough) {
+  EXPECT_EQ(acquireOrderOf(cuda::memory_order_release), cuda::memory_order_relaxed);
+  EXPECT_EQ(acquireOrderOf(cuda::memory_order_acq_rel), cuda::memory_order_acquire);
+  EXPECT_EQ(acquireOrderOf(cuda::memory_order_relaxed), cuda::memory_order_relaxed);
+  EXPECT_EQ(acquireOrderOf(cuda::memory_order_acquire), cuda::memory_order_acquire);
+  EXPECT_EQ(acquireOrderOf(cuda::memory_order_seq_cst), cuda::memory_order_seq_cst);
+}
+
+TEST(NcclUtilityMicrotest, ReleaseOrderOfCuda_DowngradesAcquireAndAcqRelPassesEverythingElseThrough) {
+  EXPECT_EQ(releaseOrderOf(cuda::memory_order_acquire), cuda::memory_order_relaxed);
+  EXPECT_EQ(releaseOrderOf(cuda::memory_order_acq_rel), cuda::memory_order_release);
+  EXPECT_EQ(releaseOrderOf(cuda::memory_order_relaxed), cuda::memory_order_relaxed);
+  EXPECT_EQ(releaseOrderOf(cuda::memory_order_release), cuda::memory_order_release);
+  EXPECT_EQ(releaseOrderOf(cuda::memory_order_seq_cst), cuda::memory_order_seq_cst);
+}
+
+TEST(NcclUtilityMicrotest, ToCudaOrder_MapsEveryStdOrderAndFallsBackToSeqCstForConsume) {
+  EXPECT_EQ(toCudaOrder(std::memory_order_relaxed), cuda::memory_order_relaxed);
+  EXPECT_EQ(toCudaOrder(std::memory_order_acquire), cuda::memory_order_acquire);
+  EXPECT_EQ(toCudaOrder(std::memory_order_release), cuda::memory_order_release);
+  EXPECT_EQ(toCudaOrder(std::memory_order_acq_rel), cuda::memory_order_acq_rel);
+  EXPECT_EQ(toCudaOrder(std::memory_order_seq_cst), cuda::memory_order_seq_cst);
+  EXPECT_EQ(toCudaOrder(std::memory_order_consume), cuda::memory_order_seq_cst);
+}
+
+TEST(NcclUtilityMicrotest, Present_GetByIndexReturnsEachArgumentInDeclarationOrder) {
+  // Lvalues, so p holds references to a, b, c rather than to dangling temporaries.
+  int a = 10, b = 20, c = 30;
+  auto p = present(a, b, c);
+  EXPECT_EQ(p.get(nccl::utility::IntSeq<0>{}), 10);
+  EXPECT_EQ(p.get(nccl::utility::IntSeq<1>{}), 20);
+  EXPECT_EQ(p.get(nccl::utility::IntSeq<2>{}), 30);
+}