#ifndef _NTINTSAFE_H_INCLUDED_
#if WINAPI_FAMILY_PARTITION(WINAPI_PARTITION_APP | WINAPI_PARTITION_SYSTEM | WINAPI_PARTITION_GAMES)
#if defined(_USE_INTRINSIC_MULTIPLY128)
#if defined(_M_X64) && !defined(_M_ARM64EC)

#define __RtlpUnsignedMultiply128 _umul128

#endif
#endif
#endif
#endif
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#ifndef _NTINTSAFE_H_INCLUDED_
#if WINAPI_FAMILY_PARTITION(WINAPI_PARTITION_APP | WINAPI_PARTITION_SYSTEM | WINAPI_PARTITION_GAMES)
#if defined(_USE_INTRINSIC_MULTIPLY128)

ULONG64
__RtlpUnsignedMultiply128(
    _In_ ULONG64 Multiplicand,
    _In_ ULONG64 Multiplier,
    _Out_ _Deref_out_range_(==, Multiplicand * Multiplier) ULONG64* HighProduct);

#endif
#endif
#endif
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#ifndef _NTINTSAFE_H_INCLUDED_
#if WINAPI_FAMILY_PARTITION(WINAPI_PARTITION_APP | WINAPI_PARTITION_SYSTEM | WINAPI_PARTITION_GAMES)
#if defined(_USE_INTRINSIC_MULTIPLY128)
#if defined(_M_ARM64) || defined(_M_ARM64EC) || defined(_M_HYBRID_X86_ARM64)

__inline
ULONG64
__RtlpUnsignedMultiply128(
    _In_ ULONG64 Multiplicand,
    _In_ ULONG64 Multiplier,
    _Out_ _Deref_out_range_(==, Multiplicand * Multiplier) ULONG64* HighProduct)
{
    *HighProduct = __umulh(Multiplier, Multiplicand);
    return Multiplier * Multiplicand;
}

#endif
#endif
#endif
#endif
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// ntintsafe.h

ULONG64 __RtlpUnsignedMultiply128(
  ULONG64 Multiplicand,
  ULONG64 Multiplier,
  ULONG64 *HighProduct
);
View the official Windows Driver Kit DDI reference