BigNumWordMul64.c

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#include "../BigNumLib.h"
 
OC_BN_WORD
BigNumWordMul64 (
  OUT OC_BN_WORD  *Hi,
  IN  OC_BN_WORD  A,
  IN  OC_BN_WORD  B
  )
{
  ASSERT (OC_BN_WORD_SIZE == sizeof (UINT64));
  ASSERT (Hi != NULL);
  //
  // This is to be used when the used compiler lacks support for both the
  // __int128 type and a suiting intrinsic to perform the calculation.
  //
  // Source: https://stackoverflow.com/a/31662911
  //
  CONST OC_BN_WORD  SubWordShift = OC_BN_WORD_NUM_BITS / 2;
  CONST OC_BN_WORD  SubWordMask  = ((OC_BN_WORD)1U << SubWordShift) - 1;
 
  OC_BN_WORD  ALo;
  OC_BN_WORD  AHi;
  OC_BN_WORD  BLo;
  OC_BN_WORD  BHi;
 
  OC_BN_WORD  P0;
  OC_BN_WORD  P1;
  OC_BN_WORD  P2;
  OC_BN_WORD  P3;
 
  OC_BN_WORD  Cy;
 
  ALo = A & SubWordMask;
  AHi = A >> SubWordShift;
  BLo = B & SubWordMask;
  BHi = B >> SubWordShift;
 
  P0 = ALo * BLo;
  P1 = ALo * BHi;
  P2 = AHi * BLo;
  P3 = AHi * BHi;
 
  Cy = (((P0 >> SubWordShift) + (P1 & SubWordMask) + (P2 & SubWordMask)) >> SubWordShift) & SubWordMask;
 
  *Hi = P3 + (P1 >> SubWordShift) + (P2 >> SubWordShift) + Cy;
  return P0 + (P1 << SubWordShift) + (P2 << SubWordShift);
}