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Implemented RtlExtendedMagicDivide(). Patch by Marc Schuetz.

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svn path=/trunk/; revision=4814
This commit is contained in:
Eric Kohl 2003-06-01 18:13:23 +00:00
parent 8eb47000b7
commit b036cea0eb
1 changed files with 70 additions and 10 deletions
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72
reactos/ntoskrnl/rtl/largeint.c
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@ -1,4 +1,4 @@
/* $Id: largeint.c,v 1.13 2002/12/08 16:23:32 robd Exp $
/* $Id: largeint.c,v 1.14 2003/06/01 18:13:23 ekohl Exp $
*
* COPYRIGHT: See COPYING in the top level directory
* PROJECT: ReactOS kernel
@ -119,17 +119,77 @@ RtlExtendedLargeIntegerDivide (
return RC;
}
LARGE_INTEGER
STDCALL
/******************************************************************************
* RtlExtendedMagicDivide
*
* Allows replacing a division by a longlong constant with a multiplication by
* the inverse constant.
*
* RETURNS
* (Dividend * MagicDivisor) >> (64 + ShiftCount)
*
* NOTES
* If the divisor of a division is constant, the constants MagicDivisor and
* shift must be chosen such that
* MagicDivisor = 2^(64 + ShiftCount) / Divisor.
*
* Then we have RtlExtendedMagicDivide(Dividend,MagicDivisor,ShiftCount) ==
* Dividend * MagicDivisor / 2^(64 + ShiftCount) == Dividend / Divisor.
*
* The Parameter MagicDivisor although defined as LONGLONG is used as
* ULONGLONG.
*/
#define LOWER_32(A) ((A) & 0xffffffff)
#define UPPER_32(A) ((A) >> 32)
LARGE_INTEGER STDCALL
RtlExtendedMagicDivide (LARGE_INTEGER Dividend,
LARGE_INTEGER MagicDivisor,
CCHAR ShiftCount)
{
LARGE_INTEGER Result;
ULONGLONG dividend_high;
ULONGLONG dividend_low;
ULONGLONG inverse_divisor_high;
ULONGLONG inverse_divisor_low;
ULONGLONG ah_bl;
ULONGLONG al_bh;
LARGE_INTEGER result;
BOOLEAN positive;
Result.QuadPart = (Dividend.QuadPart * MagicDivisor.QuadPart) >> ShiftCount;
return(Result);
if (Dividend.QuadPart < 0)
{
dividend_high = UPPER_32((ULONGLONG) -Dividend.QuadPart);
dividend_low = LOWER_32((ULONGLONG) -Dividend.QuadPart);
positive = FALSE;
}
else
{
dividend_high = UPPER_32((ULONGLONG) Dividend.QuadPart);
dividend_low = LOWER_32((ULONGLONG) Dividend.QuadPart);
positive = TRUE;
}
inverse_divisor_high = UPPER_32((ULONGLONG) MagicDivisor.QuadPart);
inverse_divisor_low = LOWER_32((ULONGLONG) MagicDivisor.QuadPart);
ah_bl = dividend_high * inverse_divisor_low;
al_bh = dividend_low * inverse_divisor_high;
result.QuadPart =
(LONGLONG) ((dividend_high * inverse_divisor_high +
UPPER_32(ah_bl) +
UPPER_32(al_bh) +
UPPER_32(LOWER_32(ah_bl) + LOWER_32(al_bh) +
UPPER_32(dividend_low * inverse_divisor_low))) >> ShiftCount);
if (!positive)
{
result.QuadPart = -result.QuadPart;
}
return result;
}
LARGE_INTEGER
STDCALL