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switch to non-branching code in AlmostInverse()

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weidai 2010-06-28 22:42:31 +00:00
parent a8b398e042
commit 1435784dee
1 changed files with 23 additions and 35 deletions
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54
integer.cpp
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@ -2633,7 +2633,8 @@ unsigned int AlmostInverse(word *R, word *T, const word *A, size_t NA, const wor
word *f = T+2*N;
word *g = T+3*N;
size_t bcLen=2, fgLen=EvenWordCount(M, N);
unsigned int k=0, s=0;
unsigned int k=0;
bool s=false;
SetWords(T, 0, 3*N);
b[0]=1;
@ -2652,58 +2653,45 @@ unsigned int AlmostInverse(word *R, word *T, const word *A, size_t NA, const wor
}
ShiftWordsRightByWords(f, fgLen, 1);
if (c[bcLen-1]) bcLen+=2;
bcLen += 2 * (c[bcLen-1] != 0);
assert(bcLen <= N);
ShiftWordsLeftByWords(c, bcLen, 1);
k+=WORD_BITS;
t=f[0];
}
unsigned int i=0;
while (t%2 == 0)
{
t>>=1;
i++;
}
k+=i;
unsigned int i = TrailingZeros(t);
t >>= i;
k += i;
if (t==1 && f[1]==0 && EvenWordCount(f, fgLen)==2)
if (t==1 && f[1]==0 && EvenWordCount(f+2, fgLen-2)==0)
{
if (s%2==0)
CopyWords(R, b, N);
else
if (s)
Subtract(R, M, b, N);
else
CopyWords(R, b, N);
return k;
}
ShiftWordsRightByBits(f, fgLen, i);
t=ShiftWordsLeftByBits(c, bcLen, i);
if (t)
{
c[bcLen] = t;
bcLen+=2;
t = ShiftWordsLeftByBits(c, bcLen, i);
c[bcLen] += t;
bcLen += 2 * (t!=0);
assert(bcLen <= N);
}
if (f[fgLen-2]==0 && g[fgLen-2]==0 && f[fgLen-1]==0 && g[fgLen-1]==0)
fgLen-=2;
bool swap = Compare(f, g, fgLen)==-1;
ConditionalSwapPointers(swap, f, g);
ConditionalSwapPointers(swap, b, c);
s ^= swap;
if (Compare(f, g, fgLen)==-1)
{
std::swap(f, g);
std::swap(b, c);
s++;
}
fgLen -= 2 * !(f[fgLen-2] | f[fgLen-1]);
Subtract(f, f, g, fgLen);
if (Add(b, b, c, bcLen))
{
b[bcLen] = 1;
bcLen+=2;
t = Add(b, b, c, bcLen);
b[bcLen] += t;
bcLen += 2*t;
assert(bcLen <= N);
}
}
}
// R[N] - result = A/(2^k) mod M