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Simplify bn_mul_part_recursive.

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The loop and the outermost special-cases are basically the same.

Change-Id: I5e3ca60ad9a04efa66b479eebf8c3637a11cdceb
Reviewed-on: https://boringssl-review.googlesource.com/25406
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>
This commit is contained in:
David Benjamin 2018-01-26 14:12:18 -05:00 committed by CQ bot account: commit-bot@chromium.org
parent 6488f4e2ba
commit ac383701b7
1 changed files with 24 additions and 40 deletions
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38
crypto/fipsmodule/bn/mul.c
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@ -465,48 +465,32 @@ static void bn_mul_part_recursive(BN_ULONG *r, const BN_ULONG *a,
bn_mul_recursive(&t[n2], t, &t[n], n, 0, 0, p);
bn_mul_recursive(r, a, b, n, 0, 0, p);
int i = n / 2, j;
if (tna > tnb) {
j = tna - i;
} else {
j = tnb - i;
}
if (j == 0) {
// If there is only a bottom half to the number, just do it. We know the
// larger of |tna - i| and |tnb - i| is zero. The other is zero or -1
// because |tna| and |tnb| differ by at most one.
bn_mul_recursive(&r[n2], &a[n], &b[n], i, tna - i, tnb - i, p);
// |bn_mul_recursive| only writes the bottom |i|*2 words.
OPENSSL_memset(&r[n2 + i * 2], 0, sizeof(BN_ULONG) * (n2 - i * 2));
} else if (j > 0) {
// E.g,, n == 16, i == 8 and tna == 11.
// |tna| and |tnb| are within one of each other, so if |tna| is larger and
// tna > i, then we know tnb >= i, and this call is valid.
bn_mul_part_recursive(&r[n2], &a[n], &b[n], i, tna - i, tnb - i, p);
} else {
// (j < 0) E.g., n == 16, i == 8 and tn == 5
OPENSSL_memset(&r[n2], 0, sizeof(BN_ULONG) * n2);
if (tna < BN_MUL_RECURSIVE_SIZE_NORMAL &&
tnb < BN_MUL_RECURSIVE_SIZE_NORMAL) {
bn_mul_normal(&r[n2], &a[n], tna, &b[n], tnb);
} else {
int i = n;
for (;;) {
i /= 2;
// These simplified conditions work exclusively because difference
// between |tna| and |tnb| is 1 or 0.
//
// TODO(davidben): This loop condition is exactly the same as the
// |j > 0| one but more complicated. Merge them.
if (i < tna || i < tnb) {
// E.g., n == 16, i == 8 and tna == 11. |tna| and |tnb| are within one
// of each other, so if |tna| is larger and tna > i, then we know
// tnb >= i, and this call is valid.
bn_mul_part_recursive(&r[n2], &a[n], &b[n], i, tna - i, tnb - i, p);
break;
}
if (i == tna || i == tnb) {
// If there is only a bottom half to the number, just do it. We know
// the larger of |tna - i| and |tnb - i| is zero. The other is zero or
// -1 by because of |tna| and |tnb| differ by at most one.
bn_mul_recursive(&r[n2], &a[n], &b[n], i, tna - i, tnb - i, p);
break;
}
}
// This loop will eventually terminate when |i| falls below
// |BN_MUL_RECURSIVE_SIZE_NORMAL| because we know one of |tna| and |tnb|
// exceeds that.
}
}
}