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diff --git a/third_party/bearssl/src/i31_muladd.c b/third_party/bearssl/src/i31_muladd.c
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+++ b/third_party/bearssl/src/i31_muladd.c
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+/*
+ * Copyright (c) 2016 Thomas Pornin <[email protected]>
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining 
+ * a copy of this software and associated documentation files (the
+ * "Software"), to deal in the Software without restriction, including
+ * without limitation the rights to use, copy, modify, merge, publish,
+ * distribute, sublicense, and/or sell copies of the Software, and to
+ * permit persons to whom the Software is furnished to do so, subject to
+ * the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be 
+ * included in all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, 
+ * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+ * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND 
+ * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
+ * BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
+ * ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
+ * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+#include "inner.h"
+
+/* see inner.h */
+void
+br_i31_muladd_small(uint32_t *x, uint32_t z, const uint32_t *m)
+{
+	uint32_t m_bitlen;
+	unsigned mblr;
+	size_t u, mlen;
+	uint32_t a0, a1, b0, hi, g, q, tb;
+	uint32_t under, over;
+	uint32_t cc;
+
+	/*
+	 * We can test on the modulus bit length since we accept to
+	 * leak that length.
+	 */
+	m_bitlen = m[0];
+	if (m_bitlen == 0) {
+		return;
+	}
+	if (m_bitlen <= 31) {
+		uint32_t lo;
+
+		hi = x[1] >> 1;
+		lo = (x[1] << 31) | z;
+		x[1] = br_rem(hi, lo, m[1]);
+		return;
+	}
+	mlen = (m_bitlen + 31) >> 5;
+	mblr = (unsigned)m_bitlen & 31;
+
+	/*
+	 * Principle: we estimate the quotient (x*2^31+z)/m by
+	 * doing a 64/32 division with the high words.
+	 *
+	 * Let:
+	 *   w = 2^31
+	 *   a = (w*a0 + a1) * w^N + a2
+	 *   b = b0 * w^N + b2
+	 * such that:
+	 *   0 <= a0 < w
+	 *   0 <= a1 < w
+	 *   0 <= a2 < w^N
+	 *   w/2 <= b0 < w
+	 *   0 <= b2 < w^N
+	 *   a < w*b
+	 * I.e. the two top words of a are a0:a1, the top word of b is
+	 * b0, we ensured that b0 is "full" (high bit set), and a is
+	 * such that the quotient q = a/b fits on one word (0 <= q < w).
+	 *
+	 * If a = b*q + r (with 0 <= r < q), we can estimate q by
+	 * doing an Euclidean division on the top words:
+	 *   a0*w+a1 = b0*u + v  (with 0 <= v < b0)
+	 * Then the following holds:
+	 *   0 <= u <= w
+	 *   u-2 <= q <= u
+	 */
+	hi = x[mlen];
+	if (mblr == 0) {
+		a0 = x[mlen];
+		memmove(x + 2, x + 1, (mlen - 1) * sizeof *x);
+		x[1] = z;
+		a1 = x[mlen];
+		b0 = m[mlen];
+	} else {
+		a0 = ((x[mlen] << (31 - mblr)) | (x[mlen - 1] >> mblr))
+			& 0x7FFFFFFF;
+		memmove(x + 2, x + 1, (mlen - 1) * sizeof *x);
+		x[1] = z;
+		a1 = ((x[mlen] << (31 - mblr)) | (x[mlen - 1] >> mblr))
+			& 0x7FFFFFFF;
+		b0 = ((m[mlen] << (31 - mblr)) | (m[mlen - 1] >> mblr))
+			& 0x7FFFFFFF;
+	}
+
+	/*
+	 * We estimate a divisor q. If the quotient returned by br_div()
+	 * is g:
+	 * -- If a0 == b0 then g == 0; we want q = 0x7FFFFFFF.
+	 * -- Otherwise:
+	 *    -- if g == 0 then we set q = 0;
+	 *    -- otherwise, we set q = g - 1.
+	 * The properties described above then ensure that the true
+	 * quotient is q-1, q or q+1.
+	 *
+	 * Take care that a0, a1 and b0 are 31-bit words, not 32-bit. We
+	 * must adjust the parameters to br_div() accordingly.
+	 */
+	g = br_div(a0 >> 1, a1 | (a0 << 31), b0);
+	q = MUX(EQ(a0, b0), 0x7FFFFFFF, MUX(EQ(g, 0), 0, g - 1));
+
+	/*
+	 * We subtract q*m from x (with the extra high word of value 'hi').
+	 * Since q may be off by 1 (in either direction), we may have to
+	 * add or subtract m afterwards.
+	 *
+	 * The 'tb' flag will be true (1) at the end of the loop if the
+	 * result is greater than or equal to the modulus (not counting
+	 * 'hi' or the carry).
+	 */
+	cc = 0;
+	tb = 1;
+	for (u = 1; u <= mlen; u ++) {
+		uint32_t mw, zw, xw, nxw;
+		uint64_t zl;
+
+		mw = m[u];
+		zl = MUL31(mw, q) + cc;
+		cc = (uint32_t)(zl >> 31);
+		zw = (uint32_t)zl & (uint32_t)0x7FFFFFFF;
+		xw = x[u];
+		nxw = xw - zw;
+		cc += nxw >> 31;
+		nxw &= 0x7FFFFFFF;
+		x[u] = nxw;
+		tb = MUX(EQ(nxw, mw), tb, GT(nxw, mw));
+	}
+
+	/*
+	 * If we underestimated q, then either cc < hi (one extra bit
+	 * beyond the top array word), or cc == hi and tb is true (no
+	 * extra bit, but the result is not lower than the modulus). In
+	 * these cases we must subtract m once.
+	 *
+	 * Otherwise, we may have overestimated, which will show as
+	 * cc > hi (thus a negative result). Correction is adding m once.
+	 */
+	over = GT(cc, hi);
+	under = ~over & (tb | LT(cc, hi));
+	br_i31_add(x, m, over);
+	br_i31_sub(x, m, under);
+}