1 files changed, 83 insertions, 0 deletions
diff --git a/src/crypto/internal/boring/div_test.c b/src/crypto/internal/boring/div_test.c
new file mode 100644
index 0000000..f909cc9
--- /dev/null
+++ b/src/crypto/internal/boring/div_test.c
@@ -0,0 +1,83 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// This file is a self-contained test for a copy of
+// the division algorithm in build-goboring.sh,
+// to verify that is correct. The real algorithm uses u128
+// but this copy uses u32 for easier testing.
+// s/32/128/g should be the only difference between the two.
+//
+// This is the dumbest possible division algorithm,
+// but any crypto code that depends on the speed of
+// division is equally dumb.
+
+//go:build ignore
+
+#include <stdio.h>
+#include <stdint.h>
+
+#define nelem(x) (sizeof(x)/sizeof((x)[0]))
+
+typedef uint32_t u32;
+
+static u32 div(u32 x, u32 y, u32 *rp) {
+	int n = 0;
+	while((y>>(32-1)) != 1 && y < x) {
+		y<<=1;
+		n++;
+	}
+	u32 q = 0;
+	for(;; n--, y>>=1, q<<=1) {
+		if(x>=y) {
+			x -= y;
+			q |= 1;
+		}
+		if(n == 0)
+			break;
+	}
+	if(rp)
+		*rp = x;
+	return q;
+}
+
+u32 tests[] = {
+	0,
+	1,
+	2,
+	3,
+	4,
+	5,
+	6,
+	7,
+	8,
+	9,
+	10,
+	11,
+	31,
+	0xFFF,
+	0x1000,
+	0x1001,
+	0xF0F0F0,
+	0xFFFFFF,
+	0x1000000,
+	0xF0F0F0F0,
+	0xFFFFFFFF,
+};
+
+int
+main(void)
+{
+	for(int i=0; i<nelem(tests); i++)
+	for(int j=0; j<nelem(tests); j++) {
+		u32 n = tests[i];
+		u32 d = tests[j];
+		if(d == 0)
+			continue;
+		u32 r;
+		u32 q = div(n, d, &r);
+		if(q != n/d || r != n%d)
+			printf("div(%x, %x) = %x, %x, want %x, %x\n", n, d, q, r, n/d, n%d);
+	}
+	return 0;
+}