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From: Paolo Bonzini <pbonzini@redhat.com>
To: kvm@vger.kernel.org
Cc: Alexandru Elisei <alexandru.elisei@arm.com>
Subject: [PATCH v2 kvm-unit-tests 1/2] libcflat: clean up and complete long division routines
Date: Wed, 12 May 2021 12:54:39 +0200	[thread overview]
Message-ID: <20210512105440.748153-2-pbonzini@redhat.com> (raw)
In-Reply-To: <20210512105440.748153-1-pbonzini@redhat.com>

Avoid possible uninitialized variables on machines where
division by zero does not trap.  Add __divmoddi4, and
use it in __moddi3 and __divdi3.

Reported-by: Alexandru Elisei <alexandru.elisei@arm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 lib/ldiv32.c | 40 ++++++++++++++++++++++++----------------
 1 file changed, 24 insertions(+), 16 deletions(-)

diff --git a/lib/ldiv32.c b/lib/ldiv32.c
index 96f4b35..897a4b9 100644
--- a/lib/ldiv32.c
+++ b/lib/ldiv32.c
@@ -1,6 +1,7 @@
 #include <stdint.h>
 
 extern uint64_t __udivmoddi4(uint64_t num, uint64_t den, uint64_t *p_rem);
+extern int64_t __divmoddi4(int64_t num, int64_t den, int64_t *p_rem);
 extern int64_t __moddi3(int64_t num, int64_t den);
 extern int64_t __divdi3(int64_t num, int64_t den);
 extern uint64_t __udivdi3(uint64_t num, uint64_t den);
@@ -11,8 +12,11 @@ uint64_t __udivmoddi4(uint64_t num, uint64_t den, uint64_t *p_rem)
 	uint64_t quot = 0;
 
 	/* Trigger a division by zero at run time (trick taken from iPXE).  */
-	if (den == 0)
+	if (den == 0) {
+		if (p_rem)
+			*p_rem = 0;
 		return 1/((unsigned)den);
+	}
 
 	if (num >= den) {
 		/* Align den to num to avoid wasting time on leftmost zero bits.  */
@@ -35,31 +39,35 @@ uint64_t __udivmoddi4(uint64_t num, uint64_t den, uint64_t *p_rem)
 	return quot;
 }
 
-int64_t __moddi3(int64_t num, int64_t den)
+int64_t __divmoddi4(int64_t num, int64_t den, int64_t *p_rem)
 {
-	uint64_t mask = num < 0 ? -1 : 0;
+	int32_t nmask = num < 0 ? -1 : 0;
+	int32_t qmask = (num ^ den) < 0 ? -1 : 0;
+	uint64_t quot;
 
 	/* Compute absolute values and do an unsigned division.  */
-	num = (num + mask) ^ mask;
+	num = (num + nmask) ^ nmask;
 	if (den < 0)
 		den = -den;
 
-	/* Copy sign of num into result.  */
-	return (__umoddi3(num, den) + mask) ^ mask;
+	/* Copy sign of num^den into quotient, sign of num into remainder.  */
+	quot = (__udivmoddi4(num, den, (uint64_t *)p_rem) + qmask) ^ qmask;
+	if (p_rem)
+		*p_rem = (*p_rem + nmask) ^ nmask;
+	return quot;
 }
 
-int64_t __divdi3(int64_t num, int64_t den)
+int64_t __moddi3(int64_t num, int64_t den)
 {
-	uint64_t mask = (num ^ den) < 0 ? -1 : 0;
-
-	/* Compute absolute values and do an unsigned division.  */
-	if (num < 0)
-		num = -num;
-	if (den < 0)
-		den = -den;
+	int64_t rem;
+	__divmoddi4(num, den, &rem);
+	return rem;
+}
 
-	/* Copy sign of num^den into result.  */
-	return (__udivdi3(num, den) + mask) ^ mask;
+int64_t __divdi3(int64_t num, int64_t den)
+{
+	int64_t rem;
+	return __divmoddi4(num, den, &rem);
 }
 
 uint64_t __udivdi3(uint64_t num, uint64_t den)
-- 
2.31.1



  reply	other threads:[~2021-05-12 10:54 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-05-12 10:54 [PATCH v2 kvm-unit-tests 0/2] fix long division routines for ARM eabi Paolo Bonzini
2021-05-12 10:54 ` Paolo Bonzini [this message]
2021-05-12 13:44   ` [PATCH v2 kvm-unit-tests 1/2] libcflat: clean up and complete long division routines Alexandru Elisei
2021-05-12 10:54 ` [PATCH v2 kvm-unit-tests 2/2] arm: add eabi version of 64-bit division functions Paolo Bonzini
2021-05-12 13:44   ` Alexandru Elisei
2021-05-12 13:51     ` Paolo Bonzini
2021-05-12 14:04       ` Alexandru Elisei
2021-05-12 14:04 ` [PATCH v2 kvm-unit-tests 0/2] fix long division routines for ARM eabi Alexandru Elisei

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