[PATCH kvm-unit-tests 0/2] fix long division routines for ARM eabi

[PATCH kvm-unit-tests 0/2] fix long division routines for ARM eabi

[Paolo Bonzini]

As reported by Alexandru, ARM follows a different convention than
x86 so it needs __aeabi_ldivmod and __aeabi_uldivmod.  Because
it does not use __divdi3 and __moddi3, it also needs __divmoddi4
to build the eabi function upon.

Paolo

Paolo Bonzini (2):
  libcflat: clean up and complete long division routines
  arm: add eabi version of 64-bit division functions

 arm/Makefile.arm  |  1 +
 lib/arm/ldivmod.S | 32 ++++++++++++++++++++++++++++++++
 lib/ldiv32.c      | 28 +++++++++++++++++++++++++---
 3 files changed, 58 insertions(+), 3 deletions(-)
 create mode 100644 lib/arm/ldivmod.S

-- 
2.31.1

[PATCH kvm-unit-tests 1/2] libcflat: clean up and complete long division routines

[Paolo Bonzini]

Avoid possible uninitialized variables on machines where
division by zero does not trap.  Add __divmoddi4, and
do not use 64-bit math unnecessarily in __moddi3 and __divdi3.

Reported-by: Alexandru Elisei <alexandru.elisei@arm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 lib/ldiv32.c | 28 +++++++++++++++++++++++++---
 1 file changed, 25 insertions(+), 3 deletions(-)

diff --git a/lib/ldiv32.c b/lib/ldiv32.c
index 96f4b35..c39fccd 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.  */
@@ -35,9 +39,27 @@ uint64_t __udivmoddi4(uint64_t num, uint64_t den, uint64_t *p_rem)
 	return quot;
 }
 
+int64_t __divmoddi4(int64_t num, int64_t den, int64_t *p_rem)
+{
+	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 + nmask) ^ nmask;
+	if (den < 0)
+		den = -den;
+
+	/* Copy sign of num^den into quotient, sign of num into remainder.  */
+	quot = (__divmoddi4(num, den, p_rem) + qmask) ^ qmask;
+	if (p_rem)
+		*p_rem = (*p_rem + nmask) ^ nmask;
+	return quot;
+}
+
 int64_t __moddi3(int64_t num, int64_t den)
 {
-	uint64_t mask = num < 0 ? -1 : 0;
+	int32_t mask = num < 0 ? -1 : 0;
 
 	/* Compute absolute values and do an unsigned division.  */
 	num = (num + mask) ^ mask;
@@ -50,7 +72,7 @@ int64_t __moddi3(int64_t num, int64_t den)
 
 int64_t __divdi3(int64_t num, int64_t den)
 {
-	uint64_t mask = (num ^ den) < 0 ? -1 : 0;
+	int32_t mask = (num ^ den) < 0 ? -1 : 0;
 
 	/* Compute absolute values and do an unsigned division.  */
 	if (num < 0)
-- 
2.31.1

Re: [PATCH kvm-unit-tests 1/2] libcflat: clean up and complete long division routines

[Alexandru Elisei]

Hi Paolo,

Thanks for sending this so quickly!

On 5/11/21 6:41 PM, Paolo Bonzini wrote:
> Avoid possible uninitialized variables on machines where
> division by zero does not trap.  Add __divmoddi4, and

According to the ARM Architecture Reference Manual for ARMv7-A (ARM DDI 0406C.d),
hardware floating point support is optional (page A2-54), so initializing the
remainder to zero in the case of zero division makes sense.

> do not use 64-bit math unnecessarily in __moddi3 and __divdi3.
>
> Reported-by: Alexandru Elisei <alexandru.elisei@arm.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  lib/ldiv32.c | 28 +++++++++++++++++++++++++---
>  1 file changed, 25 insertions(+), 3 deletions(-)
>
> diff --git a/lib/ldiv32.c b/lib/ldiv32.c
> index 96f4b35..c39fccd 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.  */
> @@ -35,9 +39,27 @@ uint64_t __udivmoddi4(uint64_t num, uint64_t den, uint64_t *p_rem)
>  	return quot;
>  }
>  
> +int64_t __divmoddi4(int64_t num, int64_t den, int64_t *p_rem)
> +{
> +	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 + nmask) ^ nmask;
> +	if (den < 0)
> +		den = -den;
> +
> +	/* Copy sign of num^den into quotient, sign of num into remainder.  */
> +	quot = (__divmoddi4(num, den, p_rem) + qmask) ^ qmask;

I see no early return statement in the function, it looks to me like the function
will recurse forever. Maybe you wanted to call here __*u*divmoddi4() (emphasis
added) instead?

Other than that, the function looks correct.

Thanks,

Alex

> +	if (p_rem)
> +		*p_rem = (*p_rem + nmask) ^ nmask;
> +	return quot;
> +}
> +
>  int64_t __moddi3(int64_t num, int64_t den)
>  {
> -	uint64_t mask = num < 0 ? -1 : 0;
> +	int32_t mask = num < 0 ? -1 : 0;
>  
>  	/* Compute absolute values and do an unsigned division.  */
>  	num = (num + mask) ^ mask;
> @@ -50,7 +72,7 @@ int64_t __moddi3(int64_t num, int64_t den)
>  
>  int64_t __divdi3(int64_t num, int64_t den)
>  {
> -	uint64_t mask = (num ^ den) < 0 ? -1 : 0;
> +	int32_t mask = (num ^ den) < 0 ? -1 : 0;
>  
>  	/* Compute absolute values and do an unsigned division.  */
>  	if (num < 0)

Re: [PATCH kvm-unit-tests 1/2] libcflat: clean up and complete long division routines

[Paolo Bonzini]

On 12/05/21 12:39, Alexandru Elisei wrote:
>> +
>> +	/* Copy sign of num^den into quotient, sign of num into remainder.  */
>> +	quot = (__divmoddi4(num, den, p_rem) + qmask) ^ qmask;
> I see no early return statement in the function, it looks to me like the function
> will recurse forever. Maybe you wanted to call here __*u*divmoddi4() (emphasis
> added) instead?

Of course...

Paolo

[PATCH kvm-unit-tests 2/2] arm: add eabi version of 64-bit division functions

[Paolo Bonzini]

eabi prescribes different entry points for 64-bit division on
32-bit platforms.  Implement a wrapper for the GCC-style __divmoddi4
and __udivmoddi4 functions.

Reported-by: Alexandru Elisei <alexandru.elisei@arm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 arm/Makefile.arm  |  1 +
 lib/arm/ldivmod.S | 32 ++++++++++++++++++++++++++++++++
 2 files changed, 33 insertions(+)
 create mode 100644 lib/arm/ldivmod.S

diff --git a/arm/Makefile.arm b/arm/Makefile.arm
index 687a8ed..3a4cc6b 100644
--- a/arm/Makefile.arm
+++ b/arm/Makefile.arm
@@ -24,6 +24,7 @@ cflatobjs += lib/arm/spinlock.o
 cflatobjs += lib/arm/processor.o
 cflatobjs += lib/arm/stack.o
 cflatobjs += lib/ldiv32.o
+cflatobjs += lib/arm/ldivmod.o
 
 # arm specific tests
 tests =
diff --git a/lib/arm/ldivmod.S b/lib/arm/ldivmod.S
new file mode 100644
index 0000000..de11ac9
--- /dev/null
+++ b/lib/arm/ldivmod.S
@@ -0,0 +1,32 @@
+// EABI ldivmod and uldivmod implementation based on libcompiler-rt
+//
+// This file is dual licensed under the MIT and the University of Illinois Open
+// Source Licenses.
+
+	.syntax unified
+	.align 2
+	.globl __aeabi_uldivmod
+	.type __aeabi_uldivmod, %function
+__aeabi_uldivmod:
+	push	{r11, lr}
+	sub	sp, sp, #16
+	add	r12, sp, #8
+	str	r12, [sp]                // third argument to __udivmoddi4
+	bl	__udivmoddi4
+	ldr	r2, [sp, #8]             // remainder returned in r2-r3
+	ldr	r3, [sp, #12]
+	add	sp, sp, #16
+	pop	{r11, pc}
+
+	.globl __aeabi_ldivmod
+	.type __aeabi_ldivmod, %function
+__aeabi_ldivmod:
+	push	{r11, lr}
+	sub	sp, sp, #16
+	add	r12, sp, #8
+	str	r12, [sp]                // third argument to __divmoddi4
+	bl	__divmoddi4
+	ldr	r2, [sp, #8]             // remainder returned in r2-r3
+	ldr	r3, [sp, #12]
+	add	sp, sp, #16
+	pop	{r11, pc}
-- 
2.31.1