* [Qemu-devel] [PATCH] target/ppc: simplify bcdadd/sub functions
@ 2018-07-24 12:13 Yasmin Beatriz
2018-07-26 1:40 ` David Gibson
0 siblings, 1 reply; 4+ messages in thread
From: Yasmin Beatriz @ 2018-07-24 12:13 UTC (permalink / raw)
To: qemu-ppc; +Cc: qemu-devel, david, rth
After solving a corner case in bcdsub, this patch simplifies the logic
of both bcdadd/sub instructions by removing some unnecessary local flags.
Signed-off-by: Yasmin Beatriz <yasmins@linux.ibm.com>
---
target/ppc/int_helper.c | 33 +++++++++------------------------
1 file changed, 9 insertions(+), 24 deletions(-)
diff --git a/target/ppc/int_helper.c b/target/ppc/int_helper.c
index fa18e6e..b8ac4bb 100644
--- a/target/ppc/int_helper.c
+++ b/target/ppc/int_helper.c
@@ -2671,16 +2671,14 @@ static int bcd_cmp_mag(ppc_avr_t *a, ppc_avr_t *b)
return 0;
}
-static int bcd_add_mag(ppc_avr_t *t, ppc_avr_t *a, ppc_avr_t *b, int *invalid,
+static void bcd_add_mag(ppc_avr_t *t, ppc_avr_t *a, ppc_avr_t *b, int *invalid,
int *overflow)
{
int carry = 0;
int i;
- int is_zero = 1;
for (i = 1; i <= 31; i++) {
uint8_t digit = bcd_get_digit(a, i, invalid) +
bcd_get_digit(b, i, invalid) + carry;
- is_zero &= (digit == 0);
if (digit > 9) {
carry = 1;
digit -= 10;
@@ -2689,26 +2687,20 @@ static int bcd_add_mag(ppc_avr_t *t, ppc_avr_t *a, ppc_avr_t *b, int *invalid,
}
bcd_put_digit(t, digit, i);
-
- if (unlikely(*invalid)) {
- return -1;
- }
}
*overflow = carry;
- return is_zero;
}
-static int bcd_sub_mag(ppc_avr_t *t, ppc_avr_t *a, ppc_avr_t *b, int *invalid,
+static void bcd_sub_mag(ppc_avr_t *t, ppc_avr_t *a, ppc_avr_t *b, int *invalid,
int *overflow)
{
int carry = 0;
int i;
- int is_zero = 1;
+
for (i = 1; i <= 31; i++) {
uint8_t digit = bcd_get_digit(a, i, invalid) -
bcd_get_digit(b, i, invalid) + carry;
- is_zero &= (digit == 0);
if (digit & 0x80) {
carry = -1;
digit += 10;
@@ -2717,14 +2709,9 @@ static int bcd_sub_mag(ppc_avr_t *t, ppc_avr_t *a, ppc_avr_t *b, int *invalid,
}
bcd_put_digit(t, digit, i);
-
- if (unlikely(*invalid)) {
- return -1;
- }
}
*overflow = carry;
- return is_zero;
}
uint32_t helper_bcdadd(ppc_avr_t *r, ppc_avr_t *a, ppc_avr_t *b, uint32_t ps)
@@ -2734,25 +2721,25 @@ uint32_t helper_bcdadd(ppc_avr_t *r, ppc_avr_t *a, ppc_avr_t *b, uint32_t ps)
int sgnb = bcd_get_sgn(b);
int invalid = (sgna == 0) || (sgnb == 0);
int overflow = 0;
- int zero = 0;
uint32_t cr = 0;
ppc_avr_t result = { .u64 = { 0, 0 } };
if (!invalid) {
if (sgna == sgnb) {
result.u8[BCD_DIG_BYTE(0)] = bcd_preferred_sgn(sgna, ps);
- zero = bcd_add_mag(&result, a, b, &invalid, &overflow);
- cr = (sgna > 0) ? CRF_GT : CRF_LT;
+ bcd_add_mag(&result, a, b, &invalid, &overflow);
+ cr = bcd_cmp_zero(&result);
} else if (bcd_cmp_mag(a, b) > 0) {
result.u8[BCD_DIG_BYTE(0)] = bcd_preferred_sgn(sgna, ps);
- zero = bcd_sub_mag(&result, a, b, &invalid, &overflow);
+ bcd_sub_mag(&result, a, b, &invalid, &overflow);
cr = (sgna > 0) ? CRF_GT : CRF_LT;
} else if (bcd_cmp_mag(a, b) == 0) {
result.u8[BCD_DIG_BYTE(0)] = bcd_preferred_sgn(0, ps);
- zero = bcd_sub_mag(&result, b, a, &invalid, &overflow);
+ bcd_sub_mag(&result, b, a, &invalid, &overflow);
+ cr = CRF_EQ;
} else {
result.u8[BCD_DIG_BYTE(0)] = bcd_preferred_sgn(sgnb, ps);
- zero = bcd_sub_mag(&result, b, a, &invalid, &overflow);
+ bcd_sub_mag(&result, b, a, &invalid, &overflow);
cr = (sgnb > 0) ? CRF_GT : CRF_LT;
}
}
@@ -2762,8 +2749,6 @@ uint32_t helper_bcdadd(ppc_avr_t *r, ppc_avr_t *a, ppc_avr_t *b, uint32_t ps)
cr = CRF_SO;
} else if (overflow) {
cr |= CRF_SO;
- } else if (zero) {
- cr = CRF_EQ;
}
*r = result;
--
1.8.3.1
^ permalink raw reply related [flat|nested] 4+ messages in thread
* Re: [Qemu-devel] [PATCH] target/ppc: simplify bcdadd/sub functions
2018-07-24 12:13 [Qemu-devel] [PATCH] target/ppc: simplify bcdadd/sub functions Yasmin Beatriz
@ 2018-07-26 1:40 ` David Gibson
2018-07-26 19:44 ` Yasmin Beatriz
0 siblings, 1 reply; 4+ messages in thread
From: David Gibson @ 2018-07-26 1:40 UTC (permalink / raw)
To: Yasmin Beatriz; +Cc: qemu-ppc, qemu-devel, rth
[-- Attachment #1: Type: text/plain, Size: 4850 bytes --]
On Tue, Jul 24, 2018 at 12:13:04PM +0000, Yasmin Beatriz wrote:
> After solving a corner case in bcdsub, this patch simplifies the logic
> of both bcdadd/sub instructions by removing some unnecessary local flags.
>
> Signed-off-by: Yasmin Beatriz <yasmins@linux.ibm.com>
> ---
> target/ppc/int_helper.c | 33 +++++++++------------------------
> 1 file changed, 9 insertions(+), 24 deletions(-)
>
> diff --git a/target/ppc/int_helper.c b/target/ppc/int_helper.c
> index fa18e6e..b8ac4bb 100644
> --- a/target/ppc/int_helper.c
> +++ b/target/ppc/int_helper.c
> @@ -2671,16 +2671,14 @@ static int bcd_cmp_mag(ppc_avr_t *a, ppc_avr_t *b)
> return 0;
> }
>
> -static int bcd_add_mag(ppc_avr_t *t, ppc_avr_t *a, ppc_avr_t *b, int *invalid,
> +static void bcd_add_mag(ppc_avr_t *t, ppc_avr_t *a, ppc_avr_t *b, int *invalid,
> int *overflow)
> {
> int carry = 0;
> int i;
> - int is_zero = 1;
> for (i = 1; i <= 31; i++) {
> uint8_t digit = bcd_get_digit(a, i, invalid) +
> bcd_get_digit(b, i, invalid) + carry;
> - is_zero &= (digit == 0);
> if (digit > 9) {
> carry = 1;
> digit -= 10;
> @@ -2689,26 +2687,20 @@ static int bcd_add_mag(ppc_avr_t *t, ppc_avr_t *a, ppc_avr_t *b, int *invalid,
> }
>
> bcd_put_digit(t, digit, i);
> -
> - if (unlikely(*invalid)) {
> - return -1;
> - }
> }
>
> *overflow = carry;
> - return is_zero;
> }
>
> -static int bcd_sub_mag(ppc_avr_t *t, ppc_avr_t *a, ppc_avr_t *b, int *invalid,
> +static void bcd_sub_mag(ppc_avr_t *t, ppc_avr_t *a, ppc_avr_t *b, int *invalid,
> int *overflow)
> {
> int carry = 0;
> int i;
> - int is_zero = 1;
> +
> for (i = 1; i <= 31; i++) {
> uint8_t digit = bcd_get_digit(a, i, invalid) -
> bcd_get_digit(b, i, invalid) + carry;
> - is_zero &= (digit == 0);
> if (digit & 0x80) {
> carry = -1;
> digit += 10;
> @@ -2717,14 +2709,9 @@ static int bcd_sub_mag(ppc_avr_t *t, ppc_avr_t *a, ppc_avr_t *b, int *invalid,
> }
>
> bcd_put_digit(t, digit, i);
> -
> - if (unlikely(*invalid)) {
> - return -1;
> - }
> }
>
> *overflow = carry;
> - return is_zero;
> }
>
> uint32_t helper_bcdadd(ppc_avr_t *r, ppc_avr_t *a, ppc_avr_t *b, uint32_t ps)
> @@ -2734,25 +2721,25 @@ uint32_t helper_bcdadd(ppc_avr_t *r, ppc_avr_t *a, ppc_avr_t *b, uint32_t ps)
> int sgnb = bcd_get_sgn(b);
> int invalid = (sgna == 0) || (sgnb == 0);
> int overflow = 0;
> - int zero = 0;
> uint32_t cr = 0;
> ppc_avr_t result = { .u64 = { 0, 0 } };
>
> if (!invalid) {
> if (sgna == sgnb) {
> result.u8[BCD_DIG_BYTE(0)] = bcd_preferred_sgn(sgna, ps);
> - zero = bcd_add_mag(&result, a, b, &invalid, &overflow);
> - cr = (sgna > 0) ? CRF_GT : CRF_LT;
> + bcd_add_mag(&result, a, b, &invalid, &overflow);
> + cr = bcd_cmp_zero(&result);
> } else if (bcd_cmp_mag(a, b) > 0) {
> result.u8[BCD_DIG_BYTE(0)] = bcd_preferred_sgn(sgna, ps);
> - zero = bcd_sub_mag(&result, a, b, &invalid, &overflow);
> + bcd_sub_mag(&result, a, b, &invalid, &overflow);
> cr = (sgna > 0) ? CRF_GT : CRF_LT;
> } else if (bcd_cmp_mag(a, b) == 0) {
> result.u8[BCD_DIG_BYTE(0)] = bcd_preferred_sgn(0, ps);
> - zero = bcd_sub_mag(&result, b, a, &invalid, &overflow);
> + bcd_sub_mag(&result, b, a, &invalid, &overflow);
I don't think you actually need the sub here, since you know the
result is going to be zero.
Although.. in all of the different-sign cases aren't we effectively
doing the subtraction twice - once in bcd_cmp_mag() then again in
bcd_sub_mag()?
> + cr = CRF_EQ;
> } else {
> result.u8[BCD_DIG_BYTE(0)] = bcd_preferred_sgn(sgnb, ps);
> - zero = bcd_sub_mag(&result, b, a, &invalid, &overflow);
> + bcd_sub_mag(&result, b, a, &invalid, &overflow);
> cr = (sgnb > 0) ? CRF_GT : CRF_LT;
> }
> }
> @@ -2762,8 +2749,6 @@ uint32_t helper_bcdadd(ppc_avr_t *r, ppc_avr_t *a, ppc_avr_t *b, uint32_t ps)
> cr = CRF_SO;
> } else if (overflow) {
> cr |= CRF_SO;
> - } else if (zero) {
> - cr = CRF_EQ;
> }
>
> *r = result;
--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson
[-- Attachment #2: signature.asc --]
[-- Type: application/pgp-signature, Size: 833 bytes --]
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [Qemu-devel] [PATCH] target/ppc: simplify bcdadd/sub functions
2018-07-26 1:40 ` David Gibson
@ 2018-07-26 19:44 ` Yasmin Beatriz
2018-07-27 0:48 ` David Gibson
0 siblings, 1 reply; 4+ messages in thread
From: Yasmin Beatriz @ 2018-07-26 19:44 UTC (permalink / raw)
To: David Gibson; +Cc: qemu-ppc, qemu-devel, rth
On Thu, Jul 26, 2018 at 11:40:20AM +1000, David Gibson wrote:
> On Tue, Jul 24, 2018 at 12:13:04PM +0000, Yasmin Beatriz wrote:
> > After solving a corner case in bcdsub, this patch simplifies the logic
> > of both bcdadd/sub instructions by removing some unnecessary local flags.
> >
> > Signed-off-by: Yasmin Beatriz <yasmins@linux.ibm.com>
> > ---
> > target/ppc/int_helper.c | 33 +++++++++------------------------
> > 1 file changed, 9 insertions(+), 24 deletions(-)
> >
> > diff --git a/target/ppc/int_helper.c b/target/ppc/int_helper.c
> > index fa18e6e..b8ac4bb 100644
> > --- a/target/ppc/int_helper.c
> > +++ b/target/ppc/int_helper.c
> > @@ -2671,16 +2671,14 @@ static int bcd_cmp_mag(ppc_avr_t *a, ppc_avr_t *b)
> > return 0;
> > }
> >
> > -static int bcd_add_mag(ppc_avr_t *t, ppc_avr_t *a, ppc_avr_t *b, int *invalid,
> > +static void bcd_add_mag(ppc_avr_t *t, ppc_avr_t *a, ppc_avr_t *b, int *invalid,
> > int *overflow)
> > {
> > int carry = 0;
> > int i;
> > - int is_zero = 1;
> > for (i = 1; i <= 31; i++) {
> > uint8_t digit = bcd_get_digit(a, i, invalid) +
> > bcd_get_digit(b, i, invalid) + carry;
> > - is_zero &= (digit == 0);
> > if (digit > 9) {
> > carry = 1;
> > digit -= 10;
> > @@ -2689,26 +2687,20 @@ static int bcd_add_mag(ppc_avr_t *t, ppc_avr_t *a, ppc_avr_t *b, int *invalid,
> > }
> >
> > bcd_put_digit(t, digit, i);
> > -
> > - if (unlikely(*invalid)) {
> > - return -1;
> > - }
> > }
> >
> > *overflow = carry;
> > - return is_zero;
> > }
> >
> > -static int bcd_sub_mag(ppc_avr_t *t, ppc_avr_t *a, ppc_avr_t *b, int *invalid,
> > +static void bcd_sub_mag(ppc_avr_t *t, ppc_avr_t *a, ppc_avr_t *b, int *invalid,
> > int *overflow)
> > {
> > int carry = 0;
> > int i;
> > - int is_zero = 1;
> > +
> > for (i = 1; i <= 31; i++) {
> > uint8_t digit = bcd_get_digit(a, i, invalid) -
> > bcd_get_digit(b, i, invalid) + carry;
> > - is_zero &= (digit == 0);
> > if (digit & 0x80) {
> > carry = -1;
> > digit += 10;
> > @@ -2717,14 +2709,9 @@ static int bcd_sub_mag(ppc_avr_t *t, ppc_avr_t *a, ppc_avr_t *b, int *invalid,
> > }
> >
> > bcd_put_digit(t, digit, i);
> > -
> > - if (unlikely(*invalid)) {
> > - return -1;
> > - }
> > }
> >
> > *overflow = carry;
> > - return is_zero;
> > }
> >
> > uint32_t helper_bcdadd(ppc_avr_t *r, ppc_avr_t *a, ppc_avr_t *b, uint32_t ps)
> > @@ -2734,25 +2721,25 @@ uint32_t helper_bcdadd(ppc_avr_t *r, ppc_avr_t *a, ppc_avr_t *b, uint32_t ps)
> > int sgnb = bcd_get_sgn(b);
> > int invalid = (sgna == 0) || (sgnb == 0);
> > int overflow = 0;
> > - int zero = 0;
> > uint32_t cr = 0;
> > ppc_avr_t result = { .u64 = { 0, 0 } };
> >
> > if (!invalid) {
> > if (sgna == sgnb) {
> > result.u8[BCD_DIG_BYTE(0)] = bcd_preferred_sgn(sgna, ps);
> > - zero = bcd_add_mag(&result, a, b, &invalid, &overflow);
> > - cr = (sgna > 0) ? CRF_GT : CRF_LT;
> > + bcd_add_mag(&result, a, b, &invalid, &overflow);
> > + cr = bcd_cmp_zero(&result);
> > } else if (bcd_cmp_mag(a, b) > 0) {
> > result.u8[BCD_DIG_BYTE(0)] = bcd_preferred_sgn(sgna, ps);
> > - zero = bcd_sub_mag(&result, a, b, &invalid, &overflow);
> > + bcd_sub_mag(&result, a, b, &invalid, &overflow);
> > cr = (sgna > 0) ? CRF_GT : CRF_LT;
> > } else if (bcd_cmp_mag(a, b) == 0) {
> > result.u8[BCD_DIG_BYTE(0)] = bcd_preferred_sgn(0, ps);
> > - zero = bcd_sub_mag(&result, b, a, &invalid, &overflow);
> > + bcd_sub_mag(&result, b, a, &invalid, &overflow);
>
> I don't think you actually need the sub here, since you know the
> result is going to be zero.
Right. Will fix this in v2.
> Although.. in all of the different-sign cases aren't we effectively
> doing the subtraction twice - once in bcd_cmp_mag() then again in
> bcd_sub_mag()?
Actually no, bcd_cmp_mag() compares the magnitude between 'a' and 'b'
starting from the most significant digit and returns as soon as it finds
a difference between the two of them. It helps to decide whether we're
going to perform a - b or b - a.
Anyway, I think the following code can make it easier to understand:
int magnitude;
if (sgna == sgnb) {
// same thing
} else {
magnitude = bdc_cmp_mag(a, b);
if (magnitude > 0) {
// do a - b
} else if (magnitude < 0) {
// do b - a
} else {
// 0
}
}
> > + cr = CRF_EQ;
> > } else {
> > result.u8[BCD_DIG_BYTE(0)] = bcd_preferred_sgn(sgnb, ps);
> > - zero = bcd_sub_mag(&result, b, a, &invalid, &overflow);
> > + bcd_sub_mag(&result, b, a, &invalid, &overflow);
> > cr = (sgnb > 0) ? CRF_GT : CRF_LT;
> > }
> > }
> > @@ -2762,8 +2749,6 @@ uint32_t helper_bcdadd(ppc_avr_t *r, ppc_avr_t *a, ppc_avr_t *b, uint32_t ps)
> > cr = CRF_SO;
> > } else if (overflow) {
> > cr |= CRF_SO;
> > - } else if (zero) {
> > - cr = CRF_EQ;
> > }
> >
> > *r = result;
>
> --
> David Gibson | I'll have my music baroque, and my code
> david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
> | _way_ _around_!
> http://www.ozlabs.org/~dgibson
^ permalink raw reply [flat|nested] 4+ messages in thread
* Re: [Qemu-devel] [PATCH] target/ppc: simplify bcdadd/sub functions
2018-07-26 19:44 ` Yasmin Beatriz
@ 2018-07-27 0:48 ` David Gibson
0 siblings, 0 replies; 4+ messages in thread
From: David Gibson @ 2018-07-27 0:48 UTC (permalink / raw)
To: Yasmin Beatriz; +Cc: qemu-ppc, qemu-devel, rth
[-- Attachment #1: Type: text/plain, Size: 6186 bytes --]
On Thu, Jul 26, 2018 at 04:44:43PM -0300, Yasmin Beatriz wrote:
> On Thu, Jul 26, 2018 at 11:40:20AM +1000, David Gibson wrote:
> > On Tue, Jul 24, 2018 at 12:13:04PM +0000, Yasmin Beatriz wrote:
> > > After solving a corner case in bcdsub, this patch simplifies the logic
> > > of both bcdadd/sub instructions by removing some unnecessary local flags.
> > >
> > > Signed-off-by: Yasmin Beatriz <yasmins@linux.ibm.com>
> > > ---
> > > target/ppc/int_helper.c | 33 +++++++++------------------------
> > > 1 file changed, 9 insertions(+), 24 deletions(-)
> > >
> > > diff --git a/target/ppc/int_helper.c b/target/ppc/int_helper.c
> > > index fa18e6e..b8ac4bb 100644
> > > --- a/target/ppc/int_helper.c
> > > +++ b/target/ppc/int_helper.c
> > > @@ -2671,16 +2671,14 @@ static int bcd_cmp_mag(ppc_avr_t *a, ppc_avr_t *b)
> > > return 0;
> > > }
> > >
> > > -static int bcd_add_mag(ppc_avr_t *t, ppc_avr_t *a, ppc_avr_t *b, int *invalid,
> > > +static void bcd_add_mag(ppc_avr_t *t, ppc_avr_t *a, ppc_avr_t *b, int *invalid,
> > > int *overflow)
> > > {
> > > int carry = 0;
> > > int i;
> > > - int is_zero = 1;
> > > for (i = 1; i <= 31; i++) {
> > > uint8_t digit = bcd_get_digit(a, i, invalid) +
> > > bcd_get_digit(b, i, invalid) + carry;
> > > - is_zero &= (digit == 0);
> > > if (digit > 9) {
> > > carry = 1;
> > > digit -= 10;
> > > @@ -2689,26 +2687,20 @@ static int bcd_add_mag(ppc_avr_t *t, ppc_avr_t *a, ppc_avr_t *b, int *invalid,
> > > }
> > >
> > > bcd_put_digit(t, digit, i);
> > > -
> > > - if (unlikely(*invalid)) {
> > > - return -1;
> > > - }
> > > }
> > >
> > > *overflow = carry;
> > > - return is_zero;
> > > }
> > >
> > > -static int bcd_sub_mag(ppc_avr_t *t, ppc_avr_t *a, ppc_avr_t *b, int *invalid,
> > > +static void bcd_sub_mag(ppc_avr_t *t, ppc_avr_t *a, ppc_avr_t *b, int *invalid,
> > > int *overflow)
> > > {
> > > int carry = 0;
> > > int i;
> > > - int is_zero = 1;
> > > +
> > > for (i = 1; i <= 31; i++) {
> > > uint8_t digit = bcd_get_digit(a, i, invalid) -
> > > bcd_get_digit(b, i, invalid) + carry;
> > > - is_zero &= (digit == 0);
> > > if (digit & 0x80) {
> > > carry = -1;
> > > digit += 10;
> > > @@ -2717,14 +2709,9 @@ static int bcd_sub_mag(ppc_avr_t *t, ppc_avr_t *a, ppc_avr_t *b, int *invalid,
> > > }
> > >
> > > bcd_put_digit(t, digit, i);
> > > -
> > > - if (unlikely(*invalid)) {
> > > - return -1;
> > > - }
> > > }
> > >
> > > *overflow = carry;
> > > - return is_zero;
> > > }
> > >
> > > uint32_t helper_bcdadd(ppc_avr_t *r, ppc_avr_t *a, ppc_avr_t *b, uint32_t ps)
> > > @@ -2734,25 +2721,25 @@ uint32_t helper_bcdadd(ppc_avr_t *r, ppc_avr_t *a, ppc_avr_t *b, uint32_t ps)
> > > int sgnb = bcd_get_sgn(b);
> > > int invalid = (sgna == 0) || (sgnb == 0);
> > > int overflow = 0;
> > > - int zero = 0;
> > > uint32_t cr = 0;
> > > ppc_avr_t result = { .u64 = { 0, 0 } };
> > >
> > > if (!invalid) {
> > > if (sgna == sgnb) {
> > > result.u8[BCD_DIG_BYTE(0)] = bcd_preferred_sgn(sgna, ps);
> > > - zero = bcd_add_mag(&result, a, b, &invalid, &overflow);
> > > - cr = (sgna > 0) ? CRF_GT : CRF_LT;
> > > + bcd_add_mag(&result, a, b, &invalid, &overflow);
> > > + cr = bcd_cmp_zero(&result);
> > > } else if (bcd_cmp_mag(a, b) > 0) {
> > > result.u8[BCD_DIG_BYTE(0)] = bcd_preferred_sgn(sgna, ps);
> > > - zero = bcd_sub_mag(&result, a, b, &invalid, &overflow);
> > > + bcd_sub_mag(&result, a, b, &invalid, &overflow);
> > > cr = (sgna > 0) ? CRF_GT : CRF_LT;
> > > } else if (bcd_cmp_mag(a, b) == 0) {
> > > result.u8[BCD_DIG_BYTE(0)] = bcd_preferred_sgn(0, ps);
> > > - zero = bcd_sub_mag(&result, b, a, &invalid, &overflow);
> > > + bcd_sub_mag(&result, b, a, &invalid, &overflow);
> >
> > I don't think you actually need the sub here, since you know the
> > result is going to be zero.
>
> Right. Will fix this in v2.
>
> > Although.. in all of the different-sign cases aren't we effectively
> > doing the subtraction twice - once in bcd_cmp_mag() then again in
> > bcd_sub_mag()?
>
> Actually no, bcd_cmp_mag() compares the magnitude between 'a' and 'b'
> starting from the most significant digit and returns as soon as it finds
> a difference between the two of them. It helps to decide whether we're
> going to perform a - b or b - a.
Ah, good point.
> Anyway, I think the following code can make it easier to understand:
>
> int magnitude;
> if (sgna == sgnb) {
> // same thing
> } else {
> magnitude = bdc_cmp_mag(a, b);
> if (magnitude > 0) {
> // do a - b
> } else if (magnitude < 0) {
> // do b - a
> } else {
> // 0
> }
> }
Yes, I think that's a good idea.
>
> > > + cr = CRF_EQ;
> > > } else {
> > > result.u8[BCD_DIG_BYTE(0)] = bcd_preferred_sgn(sgnb, ps);
> > > - zero = bcd_sub_mag(&result, b, a, &invalid, &overflow);
> > > + bcd_sub_mag(&result, b, a, &invalid, &overflow);
> > > cr = (sgnb > 0) ? CRF_GT : CRF_LT;
> > > }
> > > }
> > > @@ -2762,8 +2749,6 @@ uint32_t helper_bcdadd(ppc_avr_t *r, ppc_avr_t *a, ppc_avr_t *b, uint32_t ps)
> > > cr = CRF_SO;
> > > } else if (overflow) {
> > > cr |= CRF_SO;
> > > - } else if (zero) {
> > > - cr = CRF_EQ;
> > > }
> > >
> > > *r = result;
> >
>
>
--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson
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2018-07-24 12:13 [Qemu-devel] [PATCH] target/ppc: simplify bcdadd/sub functions Yasmin Beatriz
2018-07-26 1:40 ` David Gibson
2018-07-26 19:44 ` Yasmin Beatriz
2018-07-27 0:48 ` David Gibson
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