* [Qemu-devel] [PATCH v2] softfloat: Fix factor 2 error for scalbn on denormal inputs
@ 2013-12-21 21:08 Peter Maydell
2013-12-22 16:38 ` Aurelien Jarno
2013-12-22 16:41 ` Richard Henderson
0 siblings, 2 replies; 3+ messages in thread
From: Peter Maydell @ 2013-12-21 21:08 UTC (permalink / raw)
To: qemu-devel; +Cc: Richard Henderson, patches
If the input to float*_scalbn() is denormal then it represents
a number 0.[mantissabits] * 2^(1-exponentbias) (and the actual
exponent field is all zeroes). This means that when we convert
it to our unpacked encoding the unpacked exponent must be one
greater than for a normal number, which represents
1.[mantissabits] * 2^(e-exponentbias) for an exponent field e.
This meant we were giving answers too small by a factor of 2 for
all denormal inputs.
Note that the float-to-int routines also have this behaviour
of not adjusting the exponent for denormals; however there it is
harmless because denormals will all convert to integer zero anyway.
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
---
Changes v1->v2: propagated fix to 80 bit and 128 bit float functions.
These function names remind me of Ken Thompson's reply to a question
about what he'd do differently if he were redesigning UNIX:
"I'd spell creat with an e."
fpu/softfloat.c | 29 +++++++++++++++++++++--------
1 file changed, 21 insertions(+), 8 deletions(-)
diff --git a/fpu/softfloat.c b/fpu/softfloat.c
index dbda61b..7f5e8c7 100644
--- a/fpu/softfloat.c
+++ b/fpu/softfloat.c
@@ -6795,10 +6795,13 @@ float32 float32_scalbn( float32 a, int n STATUS_PARAM )
}
return a;
}
- if ( aExp != 0 )
+ if (aExp != 0) {
aSig |= 0x00800000;
- else if ( aSig == 0 )
+ } else if (aSig == 0) {
return a;
+ } else {
+ aExp++;
+ }
if (n > 0x200) {
n = 0x200;
@@ -6828,10 +6831,13 @@ float64 float64_scalbn( float64 a, int n STATUS_PARAM )
}
return a;
}
- if ( aExp != 0 )
+ if (aExp != 0) {
aSig |= LIT64( 0x0010000000000000 );
- else if ( aSig == 0 )
+ } else if (aSig == 0) {
return a;
+ } else {
+ aExp++;
+ }
if (n > 0x1000) {
n = 0x1000;
@@ -6861,8 +6867,12 @@ floatx80 floatx80_scalbn( floatx80 a, int n STATUS_PARAM )
return a;
}
- if (aExp == 0 && aSig == 0)
- return a;
+ if (aExp == 0) {
+ if (aSig == 0) {
+ return a;
+ }
+ aExp++;
+ }
if (n > 0x10000) {
n = 0x10000;
@@ -6891,10 +6901,13 @@ float128 float128_scalbn( float128 a, int n STATUS_PARAM )
}
return a;
}
- if ( aExp != 0 )
+ if (aExp != 0) {
aSig0 |= LIT64( 0x0001000000000000 );
- else if ( aSig0 == 0 && aSig1 == 0 )
+ } else if (aSig0 == 0 && aSig1 == 0) {
return a;
+ } else {
+ aExp++;
+ }
if (n > 0x10000) {
n = 0x10000;
--
1.8.5
^ permalink raw reply related [flat|nested] 3+ messages in thread
* Re: [Qemu-devel] [PATCH v2] softfloat: Fix factor 2 error for scalbn on denormal inputs
2013-12-21 21:08 [Qemu-devel] [PATCH v2] softfloat: Fix factor 2 error for scalbn on denormal inputs Peter Maydell
@ 2013-12-22 16:38 ` Aurelien Jarno
2013-12-22 16:41 ` Richard Henderson
1 sibling, 0 replies; 3+ messages in thread
From: Aurelien Jarno @ 2013-12-22 16:38 UTC (permalink / raw)
To: Peter Maydell; +Cc: patches, qemu-devel, Richard Henderson
On Sat, Dec 21, 2013 at 09:08:19PM +0000, Peter Maydell wrote:
> If the input to float*_scalbn() is denormal then it represents
> a number 0.[mantissabits] * 2^(1-exponentbias) (and the actual
> exponent field is all zeroes). This means that when we convert
> it to our unpacked encoding the unpacked exponent must be one
> greater than for a normal number, which represents
> 1.[mantissabits] * 2^(e-exponentbias) for an exponent field e.
>
> This meant we were giving answers too small by a factor of 2 for
> all denormal inputs.
>
> Note that the float-to-int routines also have this behaviour
> of not adjusting the exponent for denormals; however there it is
> harmless because denormals will all convert to integer zero anyway.
>
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> ---
> Changes v1->v2: propagated fix to 80 bit and 128 bit float functions.
>
> These function names remind me of Ken Thompson's reply to a question
> about what he'd do differently if he were redesigning UNIX:
> "I'd spell creat with an e."
>
> fpu/softfloat.c | 29 +++++++++++++++++++++--------
> 1 file changed, 21 insertions(+), 8 deletions(-)
>
> diff --git a/fpu/softfloat.c b/fpu/softfloat.c
> index dbda61b..7f5e8c7 100644
> --- a/fpu/softfloat.c
> +++ b/fpu/softfloat.c
> @@ -6795,10 +6795,13 @@ float32 float32_scalbn( float32 a, int n STATUS_PARAM )
> }
> return a;
> }
> - if ( aExp != 0 )
> + if (aExp != 0) {
> aSig |= 0x00800000;
> - else if ( aSig == 0 )
> + } else if (aSig == 0) {
> return a;
> + } else {
> + aExp++;
> + }
>
> if (n > 0x200) {
> n = 0x200;
> @@ -6828,10 +6831,13 @@ float64 float64_scalbn( float64 a, int n STATUS_PARAM )
> }
> return a;
> }
> - if ( aExp != 0 )
> + if (aExp != 0) {
> aSig |= LIT64( 0x0010000000000000 );
> - else if ( aSig == 0 )
> + } else if (aSig == 0) {
> return a;
> + } else {
> + aExp++;
> + }
>
> if (n > 0x1000) {
> n = 0x1000;
> @@ -6861,8 +6867,12 @@ floatx80 floatx80_scalbn( floatx80 a, int n STATUS_PARAM )
> return a;
> }
>
> - if (aExp == 0 && aSig == 0)
> - return a;
> + if (aExp == 0) {
> + if (aSig == 0) {
> + return a;
> + }
> + aExp++;
> + }
>
> if (n > 0x10000) {
> n = 0x10000;
> @@ -6891,10 +6901,13 @@ float128 float128_scalbn( float128 a, int n STATUS_PARAM )
> }
> return a;
> }
> - if ( aExp != 0 )
> + if (aExp != 0) {
> aSig0 |= LIT64( 0x0001000000000000 );
> - else if ( aSig0 == 0 && aSig1 == 0 )
> + } else if (aSig0 == 0 && aSig1 == 0) {
> return a;
> + } else {
> + aExp++;
> + }
>
> if (n > 0x10000) {
> n = 0x10000;
Reviewed-by: Aurelien Jarno <aurelien@aurel32.net>
--
Aurelien Jarno GPG: 1024D/F1BCDB73
aurelien@aurel32.net http://www.aurel32.net
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [Qemu-devel] [PATCH v2] softfloat: Fix factor 2 error for scalbn on denormal inputs
2013-12-21 21:08 [Qemu-devel] [PATCH v2] softfloat: Fix factor 2 error for scalbn on denormal inputs Peter Maydell
2013-12-22 16:38 ` Aurelien Jarno
@ 2013-12-22 16:41 ` Richard Henderson
1 sibling, 0 replies; 3+ messages in thread
From: Richard Henderson @ 2013-12-22 16:41 UTC (permalink / raw)
To: Peter Maydell, qemu-devel; +Cc: patches
On 12/21/2013 01:08 PM, Peter Maydell wrote:
> If the input to float*_scalbn() is denormal then it represents
> a number 0.[mantissabits] * 2^(1-exponentbias) (and the actual
> exponent field is all zeroes). This means that when we convert
> it to our unpacked encoding the unpacked exponent must be one
> greater than for a normal number, which represents
> 1.[mantissabits] * 2^(e-exponentbias) for an exponent field e.
>
> This meant we were giving answers too small by a factor of 2 for
> all denormal inputs.
>
> Note that the float-to-int routines also have this behaviour
> of not adjusting the exponent for denormals; however there it is
> harmless because denormals will all convert to integer zero anyway.
>
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> ---
> Changes v1->v2: propagated fix to 80 bit and 128 bit float functions.
>
> These function names remind me of Ken Thompson's reply to a question
> about what he'd do differently if he were redesigning UNIX:
> "I'd spell creat with an e."
>
> fpu/softfloat.c | 29 +++++++++++++++++++++--------
> 1 file changed, 21 insertions(+), 8 deletions(-)
Reviewed-by: Richard Henderson <rth@twiddle.net>
r~
^ permalink raw reply [flat|nested] 3+ messages in thread
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2013-12-22 16:41 ` Richard Henderson
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