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[Qemu-devel] [PATCH v2] softfloat: Fix factor 2 error for scalbn on deno
From: |
Peter Maydell |
Subject: |
[Qemu-devel] [PATCH v2] softfloat: Fix factor 2 error for scalbn on denormal inputs |
Date: |
Sat, 21 Dec 2013 21:08:19 +0000 |
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 <address@hidden>
---
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
- [Qemu-devel] [PATCH v2] softfloat: Fix factor 2 error for scalbn on denormal inputs,
Peter Maydell <=