[Qemu-devel] [PATCH v2] softfloat: Fix factor 2 error for scalbn on denormal inputs

[Peter Maydell <peter.maydell@linaro.org>]

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