Re: [Qemu-devel] [PATCH] Fix NaN handling in softfloat

[Thiemo Seufer]

Aurelien Jarno wrote:
> On Sat, Nov 03, 2007 at 02:06:04PM -0400, Daniel Jacobowitz wrote:
> > On Sat, Nov 03, 2007 at 06:35:48PM +0100, Aurelien Jarno wrote:
> > > Hi all,
> > > 
> > > The current softfloat implementation changes qNaN into sNaN when 
> > > converting between formats, for no reason. The attached patch fixes
> > > that. It also fixes an off-by-one in the extended double precision
> > > format (aka floatx80), the mantissa is 64-bit long and not 63-bit
> > > long.
> > > 
> > > With this patch applied all the glibc 2.7 floating point tests
> > > are successfull on MIPS and MIPSEL.
> > 
> > FYI, I posted a similar patch and haven't had time to get back to it.
> > Andreas reminded me that we need to make sure at least one mantissa
> > bit is set.  If we're confident that the common NaN format will
> > already have some bit other than the qnan/snan bit set, this is fine;
> > otherwise, we might want to forcibly set some other mantissa bit.
> > 
> 
> Please find an updated patch below. I have tried to match real x86, MIPS,
> HPPA, PowerPC and SPARC hardware when all mantissa bits are cleared.

The default NaN pattern for MIPS and HPPA were broken. (Fabrice spotted
this.) I currently have the appended patch, it assumes HPPA is similiar
to MIPS, which is probably incorrect.

> Index: fpu/softfloat-specialize.h
> ===================================================================
> RCS file: /sources/qemu/qemu/fpu/softfloat-specialize.h,v
> retrieving revision 1.3
> diff -u -d -p -r1.3 softfloat-specialize.h
> --- fpu/softfloat-specialize.h        11 May 2007 17:10:14 -0000      1.3
> +++ fpu/softfloat-specialize.h        3 Nov 2007 21:17:57 -0000
> @@ -120,9 +120,17 @@ static commonNaNT float32ToCommonNaN( fl
>  
>  static float32 commonNaNToFloat32( commonNaNT a )
>  {
> -
> -    return ( ( (bits32) a.sign )<<31 ) | 0x7FC00000 | ( a.high>>41 );
> -
> +    bits32 mantissa = a.high>>41;
> +    if (mantissa)
> +        return ( ( (bits32) a.sign )<<31 ) | 0x7F800000 | ( mantissa );
> +    else
> +#if defined(TARGET_MIPS)
> +        return ( ( (bits32) a.sign )<<31 ) | 0x7FBFFFFF | ( mantissa );
> +#elif defined(TARGET_HPPA)
> +        return ( ( (bits32) a.sign )<<31 ) | 0x7FA00000 | ( mantissa );
> +#else
> +        return ( ( (bits32) a.sign )<<31 ) | 0x7FC00000;
> +#endif

This looks odd. Do MIPS and HPPA really need a specialcase here but none
for doubles? Could you re-check with my patch applied?


Thiemo


Index: qemu-work/fpu/softfloat-specialize.h
===================================================================
--- qemu-work.orig/fpu/softfloat-specialize.h   2007-06-26 21:22:50.000000000 
+0100
+++ qemu-work/fpu/softfloat-specialize.h        2007-11-10 19:49:37.000000000 
+0000
@@ -30,6 +30,12 @@
 
 =============================================================================*/
 
+#if defined(TARGET_MIPS) || defined(TARGET_HPPA)
+#define SNAN_BIT_IS_ONE                1
+#else
+#define SNAN_BIT_IS_ONE                0
+#endif
+
 /*----------------------------------------------------------------------------
 | Underflow tininess-detection mode, statically initialized to default value.
 | (The declaration in `softfloat.h' must match the `int8' type here.)
@@ -45,9 +51,7 @@
 
 void float_raise( int8 flags STATUS_PARAM )
 {
-
     STATUS(float_exception_flags) |= flags;
-
 }
 
 /*----------------------------------------------------------------------------
@@ -61,20 +65,20 @@
 /*----------------------------------------------------------------------------
 | The pattern for a default generated single-precision NaN.
 *----------------------------------------------------------------------------*/
-#if defined(TARGET_MIPS) || defined(TARGET_HPPA)
-#define float32_default_nan 0xFF800000
+#if SNAN_BIT_IS_ONE
+#define float32_default_nan 0x7FBFFFFF
 #else
 #define float32_default_nan 0xFFC00000
 #endif
 
 /*----------------------------------------------------------------------------
-| Returns 1 if the single-precision floating-point value `a' is a NaN;
-| otherwise returns 0.
+| Returns 1 if the single-precision floating-point value `a' is a quiet
+| NaN; otherwise returns 0.
 *----------------------------------------------------------------------------*/
 
 int float32_is_nan( float32 a )
 {
-#if defined(TARGET_MIPS) || defined(TARGET_HPPA)
+#if SNAN_BIT_IS_ONE
     return ( ( ( a>>22 ) & 0x1FF ) == 0x1FE ) && ( a & 0x003FFFFF );
 #else
     return ( 0xFF800000 <= (bits32) ( a<<1 ) );
@@ -88,7 +92,7 @@
 
 int float32_is_signaling_nan( float32 a )
 {
-#if defined(TARGET_MIPS) || defined(TARGET_HPPA)
+#if SNAN_BIT_IS_ONE
     return ( 0xFF800000 <= (bits32) ( a<<1 ) );
 #else
     return ( ( ( a>>22 ) & 0x1FF ) == 0x1FE ) && ( a & 0x003FFFFF );
@@ -110,7 +114,6 @@
     z.low = 0;
     z.high = ( (bits64) a )<<41;
     return z;
-
 }
 
 /*----------------------------------------------------------------------------
@@ -120,9 +123,7 @@
 
 static float32 commonNaNToFloat32( commonNaNT a )
 {
-
     return ( ( (bits32) a.sign )<<31 ) | 0x7FC00000 | ( a.high>>41 );
-
 }
 
 /*----------------------------------------------------------------------------
@@ -139,7 +140,7 @@
     aIsSignalingNaN = float32_is_signaling_nan( a );
     bIsNaN = float32_is_nan( b );
     bIsSignalingNaN = float32_is_signaling_nan( b );
-#if defined(TARGET_MIPS) || defined(TARGET_HPPA)
+#if SNAN_BIT_IS_ONE
     a &= ~0x00400000;
     b &= ~0x00400000;
 #else
@@ -161,26 +162,25 @@
     else {
         return b;
     }
-
 }
 
 /*----------------------------------------------------------------------------
 | The pattern for a default generated double-precision NaN.
 *----------------------------------------------------------------------------*/
-#if defined(TARGET_MIPS) || defined(TARGET_HPPA)
-#define float64_default_nan LIT64( 0xFFF0000000000000 )
+#if SNAN_BIT_IS_ONE
+#define float64_default_nan LIT64( 0x7FF7FFFFFFFFFFFF )
 #else
 #define float64_default_nan LIT64( 0xFFF8000000000000 )
 #endif
 
 /*----------------------------------------------------------------------------
-| Returns 1 if the double-precision floating-point value `a' is a NaN;
-| otherwise returns 0.
+| Returns 1 if the double-precision floating-point value `a' is a quiet
+| NaN; otherwise returns 0.
 *----------------------------------------------------------------------------*/
 
 int float64_is_nan( float64 a )
 {
-#if defined(TARGET_MIPS) || defined(TARGET_HPPA)
+#if SNAN_BIT_IS_ONE
     return
            ( ( ( a>>51 ) & 0xFFF ) == 0xFFE )
         && ( a & LIT64( 0x0007FFFFFFFFFFFF ) );
@@ -196,7 +196,7 @@
 
 int float64_is_signaling_nan( float64 a )
 {
-#if defined(TARGET_MIPS) || defined(TARGET_HPPA)
+#if SNAN_BIT_IS_ONE
     return ( LIT64( 0xFFF0000000000000 ) <= (bits64) ( a<<1 ) );
 #else
     return
@@ -220,7 +220,6 @@
     z.low = 0;
     z.high = a<<12;
     return z;
-
 }
 
 /*----------------------------------------------------------------------------
@@ -230,12 +229,10 @@
 
 static float64 commonNaNToFloat64( commonNaNT a )
 {
-
     return
           ( ( (bits64) a.sign )<<63 )
         | LIT64( 0x7FF8000000000000 )
         | ( a.high>>12 );
-
 }
 
 /*----------------------------------------------------------------------------
@@ -252,7 +249,7 @@
     aIsSignalingNaN = float64_is_signaling_nan( a );
     bIsNaN = float64_is_nan( b );
     bIsSignalingNaN = float64_is_signaling_nan( b );
-#if defined(TARGET_MIPS) || defined(TARGET_HPPA)
+#if SNAN_BIT_IS_ONE
     a &= ~LIT64( 0x0008000000000000 );
     b &= ~LIT64( 0x0008000000000000 );
 #else
@@ -274,7 +271,6 @@
     else {
         return b;
     }
-
 }
 
 #ifdef FLOATX80
@@ -284,19 +280,32 @@
 | `high' and `low' values hold the most- and least-significant bits,
 | respectively.
 *----------------------------------------------------------------------------*/
+#if SNAN_BIT_IS_ONE
+#define floatx80_default_nan_high 0x7FFF
+#define floatx80_default_nan_low  LIT64( 0xBFFFFFFFFFFFFFFF )
+#else
 #define floatx80_default_nan_high 0xFFFF
 #define floatx80_default_nan_low  LIT64( 0xC000000000000000 )
+#endif
 
 /*----------------------------------------------------------------------------
 | Returns 1 if the extended double-precision floating-point value `a' is a
-| NaN; otherwise returns 0.
+| quiet NaN; otherwise returns 0.
 *----------------------------------------------------------------------------*/
 
 int floatx80_is_nan( floatx80 a )
 {
+#if SNAN_BIT_IS_ONE
+    bits64 aLow;
 
+    aLow = a.low & ~ LIT64( 0x4000000000000000 );
+    return
+           ( ( a.high & 0x7FFF ) == 0x7FFF )
+        && (bits64) ( aLow<<1 )
+        && ( a.low == aLow );
+#else
     return ( ( a.high & 0x7FFF ) == 0x7FFF ) && (bits64) ( a.low<<1 );
-
+#endif
 }
 
 /*----------------------------------------------------------------------------
@@ -306,6 +315,9 @@
 
 int floatx80_is_signaling_nan( floatx80 a )
 {
+#if SNAN_BIT_IS_ONE
+    return ( ( a.high & 0x7FFF ) == 0x7FFF ) && (bits64) ( a.low<<1 );
+#else
     bits64 aLow;
 
     aLow = a.low & ~ LIT64( 0x4000000000000000 );
@@ -313,7 +325,7 @@
            ( ( a.high & 0x7FFF ) == 0x7FFF )
         && (bits64) ( aLow<<1 )
         && ( a.low == aLow );
-
+#endif
 }
 
 /*----------------------------------------------------------------------------
@@ -331,7 +343,6 @@
     z.low = 0;
     z.high = a.low<<1;
     return z;
-
 }
 
 /*----------------------------------------------------------------------------
@@ -346,7 +357,6 @@
     z.low = LIT64( 0xC000000000000000 ) | ( a.high>>1 );
     z.high = ( ( (bits16) a.sign )<<15 ) | 0x7FFF;
     return z;
-
 }
 
 /*----------------------------------------------------------------------------
@@ -363,8 +373,13 @@
     aIsSignalingNaN = floatx80_is_signaling_nan( a );
     bIsNaN = floatx80_is_nan( b );
     bIsSignalingNaN = floatx80_is_signaling_nan( b );
+#if SNAN_BIT_IS_ONE
+    a.low &= ~LIT64( 0xC000000000000000 );
+    b.low &= ~LIT64( 0xC000000000000000 );
+#else
     a.low |= LIT64( 0xC000000000000000 );
     b.low |= LIT64( 0xC000000000000000 );
+#endif
     if ( aIsSignalingNaN | bIsSignalingNaN ) float_raise( float_flag_invalid 
STATUS_VAR);
     if ( aIsSignalingNaN ) {
         if ( bIsSignalingNaN ) goto returnLargerSignificand;
@@ -380,7 +395,6 @@
     else {
         return b;
     }
-
 }
 
 #endif
@@ -391,21 +405,30 @@
 | The pattern for a default generated quadruple-precision NaN.  The `high' and
 | `low' values hold the most- and least-significant bits, respectively.
 *----------------------------------------------------------------------------*/
+#if SNAN_BIT_IS_ONE
+#define float128_default_nan_high LIT64( 0x7FFF7FFFFFFFFFFF )
+#define float128_default_nan_low  LIT64( 0xFFFFFFFFFFFFFFFF )
+#else
 #define float128_default_nan_high LIT64( 0xFFFF800000000000 )
 #define float128_default_nan_low  LIT64( 0x0000000000000000 )
+#endif
 
 /*----------------------------------------------------------------------------
-| Returns 1 if the quadruple-precision floating-point value `a' is a NaN;
-| otherwise returns 0.
+| Returns 1 if the quadruple-precision floating-point value `a' is a quiet
+| NaN; otherwise returns 0.
 *----------------------------------------------------------------------------*/
 
 int float128_is_nan( float128 a )
 {
-
+#if SNAN_BIT_IS_ONE
+    return
+           ( ( ( a.high>>47 ) & 0xFFFF ) == 0xFFFE )
+        && ( a.low || ( a.high & LIT64( 0x00007FFFFFFFFFFF ) ) );
+#else
     return
            ( LIT64( 0xFFFE000000000000 ) <= (bits64) ( a.high<<1 ) )
         && ( a.low || ( a.high & LIT64( 0x0000FFFFFFFFFFFF ) ) );
-
+#endif
 }
 
 /*----------------------------------------------------------------------------
@@ -415,11 +438,15 @@
 
 int float128_is_signaling_nan( float128 a )
 {
-
+#if SNAN_BIT_IS_ONE
+    return
+           ( LIT64( 0xFFFE000000000000 ) <= (bits64) ( a.high<<1 ) )
+        && ( a.low || ( a.high & LIT64( 0x0000FFFFFFFFFFFF ) ) );
+#else
     return
            ( ( ( a.high>>47 ) & 0xFFFF ) == 0xFFFE )
         && ( a.low || ( a.high & LIT64( 0x00007FFFFFFFFFFF ) ) );
-
+#endif
 }
 
 /*----------------------------------------------------------------------------
@@ -436,7 +463,6 @@
     z.sign = a.high>>63;
     shortShift128Left( a.high, a.low, 16, &z.high, &z.low );
     return z;
-
 }
 
 /*----------------------------------------------------------------------------
@@ -451,7 +477,6 @@
     shift128Right( a.high, a.low, 16, &z.high, &z.low );
     z.high |= ( ( (bits64) a.sign )<<63 ) | LIT64( 0x7FFF800000000000 );
     return z;
-
 }
 
 /*----------------------------------------------------------------------------
@@ -468,8 +493,13 @@
     aIsSignalingNaN = float128_is_signaling_nan( a );
     bIsNaN = float128_is_nan( b );
     bIsSignalingNaN = float128_is_signaling_nan( b );
+#if SNAN_BIT_IS_ONE
+    a.high &= ~LIT64( 0x0000800000000000 );
+    b.high &= ~LIT64( 0x0000800000000000 );
+#else
     a.high |= LIT64( 0x0000800000000000 );
     b.high |= LIT64( 0x0000800000000000 );
+#endif
     if ( aIsSignalingNaN | bIsSignalingNaN ) float_raise( float_flag_invalid 
STATUS_VAR);
     if ( aIsSignalingNaN ) {
         if ( bIsSignalingNaN ) goto returnLargerSignificand;
@@ -485,8 +515,6 @@
     else {
         return b;
     }
-
 }
 
 #endif
-