MIPS: math-emu: Add IEEE Std 754-2008 NaN encoding emulation

Implement IEEE Std 754-2008 NaN encoding wired to the state of the
FCSR.NAN2008 bit.  Make the interpretation of the quiet bit in NaN data
as follows:

* in the legacy mode originally defined by the MIPS architecture the
  value of 1 denotes an sNaN whereas the value of 0 denotes a qNaN,

* in the 2008 mode introduced with revision 5 of the MIPS architecture
  the value of 0 denotes an sNaN whereas the value of 1 denotes a qNaN,
  following the definition of the preferred NaN encoding introduced with
  IEEE Std 754-2008.

In the 2008 mode, following the requirement of the said standard, quiet
an sNaN where needed by setting the quiet bit to 1 and leaving all the
NaN payload bits unchanged.

Update format conversion operations according to the rules set by IEEE
Std 754-2008 and the MIPS architecture.  Specifically:

* propagate NaN payload bits through conversions between floating-point
  formats such that as much information as possible is preserved and
  specifically a conversion from a narrower format to a wider format and
  then back to the original format does not change a qNaN payload in any
  way,

* conversions from a floating-point to an integer format where the
  source is a NaN, infinity or a value that would convert to an integer
  outside the range of the result format produce, under the default
  exception handling, the respective values defined by the MIPS
  architecture.

In full FPU emulation set the FIR.HAS2008 bit to 1, however do not make
any further FCSR bits writable.

Signed-off-by: Maciej W. Rozycki <macro@imgtec.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Matthew Fortune <Matthew.Fortune@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/11477/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

1 files changed, 6 insertions, 3 deletions

diff --git a/arch/mips/math-emu/sp_tint.c b/arch/mips/math-emu/sp_tint.c
index 091299a31798..f4b4cabfe2e1 100644
--- a/arch/mips/math-emu/sp_tint.c
+++ b/arch/mips/math-emu/sp_tint.c
@@ -38,10 +38,13 @@ int ieee754sp_tint(union ieee754sp x)
 	switch (xc) {
 	case IEEE754_CLASS_SNAN:
 	case IEEE754_CLASS_QNAN:
-	case IEEE754_CLASS_INF:
 		ieee754_setcx(IEEE754_INVALID_OPERATION);
 		return ieee754si_indef();
 
+	case IEEE754_CLASS_INF:
+		ieee754_setcx(IEEE754_INVALID_OPERATION);
+		return ieee754si_overflow(xs);
+
 	case IEEE754_CLASS_ZERO:
 		return 0;
 
@@ -56,7 +59,7 @@ int ieee754sp_tint(union ieee754sp x)
 		/* Set invalid. We will only use overflow for floating
 		   point overflow */
 		ieee754_setcx(IEEE754_INVALID_OPERATION);
-		return ieee754si_indef();
+		return ieee754si_overflow(xs);
 	}
 	/* oh gawd */
 	if (xe > SP_FBITS) {
@@ -97,7 +100,7 @@ int ieee754sp_tint(union ieee754sp x)
 		if ((xm >> 31) != 0) {
 			/* This can happen after rounding */
 			ieee754_setcx(IEEE754_INVALID_OPERATION);
-			return ieee754si_indef();
+			return ieee754si_overflow(xs);
 		}
 		if (round || sticky)
 			ieee754_setcx(IEEE754_INEXACT);