MIPS: math-emu: <MADDF|MSUBF>.<D|S>: Fix some cases of infinite inputs

Fix the cases of <MADDF|MSUBF>.<D|S> when any of two multiplicands is
infinity. The correct behavior in such cases is affected by the nature
of third input. Cases of addition of infinities with opposite signs
and subtraction of infinities with same signs may arise and must be
handles separately. Also, the value od flags argument (that determines
whether the instruction is MADDF or MSUBF) affects the outcome.

Relevant examples:

MADDF.S fd,fs,ft:
  If fs contains +inf, ft contains +inf, and fd contains -inf, fd is
  going to contain indef (without this patch, it used to contain
  -inf).

MSUBF.S fd,fs,ft:
  If fs contains +inf, ft contains 1.0, and fd contains +0.0, fd is
  going to contain -inf (without this patch, it used to contain +inf).

Fixes: e24c3bec3e ("MIPS: math-emu: Add support for the MIPS R6 MADDF FPU instruction")
Fixes: 83d43305a1 ("MIPS: math-emu: Add support for the MIPS R6 MSUBF FPU instruction")

Signed-off-by: Douglas Leung <douglas.leung@imgtec.com>
Signed-off-by: Miodrag Dinic <miodrag.dinic@imgtec.com>
Signed-off-by: Goran Ferenc <goran.ferenc@imgtec.com>
Signed-off-by: Aleksandar Markovic <aleksandar.markovic@imgtec.com>
Reviewed-by: James Hogan <james.hogan@imgtec.com>
Cc: Douglas Leung <douglas.leung@imgtec.com>
Cc: Bo Hu <bohu@google.com>
Cc: Jin Qian <jinqian@google.com>
Cc: Paul Burton <paul.burton@imgtec.com>
Cc: Petar Jovanovic <petar.jovanovic@imgtec.com>
Cc: Raghu Gandham <raghu.gandham@imgtec.com>
Cc: <stable@vger.kernel.org> # 4.7+
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/16887/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/math-emu/dp_maddf.c

@@ -84,7 +84,27 @@ static union ieee754dp _dp_maddf(union ieee754dp z, union ieee754dp x,
 	case CLPAIR(IEEE754_CLASS_INF, IEEE754_CLASS_NORM):
 	case CLPAIR(IEEE754_CLASS_INF, IEEE754_CLASS_DNORM):
 	case CLPAIR(IEEE754_CLASS_INF, IEEE754_CLASS_INF):
-		return ieee754dp_inf(xs ^ ys);
+		if ((zc == IEEE754_CLASS_INF) &&
+		    ((!(flags & maddf_negate_product) && (zs != (xs ^ ys))) ||
+		     ((flags & maddf_negate_product) && (zs == (xs ^ ys))))) {
+			/*
+			 * Cases of addition of infinities with opposite signs
+			 * or subtraction of infinities with same signs.
+			 */
+			ieee754_setcx(IEEE754_INVALID_OPERATION);
+			return ieee754dp_indef();
+		}
+		/*
+		 * z is here either not an infinity, or an infinity having the
+		 * same sign as product (x*y) (in case of MADDF.D instruction)
+		 * or product -(x*y) (in MSUBF.D case). The result must be an
+		 * infinity, and its sign is determined only by the value of
+		 * (flags & maddf_negate_product) and the signs of x and y.
+		 */
+		if (flags & maddf_negate_product)
+			return ieee754dp_inf(1 ^ (xs ^ ys));
+		else
+			return ieee754dp_inf(xs ^ ys);
 
 	case CLPAIR(IEEE754_CLASS_ZERO, IEEE754_CLASS_ZERO):
 	case CLPAIR(IEEE754_CLASS_ZERO, IEEE754_CLASS_NORM):

arch/mips/math-emu/sp_maddf.c

@@ -85,7 +85,27 @@ static union ieee754sp _sp_maddf(union ieee754sp z, union ieee754sp x,
 	case CLPAIR(IEEE754_CLASS_INF, IEEE754_CLASS_NORM):
 	case CLPAIR(IEEE754_CLASS_INF, IEEE754_CLASS_DNORM):
 	case CLPAIR(IEEE754_CLASS_INF, IEEE754_CLASS_INF):
-		return ieee754sp_inf(xs ^ ys);
+		if ((zc == IEEE754_CLASS_INF) &&
+		    ((!(flags & maddf_negate_product) && (zs != (xs ^ ys))) ||
+		     ((flags & maddf_negate_product) && (zs == (xs ^ ys))))) {
+			/*
+			 * Cases of addition of infinities with opposite signs
+			 * or subtraction of infinities with same signs.
+			 */
+			ieee754_setcx(IEEE754_INVALID_OPERATION);
+			return ieee754sp_indef();
+		}
+		/*
+		 * z is here either not an infinity, or an infinity having the
+		 * same sign as product (x*y) (in case of MADDF.D instruction)
+		 * or product -(x*y) (in MSUBF.D case). The result must be an
+		 * infinity, and its sign is determined only by the value of
+		 * (flags & maddf_negate_product) and the signs of x and y.
+		 */
+		if (flags & maddf_negate_product)
+			return ieee754sp_inf(1 ^ (xs ^ ys));
+		else
+			return ieee754sp_inf(xs ^ ys);
 
 	case CLPAIR(IEEE754_CLASS_ZERO, IEEE754_CLASS_ZERO):
 	case CLPAIR(IEEE754_CLASS_ZERO, IEEE754_CLASS_NORM):