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

Fix the cases of <MADDF|MSUBF>.<D|S> when any of two multiplicands is
+0 or -0, and the third input is also +0 or -0. Depending on the signs
of inputs, certain special cases must be handled.

A relevant example:

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

Fixes: e24c3bec3e8e ("MIPS: math-emu: Add support for the MIPS R6 MADDF FPU instruction")
Fixes: 83d43305a1df ("MIPS: math-emu: Add support for the MIPS R6 MSUBF FPU instruction")
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: Bo Hu <bohu@google.com>
Cc: Douglas Leung <douglas.leung@imgtec.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/16888/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

diff --git a/arch/mips/math-emu/dp_maddf.c b/arch/mips/math-emu/dp_maddf.c
index 557a0a11882702709ec1d767337d257c6f7e8c14..c38fe1bde8753b2c7b6d79673ceb699e02e5ec27 100644 (file)
--- a/arch/mips/math-emu/dp_maddf.c
+++ b/arch/mips/math-emu/dp_maddf.c
@@ -113,7 +113,23 @@ static union ieee754dp _dp_maddf(union ieee754dp z, union ieee754dp x,
        case CLPAIR(IEEE754_CLASS_DNORM, IEEE754_CLASS_ZERO):
                if (zc == IEEE754_CLASS_INF)
                        return ieee754dp_inf(zs);
-               /* Multiplication is 0 so just return z */
+               if (zc == IEEE754_CLASS_ZERO) {
+                       /* Handle cases +0 + (-0) and similar ones. */
+                       if ((!(flags & maddf_negate_product)
+                                       && (zs == (xs ^ ys))) ||
+                           ((flags & maddf_negate_product)
+                                       && (zs != (xs ^ ys))))
+                               /*
+                                * Cases of addition of zeros of equal signs
+                                * or subtraction of zeroes of opposite signs.
+                                * The sign of the resulting zero is in any
+                                * such case determined only by the sign of z.
+                                */
+                               return z;
+
+                       return ieee754dp_zero(ieee754_csr.rm == FPU_CSR_RD);
+               }
+               /* x*y is here 0, and z is not 0, so just return z */
                return z;
 
        case CLPAIR(IEEE754_CLASS_DNORM, IEEE754_CLASS_DNORM):

diff --git a/arch/mips/math-emu/sp_maddf.c b/arch/mips/math-emu/sp_maddf.c
index 0d8d25f4b737f209bbcd13b889439507b5e7dee1..4241ec19df1253fdea51273fae4f7ee09d5a02d0 100644 (file)
--- a/arch/mips/math-emu/sp_maddf.c
+++ b/arch/mips/math-emu/sp_maddf.c
@@ -114,7 +114,23 @@ static union ieee754sp _sp_maddf(union ieee754sp z, union ieee754sp x,
        case CLPAIR(IEEE754_CLASS_DNORM, IEEE754_CLASS_ZERO):
                if (zc == IEEE754_CLASS_INF)
                        return ieee754sp_inf(zs);
-               /* Multiplication is 0 so just return z */
+               if (zc == IEEE754_CLASS_ZERO) {
+                       /* Handle cases +0 + (-0) and similar ones. */
+                       if ((!(flags & maddf_negate_product)
+                                       && (zs == (xs ^ ys))) ||
+                           ((flags & maddf_negate_product)
+                                       && (zs != (xs ^ ys))))
+                               /*
+                                * Cases of addition of zeros of equal signs
+                                * or subtraction of zeroes of opposite signs.
+                                * The sign of the resulting zero is in any
+                                * such case determined only by the sign of z.
+                                */
+                               return z;
+
+                       return ieee754sp_zero(ieee754_csr.rm == FPU_CSR_RD);
+               }
+               /* x*y is here 0, and z is not 0, so just return z */
                return z;
 
        case CLPAIR(IEEE754_CLASS_DNORM, IEEE754_CLASS_DNORM):