linux-kernel - [PATCH v3 13/16] MIPS: math-emu: <MADDF|MSUBF>.<D|S>: Fix some cases of zero inputs

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Date:   Fri, 21 Jul 2017 16:09:11 +0200
From:   Aleksandar Markovic <aleksandar.markovic@...rk.com>
To:     linux-mips@...ux-mips.org
Cc:     Aleksandar Markovic <aleksandar.markovic@...tec.com>,
        Miodrag Dinic <miodrag.dinic@...tec.com>,
        Goran Ferenc <goran.ferenc@...tec.com>,
        Douglas Leung <douglas.leung@...tec.com>,
        James Hogan <james.hogan@...tec.com>,
        linux-kernel@...r.kernel.org, Paul Burton <paul.burton@...tec.com>,
        Petar Jovanovic <petar.jovanovic@...tec.com>,
        Raghu Gandham <raghu.gandham@...tec.com>,
        Ralf Baechle <ralf@...ux-mips.org>
Subject: [PATCH v3 13/16] MIPS: math-emu: <MADDF|MSUBF>.<D|S>: Fix some cases of zero inputs

From: Aleksandar Markovic <aleksandar.markovic@...tec.com>

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.

The 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).

Signed-off-by: Miodrag Dinic <miodrag.dinic@...tec.com>
Signed-off-by: Goran Ferenc <goran.ferenc@...tec.com>
Signed-off-by: Aleksandar Markovic <aleksandar.markovic@...tec.com>
---
 arch/mips/math-emu/dp_maddf.c | 8 ++++++++
 arch/mips/math-emu/sp_maddf.c | 8 ++++++++
 2 files changed, 16 insertions(+)

diff --git a/arch/mips/math-emu/dp_maddf.c b/arch/mips/math-emu/dp_maddf.c
index 45f815d..b8b2c17 100644
--- a/arch/mips/math-emu/dp_maddf.c
+++ b/arch/mips/math-emu/dp_maddf.c
@@ -113,6 +113,14 @@ 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);
+		/* Handle cases +0 + (-0) and similar ones. */
+		if (zc == IEEE754_CLASS_ZERO) {
+			if ((!(flags & maddf_negate_product) && (zs == (xs ^ ys))) ||
+			    ((flags & maddf_negate_product) && (zs != (xs ^ ys))))
+				return z;
+			else
+				return ieee754dp_zero(ieee754_csr.rm == FPU_CSR_RD);
+		}
 		/* Multiplication is 0 so just return z */
 		return z;
 
diff --git a/arch/mips/math-emu/sp_maddf.c b/arch/mips/math-emu/sp_maddf.c
index 76856d7..cb8597b 100644
--- a/arch/mips/math-emu/sp_maddf.c
+++ b/arch/mips/math-emu/sp_maddf.c
@@ -114,6 +114,14 @@ 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);
+		/* Handle cases +0 + (-0) and similar ones. */
+		if (zc == IEEE754_CLASS_ZERO) {
+			if ((!(flags & maddf_negate_product) && (zs == (xs ^ ys))) ||
+			    ((flags & maddf_negate_product) && (zs != (xs ^ ys))))
+				return z;
+			else
+				return ieee754sp_zero(ieee754_csr.rm == FPU_CSR_RD);
+		}
 		/* Multiplication is 0 so just return z */
 		return z;
 
-- 
2.7.4