From b8b4fb3b3e79d2cd65f72941919c33bb8bd7a6ab Mon Sep 17 00:00:00 2001
From: Jeff Bezanson <jeff.bezanson@gmail.com>
Date: Fri, 5 Jan 2018 14:21:59 -0500
Subject: [PATCH] preserve sign of NaN in conversions to and from BigFloat
 (#25378)

---
 base/mpfr.jl | 27 ++++++++++++++++++++-------
 test/mpfr.jl |  7 +++++++
 2 files changed, 27 insertions(+), 7 deletions(-)

diff --git a/base/mpfr.jl b/base/mpfr.jl
index cf54f930ef252..2338581d14902 100644
--- a/base/mpfr.jl
+++ b/base/mpfr.jl
@@ -97,7 +97,7 @@ widen(::Type{BigFloat}) = BigFloat
 BigFloat(x::BigFloat) = x
 
 # convert to BigFloat
-for (fJ, fC) in ((:si,:Clong), (:ui,:Culong), (:d,:Float64))
+for (fJ, fC) in ((:si,:Clong), (:ui,:Culong))
     @eval begin
         function BigFloat(x::($fC))
             z = BigFloat()
@@ -107,6 +107,16 @@ for (fJ, fC) in ((:si,:Clong), (:ui,:Culong), (:d,:Float64))
     end
 end
 
+function BigFloat(x::Float64)
+    z = BigFloat()
+    ccall((:mpfr_set_d, :libmpfr), Int32, (Ref{BigFloat}, Float64, Int32), z, x, ROUNDING_MODE[])
+    if isnan(x) && signbit(x) != signbit(z)
+        # for some reason doing mpfr_neg in-place doesn't work here
+        return -z
+    end
+    return z
+end
+
 function BigFloat(x::BigInt)
     z = BigFloat()
     ccall((:mpfr_set_z, :libmpfr), Int32, (Ref{BigFloat}, Ref{BigInt}, Int32), z, x, ROUNDING_MODE[])
@@ -247,19 +257,22 @@ function (::Type{T})(x::BigFloat) where T<:Integer
 end
 
 ## BigFloat -> AbstractFloat
+
+_cpynansgn(x::AbstractFloat, y::BigFloat) = isnan(x) && signbit(x) != signbit(y) ? -x : x
+
 Float64(x::BigFloat) =
-    ccall((:mpfr_get_d,:libmpfr), Float64, (Ref{BigFloat}, Int32), x, ROUNDING_MODE[])
+    _cpynansgn(ccall((:mpfr_get_d,:libmpfr), Float64, (Ref{BigFloat}, Int32), x, ROUNDING_MODE[]), x)
 Float32(x::BigFloat) =
-    ccall((:mpfr_get_flt,:libmpfr), Float32, (Ref{BigFloat}, Int32), x, ROUNDING_MODE[])
+    _cpynansgn(ccall((:mpfr_get_flt,:libmpfr), Float32, (Ref{BigFloat}, Int32), x, ROUNDING_MODE[]), x)
 # TODO: avoid double rounding
-Float16(x::BigFloat) = convert(Float16, convert(Float32, x))
+Float16(x::BigFloat) = Float16(Float32(x))
 
 Float64(x::BigFloat, r::RoundingMode) =
-    ccall((:mpfr_get_d,:libmpfr), Float64, (Ref{BigFloat}, Int32), x, to_mpfr(r))
+    _cpynansgn(ccall((:mpfr_get_d,:libmpfr), Float64, (Ref{BigFloat}, Int32), x, to_mpfr(r)), x)
 Float32(x::BigFloat, r::RoundingMode) =
-    ccall((:mpfr_get_flt,:libmpfr), Float32, (Ref{BigFloat}, Int32), x, to_mpfr(r))
+    _cpynansgn(ccall((:mpfr_get_flt,:libmpfr), Float32, (Ref{BigFloat}, Int32), x, to_mpfr(r)), x)
 # TODO: avoid double rounding
-Float16(x::BigFloat, r::RoundingMode) = convert(Float16, Float32(x, r))
+Float16(x::BigFloat, r::RoundingMode) = Float16(Float32(x, r))
 
 promote_rule(::Type{BigFloat}, ::Type{<:Real}) = BigFloat
 promote_rule(::Type{BigInt}, ::Type{<:AbstractFloat}) = BigFloat
diff --git a/test/mpfr.jl b/test/mpfr.jl
index 7511676c5a16b..cd90a75165346 100644
--- a/test/mpfr.jl
+++ b/test/mpfr.jl
@@ -353,6 +353,7 @@ end
     y = BigFloat(-1)
     @test copysign(x, y) == y
     @test copysign(y, x) == x
+    @test copysign(1.0, BigFloat(NaN)) == 1.0
 end
 @testset "isfinite / isinf / isnan" begin
     x = BigFloat(Inf)
@@ -380,6 +381,9 @@ end
     @test typeof(convert(BigFloat, parse(BigInt,"9223372036854775808"))) == BigFloat
     @test convert(AbstractFloat, parse(BigInt,"9223372036854775808")) == parse(BigFloat,"9223372036854775808")
     @test typeof(convert(AbstractFloat, parse(BigInt,"9223372036854775808"))) == BigFloat
+
+    @test signbit(BigFloat(NaN)) == 0
+    @test signbit(BigFloat(-NaN)) == 1
 end
 @testset "convert from BigFloat" begin
     @test convert(Float64, BigFloat(0.5)) == 0.5
@@ -388,6 +392,9 @@ end
     @test convert(Bool, BigFloat(0.0)) == false
     @test convert(Bool, BigFloat(1.0)) == true
     @test_throws InexactError convert(Bool, BigFloat(0.1))
+
+    @test signbit(Float64(BigFloat(NaN))) == 0
+    @test signbit(Float64(-BigFloat(NaN))) == 1
 end
 @testset "exponent, frexp, significand" begin
     x = BigFloat(0)