4 files changed, 108 insertions, 13 deletions
diff --git a/src/runtime/export_test.go b/src/runtime/export_test.go
index 1b5f267b8b..e0c8b17bd3 100644
--- a/src/runtime/export_test.go
+++ b/src/runtime/export_test.go
@@ -17,6 +17,7 @@ var F32to64 = f32to64
 var Fcmp64 = fcmp64
 var Fintto64 = fintto64
 var F64toint = f64toint
+var Sqrt = sqrt
 
 var Entersyscall = entersyscall
 var Exitsyscall = exitsyscall
diff --git a/src/runtime/softfloat_arm.go b/src/runtime/softfloat_arm.go
index 8f184ccf3b..4e97e13f1a 100644
--- a/src/runtime/softfloat_arm.go
+++ b/src/runtime/softfloat_arm.go
@@ -437,8 +437,7 @@ stage3: // regd, regm are 4bit variables
 		break
 
 	case 0xeeb10bc0: // D[regd] = sqrt D[regm]
-		uval = float64bits(sqrt(float64frombits(fgetd(regm))))
-		fputd(regd, uval)
+		fputd(regd, sqrt(fgetd(regm)))
 
 		if fptrace > 0 {
 			print("*** D[", regd, "] = sqrt D[", regm, "] ", hex(m.freghi[regd]), "-", hex(m.freglo[regd]), "\n")
diff --git a/src/runtime/sqrt.go b/src/runtime/sqrt.go
index d483f8a01f..7452a61f3c 100644
--- a/src/runtime/sqrt.go
+++ b/src/runtime/sqrt.go
@@ -3,11 +3,12 @@
 // license that can be found in the LICENSE file.
 
 // Copy of math/sqrt.go, here for use by ARM softfloat.
+// Modified to not use any floating point arithmetic so
+// that we don't clobber any floating-point registers
+// while emulating the sqrt instruction.
 
 package runtime
 
-import "unsafe"
-
 // The original C code and the long comment below are
 // from FreeBSD's /usr/src/lib/msun/src/e_sqrt.c and
 // came with this notice.  The go code is a simplified
@@ -89,21 +90,30 @@ const (
 	float64Mask  = 0x7FF
 	float64Shift = 64 - 11 - 1
 	float64Bias  = 1023
+	float64NaN   = 0x7FF8000000000001
+	float64Inf   = 0x7FF0000000000000
 	maxFloat64   = 1.797693134862315708145274237317043567981e+308 // 2**1023 * (2**53 - 1) / 2**52
 )
 
-func float64bits(f float64) uint64     { return *(*uint64)(unsafe.Pointer(&f)) }
-func float64frombits(b uint64) float64 { return *(*float64)(unsafe.Pointer(&b)) }
+// isnanu returns whether ix represents a NaN floating point number.
+func isnanu(ix uint64) bool {
+	exp := (ix >> float64Shift) & float64Mask
+	sig := ix << (64 - float64Shift) >> (64 - float64Shift)
+	return exp == float64Mask && sig != 0
+}
 
-func sqrt(x float64) float64 {
+func sqrt(ix uint64) uint64 {
 	// special cases
 	switch {
-	case x == 0 || x != x || x > maxFloat64:
-		return x
-	case x < 0:
-		return nan()
+	case ix == 0 || ix == 1<<63: // x == 0
+		return ix
+	case isnanu(ix): // x != x
+		return ix
+	case ix&(1<<63) != 0: // x < 0
+		return float64NaN
+	case ix == float64Inf: // x > MaxFloat
+		return ix
 	}
-	ix := float64bits(x)
 	// normalize x
 	exp := int((ix >> float64Shift) & float64Mask)
 	if exp == 0 { // subnormal x
@@ -139,5 +149,5 @@ func sqrt(x float64) float64 {
 		q += q & 1 // round according to extra bit
 	}
 	ix = q>>1 + uint64(exp-1+float64Bias)<<float64Shift // significand + biased exponent
-	return float64frombits(ix)
+	return ix
 }
diff --git a/src/runtime/sqrt_test.go b/src/runtime/sqrt_test.go
new file mode 100644
index 0000000000..f1a6e8369d
--- /dev/null
+++ b/src/runtime/sqrt_test.go
@@ -0,0 +1,85 @@
+// Copyright 2015 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// A copy of Sqrt tests from the math package to test the
+// purely integer arithmetic implementaiton in sqrt.go.
+
+package runtime_test
+
+import (
+	"math"
+	"runtime"
+	"testing"
+)
+
+func SqrtRT(x float64) float64 {
+	return math.Float64frombits(runtime.Sqrt(math.Float64bits(x)))
+}
+
+func TestSqrt(t *testing.T) {
+	for i := 0; i < len(vf); i++ {
+		a := math.Abs(vf[i])
+		if f := SqrtRT(a); sqrt[i] != f {
+			t.Errorf("Sqrt(%g) = %g, want %g", a, f, sqrt[i])
+		}
+	}
+	for i := 0; i < len(vfsqrtSC); i++ {
+		if f := SqrtRT(vfsqrtSC[i]); !alike(sqrtSC[i], f) {
+			t.Errorf("Sqrt(%g) = %g, want %g", vfsqrtSC[i], f, sqrtSC[i])
+		}
+	}
+}
+
+func alike(a, b float64) bool {
+	switch {
+	case math.IsNaN(a) && math.IsNaN(b):
+		return true
+	case a == b:
+		return math.Signbit(a) == math.Signbit(b)
+	}
+	return false
+}
+
+var vf = []float64{
+	4.9790119248836735e+00,
+	7.7388724745781045e+00,
+	-2.7688005719200159e-01,
+	-5.0106036182710749e+00,
+	9.6362937071984173e+00,
+	2.9263772392439646e+00,
+	5.2290834314593066e+00,
+	2.7279399104360102e+00,
+	1.8253080916808550e+00,
+	-8.6859247685756013e+00,
+}
+
+var sqrt = []float64{
+	2.2313699659365484748756904e+00,
+	2.7818829009464263511285458e+00,
+	5.2619393496314796848143251e-01,
+	2.2384377628763938724244104e+00,
+	3.1042380236055381099288487e+00,
+	1.7106657298385224403917771e+00,
+	2.286718922705479046148059e+00,
+	1.6516476350711159636222979e+00,
+	1.3510396336454586262419247e+00,
+	2.9471892997524949215723329e+00,
+}
+
+var vfsqrtSC = []float64{
+	math.Inf(-1),
+	-math.Pi,
+	math.Copysign(0, -1),
+	0,
+	math.Inf(1),
+	math.NaN(),
+}
+var sqrtSC = []float64{
+	math.NaN(),
+	math.NaN(),
+	math.Copysign(0, -1),
+	0,
+	math.Inf(1),
+	math.NaN(),
+}