runtime: fix software FP regs corruption when emulating SQRT on ARM

When emulating ARM FSQRT instruction, the sqrt function itself
should not use any floating point arithmetics, otherwise it will
clobber the user software FP registers.

Fortunately, the sqrt function only uses floating point instructions
to test for corner cases, so it's easy to make that function does
all it job using pure integer arithmetic only. I've verified that
after this change, runtime.stepflt and runtime.sqrt doesn't contain
any call to _sfloat. (Perhaps we should add //go:nosfloat to make
the compiler enforce this?)

Fixes #10641.

Change-Id: Ida4742c49000fae4fea4649f28afde630ce4c576
Signed-off-by: Shenghou Ma <minux@golang.org>
Reviewed-on: https://go-review.googlesource.com/9570Reviewed-by: Dave Cheney <dave@cheney.net>
Reviewed-by: Keith Randall <khr@golang.org>

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

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")

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
 }

src/runtime/sqrt_test.go

+// 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(),
+}