math: fix Abs, Copysign and Signbit benchmarks

CL 62250 makes constant folding a bit more aggressive and these
benchmarks were optimized away. This CL adds some indirection to
the function arguments to stop them being folded.

The Copysign benchmark is a bit faster because I've left one
argument as a constant and it can be partially folded.

                     old           CL 62250     this CL
Copysign             1.24ns ± 0%   0.34ns ± 2%  1.02ns ± 2%
Abs                  0.67ns ± 0%   0.35ns ± 3%  0.67ns ± 0%
Signbit              0.87ns ± 0%   0.35ns ± 2%  0.87ns ± 1%

Change-Id: I9604465a87d7aa29f4bd6009839c8ee354be3cd7
Reviewed-on: https://go-review.googlesource.com/62450
Run-TryBot: Michael Munday <mike.munday@ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/math/all_test.go

@@ -3068,10 +3068,12 @@ func BenchmarkCeil(b *testing.B) {
 	GlobalF = x
 }
 
+var copysignNeg = -1.0
+
 func BenchmarkCopysign(b *testing.B) {
 	x := 0.0
 	for i := 0; i < b.N; i++ {
-		x = Copysign(.5, -1)
+		x = Copysign(.5, copysignNeg)
 	}
 	GlobalF = x
 }
@@ -3164,10 +3166,12 @@ func BenchmarkExp2Go(b *testing.B) {
 	GlobalF = x
 }
 
+var absPos = .5
+
 func BenchmarkAbs(b *testing.B) {
 	x := 0.0
 	for i := 0; i < b.N; i++ {
-		x = Abs(.5)
+		x = Abs(absPos)
 	}
 	GlobalF = x
 
@@ -3417,10 +3421,12 @@ func BenchmarkRemainder(b *testing.B) {
 	GlobalF = x
 }
 
+var signbitPos = 2.5
+
 func BenchmarkSignbit(b *testing.B) {
 	x := false
 	for i := 0; i < b.N; i++ {
-		x = Signbit(2.5)
+		x = Signbit(signbitPos)
 	}
 	GlobalB = x
 }