Fix `float approxExp2Floor()` to work with negative arguments and arguments above 30.

include/common.hpp

@@ -164,6 +164,17 @@ namespace std {
 namespace rack {
 
 
+/** Casts a primitive, preserving its bits instead of converting. */
+template <typename To, typename From>
+To bitCast(From from) {
+	static_assert(sizeof(From) == sizeof(To), "Types must be the same size");
+	To to;
+	// This is optimized out
+	std::memcpy(&to, &from, sizeof(From));
+	return to;
+}
+
+
 /** C#-style property constructor
 Example:
 
@@ -181,6 +192,7 @@ T* construct(F f, V v, Args... args) {
 	return o;
 }
 
+
 /** Defers code until the scope is destructed
 From http://www.gingerbill.org/article/defer-in-cpp.html
 Example:
@@ -244,6 +256,7 @@ typename C::mapped_type get(const C& m, const typename C::key_type& key, const t
 	return it->second;
 }
 
+
 // config
 
 extern const std::string APP_NAME;

include/dsp/approx.hpp

@@ -39,10 +39,11 @@ inline simd::float_4 approxExp2Floor(simd::float_4 x, simd::float_4* xf) {
 
 template <>
 inline float approxExp2Floor(float x, float* xf) {
-	int xi = x;
+	int32_t xi = x;
 	if (xf)
 		*xf = x - xi;
-	return 1 << xi;
+	int32_t y = (xi + 127) << 23;
+	return bitCast<float>(y);
 }