TLV reader: Improve floating-point support

1. For some reason the Get(float&) signature was declared in the .h file
   but was never defined. Define it as well, extracting a helper
   function to share logic between Get(float&) and Get(double&).
2. It is undefined behaviour to use unions for type-punning in C++. Use
   raw memcpy calls instead, which is defined by the C++ standard
   (std::bit_cast is not available).

   Compilers will generally be able to optimize this away unless a
   freestanding implementation (with possibly non-standard memcpy) is
   used.

src/lib/core/CHIPTLVReader.cpp

@@ -207,35 +207,45 @@ CHIP_ERROR TLVReader::Get(uint64_t & v)
     return CHIP_NO_ERROR;
 }
 
-CHIP_ERROR TLVReader::Get(double & v)
+namespace {
+template <typename T>
+CHIP_ERROR GetFloatingImpl(const TLVElementType elementType, const uint64_t elemLenOrVal, T & v)
 {
-    switch (ElementType())
+    switch (elementType)
     {
     case TLVElementType::FloatingPointNumber32: {
-        union
-        {
-            uint32_t u32;
-            float f;
-        } cvt;
-        cvt.u32 = static_cast<uint32_t>(mElemLenOrVal);
-        v       = cvt.f;
+        float f;
+        auto u32 = static_cast<uint32_t>(elemLenOrVal);
+        memcpy(&f, &u32, sizeof(f));
+        v = f;
         break;
     }
     case TLVElementType::FloatingPointNumber64: {
-        union
-        {
-            uint64_t u64;
-            double d;
-        } cvt;
-        cvt.u64 = mElemLenOrVal;
-        v       = cvt.d;
+        double d;
+        memcpy(&d, &elemLenOrVal, sizeof(d));
+        v = static_cast<T>(d);
         break;
     }
     default:
         return CHIP_ERROR_WRONG_TLV_TYPE;
     }
     return CHIP_NO_ERROR;
 }
+} // namespace
+
+// Note: Unlike the integer Get functions, this code avoids doing conversions
+// between float and double wherever possible, because these conversions are
+// relatively expensive on platforms that use soft-float instruction sets.
+
+CHIP_ERROR TLVReader::Get(float & v)
+{
+    return GetFloatingImpl(ElementType(), mElemLenOrVal, v);
+}
+
+CHIP_ERROR TLVReader::Get(double & v)
+{
+    return GetFloatingImpl(ElementType(), mElemLenOrVal, v);
+}
 
 CHIP_ERROR TLVReader::GetBytes(uint8_t * buf, uint32_t bufSize)
 {

src/lib/core/tests/TestCHIPTLV.cpp

@@ -582,11 +582,12 @@ void ReadEncoding1(nlTestSuite * inSuite, TLVReader & reader)
 
         TestNext<TLVReader>(inSuite, reader2);
 
-        TestGet<TLVReader, double>(inSuite, reader2, kTLVType_FloatingPointNumber, ProfileTag(TestProfile_2, 65535),
-                                   static_cast<float>(17.9));
+        TestGet<TLVReader, float>(inSuite, reader2, kTLVType_FloatingPointNumber, ProfileTag(TestProfile_2, 65535), 17.9f);
+        TestGet<TLVReader, double>(inSuite, reader2, kTLVType_FloatingPointNumber, ProfileTag(TestProfile_2, 65535), 17.9f);
 
         TestNext<TLVReader>(inSuite, reader2);
 
+        TestGet<TLVReader, float>(inSuite, reader2, kTLVType_FloatingPointNumber, ProfileTag(TestProfile_2, 65536), 17.9f);
         TestGet<TLVReader, double>(inSuite, reader2, kTLVType_FloatingPointNumber, ProfileTag(TestProfile_2, 65536), 17.9);
 
         TestEndAndCloseContainer(inSuite, reader, reader2);
@@ -2966,11 +2967,12 @@ void TestCHIPTLVReaderDup(nlTestSuite * inSuite)
 
         TestNext<TLVReader>(inSuite, reader2);
 
-        TestGet<TLVReader, double>(inSuite, reader2, kTLVType_FloatingPointNumber, ProfileTag(TestProfile_2, 65535),
-                                   static_cast<float>(17.9));
+        TestGet<TLVReader, float>(inSuite, reader2, kTLVType_FloatingPointNumber, ProfileTag(TestProfile_2, 65535), 17.9f);
+        TestGet<TLVReader, double>(inSuite, reader2, kTLVType_FloatingPointNumber, ProfileTag(TestProfile_2, 65535), 17.9f);
 
         TestNext<TLVReader>(inSuite, reader2);
 
+        TestGet<TLVReader, float>(inSuite, reader2, kTLVType_FloatingPointNumber, ProfileTag(TestProfile_2, 65536), 17.9f);
         TestGet<TLVReader, double>(inSuite, reader2, kTLVType_FloatingPointNumber, ProfileTag(TestProfile_2, 65536), 17.9);
 
         TestEndAndCloseContainer(inSuite, reader, reader2);
@@ -2995,6 +2997,11 @@ void TestCHIPTLVReaderErrorHandling(nlTestSuite * inSuite)
     err = reader.Get(val);
     NL_TEST_ASSERT(inSuite, err == CHIP_ERROR_WRONG_TLV_TYPE);
 
+    // Get(float&)
+    float numF;
+    err = reader.Get(numF);
+    NL_TEST_ASSERT(inSuite, err == CHIP_ERROR_WRONG_TLV_TYPE);
+
     // Get(double&)
     double numD;
     err = reader.Get(numD);