[wasm] Fix build failure due to lack of _Float16 support

WebAssembly does not support _Float16 type, so we need to guard the use
of the type. Unfortunately, Clang does not provide a good way to detect
the support of _Float16 type at compile time, so just disable for wasm
targets.

Author: kateinoigakukun

include/swift/Runtime/SwiftDtoa.h

@@ -93,6 +93,16 @@
  #define SWIFT_DTOA_BINARY16_SUPPORT 1
 #endif
 
+/// Does this platform support needs to pass _Float16 as a float in
+/// C function?
+#ifndef SWIFT_DTOA_PASS_FLOAT16_AS_FLOAT
+# if !defined(FLT16_MAX) || defined(__wasm__)
+#  define SWIFT_DTOA_PASS_FLOAT16_AS_FLOAT 1
+# else
+#  define SWIFT_DTOA_PASS_FLOAT16_AS_FLOAT 0
+# endif
+#endif
+
 //
 // IEEE 754 Binary32 support (also known as "single-precision")
 //
@@ -239,7 +249,7 @@ extern "C" {
 
 #if SWIFT_DTOA_BINARY16_SUPPORT
 size_t swift_dtoa_optimal_binary16_p(const void *, char *dest, size_t length);
-#if defined FLT16_MAX
+#if !SWIFT_DTOA_PASS_FLOAT16_AS_FLOAT
 // If `_Float16` is defined, provide this convenience wrapper.
 size_t swift_dtoa_optimal_binary16(_Float16, char *dest, size_t length);
 #endif

stdlib/public/core/Runtime.swift

@@ -350,14 +350,20 @@ internal struct _Buffer72 {
 }
 
 #if !((os(macOS) || targetEnvironment(macCatalyst)) && arch(x86_64))
+#if arch(wasm32)
 // Note that this takes a Float32 argument instead of Float16, because clang
 // doesn't have _Float16 on all platforms yet.
+typealias _CFloat16Argument = Float32
+#else
+typealias _CFloat16Argument = Float16
+#endif
+
 @available(SwiftStdlib 5.3, *)
 @_silgen_name("swift_float16ToString")
 internal func _float16ToStringImpl(
   _ buffer: UnsafeMutablePointer<UTF8.CodeUnit>,
   _ bufferLength: UInt,
-  _ value: Float16,
+  _ value: _CFloat16Argument,
   _ debug: Bool
 ) -> Int
 
@@ -370,7 +376,7 @@ internal func _float16ToString(
   _internalInvariant(MemoryLayout<_Buffer32>.size == 32)
   var buffer = _Buffer32()
   let length = buffer.withBytes { (bufferPtr) in
-    _float16ToStringImpl(bufferPtr, 32, value, debug)
+    _float16ToStringImpl(bufferPtr, 32, _CFloat16Argument(value), debug)
   }
   return (buffer, length)
 }

stdlib/public/runtime/SwiftDtoa.cpp

@@ -343,7 +343,7 @@ static size_t nan_details(char *dest, size_t len, int negative, int quiet, uint6
 
 
 #if SWIFT_DTOA_BINARY16_SUPPORT
-#if defined FLT16_MAX
+#if !SWIFT_DTOA_PASS_FLOAT16_AS_FLOAT
 // Format a C `_Float16`
 size_t swift_dtoa_optimal_binary16(_Float16 d, char *dest, size_t length) {
   return swift_dtoa_optimal_binary16_p(&d, dest, length);

stdlib/public/stubs/Stubs.cpp

@@ -173,10 +173,21 @@ static locale_t getCLocale() {
 #endif
 #endif // SWIFT_STDLIB_HAS_LOCALE
 
+#if SWIFT_DTOA_PASS_FLOAT16_AS_FLOAT
+using _CFloat16Argument = float;
+#else
+using _CFloat16Argument = _Float16;
+#endif
+
 SWIFT_CC(swift) SWIFT_RUNTIME_STDLIB_API
 __swift_ssize_t swift_float16ToString(char *Buffer, size_t BufferLength,
-                                      _Float16 Value, bool Debug) {
+                                      _CFloat16Argument Value, bool Debug) {
+#if SWIFT_DTOA_PASS_FLOAT16_AS_FLOAT
+  __fp16 v = Value;
+  return swift_dtoa_optimal_binary16_p(&v, Buffer, BufferLength);
+#else
   return swift_dtoa_optimal_binary16_p(&Value, Buffer, BufferLength);
+#endif
 }
 
 SWIFT_CC(swift) SWIFT_RUNTIME_STDLIB_API

test/stdlib/PrintFloat16.swift

@@ -127,6 +127,9 @@ PrintTests.test("Printable_Float16") {
   expectEqual(Float16.infinity.debugDescription, "inf")
   expectEqual((-Float16.infinity).debugDescription, "-inf")
 
+  // Platforms without float 16 argument passing can cause NaNs to be changed
+  // while being passed.
+  #if !arch(wasm32)
   for bitPattern in (0x7c01 as UInt16) ... 0x7fff {
     expectEqual(Float16(bitPattern: bitPattern).description, "nan")
     expectEqual(Float16(bitPattern: 0x8000 | bitPattern).description, "nan")
@@ -144,6 +147,7 @@ PrintTests.test("Printable_Float16") {
     expectEqual(Float16(bitPattern: bitPattern).debugDescription, expected)
     expectEqual(Float16(bitPattern: 0x8000 | bitPattern).debugDescription, "-\(expected)")
   }
+  #endif
 #endif
 }