stdlib: excise the FP16 support routines on x64

Bump the minimum supported CPU to ~Nahelem so that we can take advantage
of the F16C/CVT16 extensions to support half floating point rounding.
This should avoid the need to replicate the compiler-rt functionality in
the runtime and the associated problems with linking compiler-rt
builtins on non-Windows targets when performing static linking.

Author: compnerd

lib/ClangImporter/ClangImporter.cpp

@@ -778,6 +778,12 @@ importer::addCommonInvocationArguments(
     // Enable double wide atomic intrinsics on every x86_64 target.
     // (This is the default on Darwin, but not so on other platforms.)
     invocationArgStrs.push_back("-mcx16");
+    // Enable the FC16/CVT16 extensions which provide half prevision floating
+    // point support on x86_64.  This bumps the minimum requirement of the CPU
+    // to ~Haswell, however, Windows 10 (at least as of 1709) requires
+    // ~Broadwell.  At this point, the complexity of supporting an older release
+    // no longer is justified.
+    invocationArgStrs.push_back("-mf16c");
   }
 
   if (!importerOpts.Optimization.empty()) {

stdlib/cmake/modules/AddSwiftStdlib.cmake

@@ -288,6 +288,18 @@ function(_add_target_variant_c_compile_flags)
     endif()
   endif()
 
+  # Avoid the need for the FP16 truncation and extension routines from
+  # compiler-rt by assuming that we have hardware capable of performing
+  # half-precision floating point conversions.  This was announced circa 2007,
+  # and is standard with ~Nahelem (~2013).
+  if(CFLAGS_ARCH STREQUAL x86_64)
+    if(SWIFT_COMPILER_IS_MSVC_LIKE)
+      list(APPEND result /clang:-mf16c)
+    else()
+      list(APPEND result -mf16c)
+    endif()
+  endif()
+
   if(CFLAGS_ENABLE_ASSERTIONS)
     list(APPEND result "-UNDEBUG")
   else()

stdlib/public/runtime/Float16Support.cpp

@@ -29,8 +29,7 @@
 
 // Android NDK <r21 do not provide `__aeabi_d2h` in the compiler runtime,
 // provide shims in that case.
-#if (defined(__ANDROID__) && defined(__ARM_ARCH_7A__) && defined(__ARM_EABI__)) || \
-  ((defined(__i386__) || defined(__i686__) || defined(__x86_64__)) && !defined(__APPLE__) && !defined(__linux__))
+#if (defined(__ANDROID__) && defined(__ARM_ARCH_7A__) && defined(__ARM_EABI__))
 
 #include "swift/shims/Visibility.h"
 
@@ -156,4 +155,4 @@ SWIFT_RUNTIME_EXPORT unsigned short __aeabi_d2h(double d) {
 }
 #endif
 
-#endif // defined(__x86_64__) && !defined(__APPLE__)
+#endif // defined(__ANDROID__) && defined(__ARM_ARCH_7A__) && defined(__ARM_EABI__)