[libc] Fix x86_64 fenv implementation for windows

[sivachandra]

Summary

All fenv functions are also enabled for windows. Since two tests,
enabled_exceptions_test and feholdexcept_test are still failing on
windows, they have been disabled.

[aeubanks]

libc/test/src/fenv/CMakeLists.txt

74

add a TODO/FIXME?

libc/config/windows/entrypoints.txt

set(TARGET_LIBC_ENTRYPOINTS
     libc.src.string.strrchr
     libc.src.string.strspn
     libc.src.string.strstr
     libc.src.string.strtok
     libc.src.string.strtok_r
 )
 
 set(TARGET_LIBM_ENTRYPOINTS
+    # fenv.h entrypoints
+    libc.src.fenv.feclearexcept
+    libc.src.fenv.fegetenv
+    libc.src.fenv.fegetexceptflag
+    libc.src.fenv.fegetround
+    libc.src.fenv.feholdexcept
+    libc.src.fenv.fesetenv
+    libc.src.fenv.fesetexceptflag
+    libc.src.fenv.fesetround
+    libc.src.fenv.feraiseexcept
+    libc.src.fenv.fetestexcept
+    libc.src.fenv.feupdateenv
+
     # math.h entrypoints
     libc.src.math.copysign
     libc.src.math.copysignf
     libc.src.math.copysignl
     libc.src.math.fdimf
     libc.src.math.fdim
     libc.src.math.fdiml
     libc.src.math.fmin

libc/test/src/fenv/CMakeLists.txt

SUITE
     libc_fenv_unittests
   SRCS
     feclearexcept_test.cpp
   DEPENDS
     libc.src.fenv.feclearexcept
     libc.utils.FPUtil.fputil
 )
 
-if (NOT LLVM_USE_SANITIZER)
+if (NOT (LLVM_USE_SANITIZER OR (${LIBC_TARGET_OS} STREQUAL "windows")))
   # Sanitizers don't like SIGFPE. So, we will run the 
   # tests which raise SIGFPE only in non-sanitizer builds.
   add_fp_unittest(
     enabled_exceptions_test
     SUITE
       libc_fenv_unittests
     SRCS
       enabled_exceptions_test.cpp

libc/utils/FPUtil/x86_64/FEnvImpl.h

 }
 
 struct X87StateDescriptor {
   uint16_t ControlWord;
   uint16_t Unused1;
   uint16_t StatusWord;
   uint16_t Unused2;
   // TODO: Elaborate the remaining 20 bytes as required.
-#if !(defined(_WIN32))
   uint32_t _[5];
-#endif
 };
 
-struct FPState {
-  X87StateDescriptor X87Status;
-#if !(defined(_WIN32))
-  uint32_t MXCSR;
-#endif
-};
-
-static_assert(
-    sizeof(fenv_t) == sizeof(FPState),
-    "Internal floating point state does not match the public fenv_t type.");
-
 static inline uint16_t getX87ControlWord() {
   uint16_t w;
   __asm__ __volatile__("fnstcw %0" : "=m"(w)::);
   SANITIZER_MEMORY_INITIALIZED(&w, sizeof(w));
   return w;
 }
 
 static inline void writeX87ControlWord(uint16_t w) {
static inline int setRound(int mode) {
   mxcsrControl =
       (mxcsrControl & ~(0x3 << internal::MXCSRRoundingControlBitPosition)) |
       mxcsrValue;
   internal::writeMXCSR(mxcsrControl);
 
   return 0;
 }
 
-#if !(defined(_WIN32))
+namespace internal {
+
+#ifdef _WIN32
+// MSVC fenv.h defines a very simple representation of the floating point state
+// which just consists of control and status words of the x87 unit.
+struct FPState {
+  uint32_t ControlWord;
+  uint32_t StatusWord;
+};
+#else
+struct FPState {
+  X87StateDescriptor X87Status;
+  uint32_t MXCSR;
+};
+#endif // _WIN32
+
+} // namespace internal
+
+static_assert(
+    sizeof(fenv_t) == sizeof(internal::FPState),
+    "Internal floating point state does not match the public fenv_t type.");
+
+#ifdef _WIN32
+static inline int getEnv(fenv_t *envp) {
+  internal::FPState *state = reinterpret_cast<internal::FPState *>(envp);
+  internal::X87StateDescriptor X87Status;
+  internal::getX87StateDescriptor(X87Status);
+  state->ControlWord = X87Status.ControlWord;
+  state->StatusWord = X87Status.StatusWord;
+  return 0;
+}
+
+static inline int setEnv(const fenv_t *envp) {
+  const internal::FPState *state =
+      reinterpret_cast<const internal::FPState *>(envp);
+  internal::X87StateDescriptor X87Status;
+  X87Status.ControlWord = state->ControlWord;
+  X87Status.StatusWord = state->StatusWord;
+  internal::writeX87StateDescriptor(X87Status);
+  return 0;
+}
+#else
 static inline int getEnv(fenv_t *envp) {
   internal::FPState *state = reinterpret_cast<internal::FPState *>(envp);
   internal::getX87StateDescriptor(state->X87Status);
   state->MXCSR = internal::getMXCSR();
   return 0;
 }
 
 static inline int setEnv(const fenv_t *envp) {