Revert "remove fldcw/fstcw from Win64 builds"

This reverts commit 60a3cb9ad840377d46286bfd703c30a4a4ee56e2.

Reason for revert: x87 instruction usage might not be as
clear cut as I would like. At the very least, llvm mingw
builds appear to having issues with emms.

Original change's description:
> remove fldcw/fstcw from Win64 builds
>
> _MCW_PC (Precision control) is not supported on x64:
> https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/control87-controlfp-control87-2
>
> The x87 FPU is not used on Win64 or ARM so setting the x87 control word
> is not necessary. The SSE/SSE2 and ARM FPUs don't have a precision
> control - the precision is embedded in each instruction - so the need to
> set the control word is also gone.

BUG=webm:1500

Change-Id: I25bcfa96bc9c860f6c7e03315d75fa6fd1d88ec5

3 files changed, 42 insertions, 5 deletions

diff --git a/vpx_ports/float_control_word.asm b/vpx_ports/float_control_word.asm
new file mode 100644
index 000000000..256dae084
--- /dev/null
+++ b/vpx_ports/float_control_word.asm
@@ -0,0 +1,33 @@
+;
+;  Copyright (c) 2010 The WebM project authors. All Rights Reserved.
+;
+;  Use of this source code is governed by a BSD-style license
+;  that can be found in the LICENSE file in the root of the source
+;  tree. An additional intellectual property rights grant can be found
+;  in the file PATENTS.  All contributing project authors may
+;  be found in the AUTHORS file in the root of the source tree.
+;
+
+
+%include "vpx_ports/x86_abi_support.asm"
+
+section .text
+
+%if LIBVPX_YASM_WIN64
+global sym(vpx_winx64_fldcw) PRIVATE
+sym(vpx_winx64_fldcw):
+    sub   rsp, 8
+    mov   [rsp], rcx ; win x64 specific
+    fldcw [rsp]
+    add   rsp, 8
+    ret
+
+
+global sym(vpx_winx64_fstcw) PRIVATE
+sym(vpx_winx64_fstcw):
+    sub   rsp, 8
+    fstcw [rsp]
+    mov   rax, [rsp]
+    add   rsp, 8
+    ret
+%endif
diff --git a/vpx_ports/vpx_ports.mk b/vpx_ports/vpx_ports.mk
index 9a6616a81..9299fa0ca 100644
--- a/vpx_ports/vpx_ports.mk
+++ b/vpx_ports/vpx_ports.mk
@@ -21,6 +21,10 @@ ifeq ($(ARCH_X86),yes)
 PORTS_SRCS-$(HAVE_MMX) += emms_mmx.c
 endif
 
+ifeq ($(ARCH_X86_64),yes)
+PORTS_SRCS-$(CONFIG_MSVS) += float_control_word.asm
+endif
+
 ifeq ($(ARCH_X86)$(ARCH_X86_64),yes)
 PORTS_SRCS-yes += x86.h
 PORTS_SRCS-yes += x86_abi_support.asm
diff --git a/vpx_ports/x86.h b/vpx_ports/x86.h
index 08295ceb7..4b053b236 100644
--- a/vpx_ports/x86.h
+++ b/vpx_ports/x86.h
@@ -295,11 +295,11 @@ static unsigned short x87_get_control_word(void) {
   return mode;
 }
 #elif ARCH_X86_64
-// Unsupported on Win64:
-// https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/control87-controlfp-control87-2
-// _MCW_PC (Precision control) (Not supported on ARM or x64 platforms.)
-static void x87_set_control_word(unsigned int mode) { (void)mode; }
-static unsigned int x87_get_control_word(void) { return 0; }
+/* No fldcw intrinsics on Windows x64, punt to external asm */
+extern void vpx_winx64_fldcw(unsigned short mode);
+extern unsigned short vpx_winx64_fstcw(void);
+#define x87_set_control_word vpx_winx64_fldcw
+#define x87_get_control_word vpx_winx64_fstcw
 #else
 static void x87_set_control_word(unsigned short mode) {
   __asm { fldcw mode }