diff --git a/third_party/x86inc/README.libvpx b/third_party/x86inc/README.libvpx
index fcb49e16c..728c0ebd8 100644
--- a/third_party/x86inc/README.libvpx
+++ b/third_party/x86inc/README.libvpx
@@ -10,3 +10,5 @@ defines that help automatically allow assembly to work cross-platform.
 Local Modifications:
 Get configuration from vpx_config.asm.
 Prefix functions with vpx by default.
+Manage name mangling (prefixing with '_') manually because 'PREFIX' does not
+  exist in libvpx.
diff --git a/third_party/x86inc/x86inc.asm.new b/third_party/x86inc/x86inc.asm.new
index fbd95a632..b4c30ca7d 100644
--- a/third_party/x86inc/x86inc.asm.new
+++ b/third_party/x86inc/x86inc.asm.new
@@ -66,10 +66,16 @@
     %endif
 %endif
 
-%ifdef PREFIX
-    %define mangle(x) _ %+ x
-%else
+%ifidn   __OUTPUT_FORMAT__,elf32
     %define mangle(x) x
+%elifidn __OUTPUT_FORMAT__,elf64
+    %define mangle(x) x
+%elifidn __OUTPUT_FORMAT__,x64
+    %define mangle(x) x
+%elifidn __OUTPUT_FORMAT__,win64
+    %define mangle(x) x
+%else
+    %define mangle(x) _ %+ x
 %endif
 
 %macro SECTION_RODATA 0-1 16