diff --git a/gfx/thebes/gfxFont.cpp b/gfx/thebes/gfxFont.cpp
index 775282052..aa75a5d29 100644
--- a/gfx/thebes/gfxFont.cpp
+++ b/gfx/thebes/gfxFont.cpp
@@ -50,6 +50,9 @@
 #include <algorithm>
 #include <cmath>
 
+#include "mozilla-config.h"
+#include "plvmx.h"
+
 using namespace mozilla;
 using namespace mozilla::gfx;
 using namespace mozilla::unicode;
@@ -2742,7 +2745,7 @@ inline static bool IsChar8Bit(char16_t aCh) { return aCh < 0x100; }
 
 inline static bool HasSpaces(const uint8_t *aString, uint32_t aLen)
 {
-    return memchr(aString, 0x20, aLen) != nullptr;
+    return VMX_MEMCHR(aString, 0x20, aLen) != nullptr;
 }
 
 inline static bool HasSpaces(const char16_t *aString, uint32_t aLen)
diff --git a/layout/generic/nsTextFrame.cpp b/layout/generic/nsTextFrame.cpp
index a7f85ddb5..349866f35 100644
--- a/layout/generic/nsTextFrame.cpp
+++ b/layout/generic/nsTextFrame.cpp
@@ -80,6 +80,9 @@
 
 #include "GeckoProfiler.h"
 
+#include "mozilla-config.h"
+#include "plvmx.h"
+
 #ifdef DEBUG
 #undef NOISY_REFLOW
 #undef NOISY_TRIM
@@ -2891,7 +2894,7 @@ static int32_t FindChar(const nsTextFragment* frag,
   } else {
     if (uint16_t(ch) <= 0xFF) {
       const char* str = frag->Get1b() + aOffset;
-      const void* p = memchr(str, ch, aLength);
+      const void* p = VMX_MEMCHR(str, ch, aLength);
       if (p)
         return (static_cast<const char*>(p) - str) + aOffset;
     }
diff --git a/xpcom/string/nsCharTraits.h b/xpcom/string/nsCharTraits.h
index d93e1f5dc..3775f10ec 100644
--- a/xpcom/string/nsCharTraits.h
+++ b/xpcom/string/nsCharTraits.h
@@ -12,6 +12,9 @@
 
 #include "nscore.h" // for |char16_t|
 
+#include "mozilla-config.h"
+#include "plvmx.h" // for |vmx_memchr| (if implemented)
+
 // This file may be used (through nsUTF8Utils.h) from non-XPCOM code, in
 // particular the standalone software updater. In that case stub out
 // the macros provided by nsDebug.h which are only usable when linking XPCOM
@@ -501,8 +504,8 @@ struct nsCharTraits<char>
   static const char_type*
   find(const char_type* aStr, size_t aN, char_type aChar)
   {
-    return reinterpret_cast<const char_type*>(memchr(aStr, to_int_type(aChar),
-                                                     aN));
+    return reinterpret_cast<const char_type*>(VMX_MEMCHR(aStr, to_int_type(aChar),
+                                                         aN));
   }
 };