diff --git a/gfx/angle/src/compiler/MapLongVariableNames.cpp b/gfx/angle/src/compiler/MapLongVariableNames.cpp
index 3852743fb47f..f99438044632 100644
--- a/gfx/angle/src/compiler/MapLongVariableNames.cpp
+++ b/gfx/angle/src/compiler/MapLongVariableNames.cpp
@@ -14,12 +14,18 @@ TString mapLongName(int id, const TString& name, bool isGlobal)
     ASSERT(name.size() > MAX_SHORTENED_IDENTIFIER_SIZE);
     TStringStream stream;
     uint64 hash = SpookyHash::Hash64(name.data(), name.length(), 0);
-    stream << "webgl_"
+
+    // We want to avoid producing a string with a double underscore,
+    // which would be an illegal GLSL identifier. We can assume that the
+    // original identifier doesn't have a double underscore, otherwise
+    // it's illegal anyway.
+    stream << (name[0] == '_' ? "webgl" : "webgl_")
            << name.substr(0, 9)
-           << "_"
+           << (name[8] == '_' ? "" : "_")
            << std::hex
            << hash;
-    ASSERT(stream.str().length() == MAX_SHORTENED_IDENTIFIER_SIZE);
+    ASSERT(stream.str().length() <= MAX_SHORTENED_IDENTIFIER_SIZE);
+    ASSERT(stream.str().length() >= MAX_SHORTENED_IDENTIFIER_SIZE - 2);
     return stream.str();
 }