only allocate trimmedName on win32

Reviewed By: yfeldblum

Differential Revision: D21921631

fbshipit-source-id: e905d6d2a96775a8c3b328fb78c8d32897f634f6

folly/system/ThreadName.cpp

@@ -138,7 +138,6 @@ Optional<std::string> getCurrentThreadName() {
 }
 
 bool setThreadName(std::thread::id tid, StringPiece name) {
-  auto trimmedName = name.subpiece(0, kMaxThreadNameLength - 1).str();
 #ifdef _WIN32
   static_assert(
       sizeof(unsigned int) == sizeof(std::thread::id),
@@ -166,6 +165,8 @@ bool setThreadName(std::thread::id tid, StringPiece name) {
   unsigned int id;
   std::memcpy(&id, &tid, sizeof(id));
 
+  auto trimmedName = name.subpiece(0, kMaxThreadNameLength - 1).str();
+
   TNIUnion tniUnion = {0x1000, trimmedName.data(), id, 0};
   // This has to be in a separate stack frame from trimmedName, which requires
   // C++ object destruction semantics.