[Graph Writer] Limit the length of the graph name because Windows can't handle it.

[Bigcheese]

Summary

Windows can't handle paths longer than 260 code points without \\?\. Even
with \\?\ it can't handle path components longer than 255 code points. So
limit graph names to the arbitrary length of 140. Random characters are still
added to the end, so it's ok if graph names collide.

[Bigcheese]

CCed llvm-commits.

[rnk]

lgtm

[aaron.ballman]

LGTM!

~Aaron

[Bigcheese]

Closed by commit rL209483 (authored by mspencer).

llvm/trunk/include/llvm/Support/GraphWriter.h

 }
 
 std::string createGraphFilename(const Twine &Name, int &FD);
 
 template <typename GraphType>
 std::string WriteGraph(const GraphType &G, const Twine &Name,
                        bool ShortNames = false, const Twine &Title = "") {
   int FD;
-  std::string Filename = createGraphFilename(Name, FD);
+  // Windows can't always handle long paths, so limit the length of the name.
+  std::string N = Name.str();
+  N = N.substr(0, std::min<std::size_t>(N.size(), 140));
+  std::string Filename = createGraphFilename(N, FD);
   raw_fd_ostream O(FD, /*shouldClose=*/ true);
 
   if (FD == -1) {
     errs() << "error opening file '" << Filename << "' for writing!\n";
     return "";
   }
 
   llvm::WriteGraph(O, G, ShortNames, Title);