[MS ABI]: Fix mangling function arguments for template types to be compatible with MSVC

[adamf]

Summary

MS name mangling supports cache for first 10 distinct function arguments.
The error was when non cached template type occurred twice (e.g. 11th and 12th).
For such case in code there is another cache table TemplateArgStrings (for performance reasons).
Then one '@' character at the end of the mangled name taken from this table was missing.
For other cases the missing '@' character was added in mangleSourceName function.

[rnk]

lgtm

I confirmed the test case matches the name that MSVC produces. I guess this regressed in rC362293 / D62746, so the bug is in clang 9.0, but not 8.0. Maybe we should merge this to 9.0.1.

[thakis]

(It regressed in D62780; I had bisected that bit.)

[thakis]

lgtm. I finally got around to understanding the patch. Sorry for the delay, thanks for the fix! Next time you upload a patch, please upload it with more context (see https://llvm.org/docs/Phabricator.html#requesting-a-review-via-the-web-interface).

Note to self: The mangleSourceName() call mentioned in the patch description is 14 lines further up. mangleSourceName(TemplateMangling); outputs the number if there's a backref, else it outputs the argument followed by a '@'. Since we only cache mangleSourceName()'s argument, we need to append '@' ourselves. It seems a tiny bit nicer to store the '@' in the cache, but as-is is fine too.

Do you need someone to commit this for you?

[adamf]

@thakis Yes please, I cannot commit this patch by myself, because I don't have commit access.
Surely, next time I will add more context in diff.
Thanks a lot for the effort.

clang/lib/AST/MicrosoftMangle.cpp

               TemplateArgStringStorage.save(TemplateMangling.str());
         }
       } else {
-        Out << Found->second; // Outputs a StringRef.
+        Out << Found->second << '@'; // Outputs a StringRef.
       }
     } else {
       Out << Found->second; // Outputs a back reference (an int).

clang/test/CodeGenCXX/mangle-ms-back-references.cpp

 void foo() { }
 // CHECK: "?foo@0@YAXXZ"
 }
+
+class T01;
+class T02;
+class T03;
+class T04;
+class T05;
+class T06;
+class T07;
+class T08;
+class T09;
+class T10;
+class T11;
+template <typename T>
+class H;
+
+void ManyParams(T01 &, T02 &, T03 &, T04 &, T05 &, T06 &, T07 &, T08 &, T09 &, T10 &, H<T11> &, H<T11> &) {}
+// CHECK: "?ManyParams@@YAXAAVT01@@AAVT02@@AAVT03@@AAVT04@@AAVT05@@AAVT06@@AAVT07@@AAVT08@@AAVT09@@AAVT10@@AAV?$H@VT11@@@@AAV?$H@VT11@@@@@Z"