[COFF] Improve the check for functions that should get an extra underscore

This fixes exporting functions starting with an underscore, and
fully decorated fastcall/vectorcall functions.

Tests will be added in the lld repo.

Differential Revision: https://reviews.llvm.org/D39168

llvm-svn: 316316

1 files changed, 3 insertions, 2 deletions

diff --git a/llvm/lib/ToolDrivers/llvm-dlltool/DlltoolDriver.cpp b/llvm/lib/ToolDrivers/llvm-dlltool/DlltoolDriver.cpp
index eb9b9c3b264..3891efae57b 100644
--- a/llvm/lib/ToolDrivers/llvm-dlltool/DlltoolDriver.cpp
+++ b/llvm/lib/ToolDrivers/llvm-dlltool/DlltoolDriver.cpp
@@ -165,8 +165,9 @@ int llvm::dlltoolDriverMain(llvm::ArrayRef<const char *> ArgsArr) {
       E.SymbolName = E.Name;
       // Trim off the trailing decoration. Symbols will always have a
       // starting prefix here (either _ for cdecl/stdcall, @ for fastcall
-      // or ? for C++ functions). (Vectorcall functions also will end up having
-      // a prefix here, even if they shouldn't.)
+      // or ? for C++ functions). Vectorcall functions won't have any
+      // fixed prefix, but the function base name will still be at least
+      // one char.
       E.Name = E.Name.substr(0, E.Name.find('@', 1));
       // By making sure E.SymbolName != E.Name for decorated symbols,
       // writeImportLibrary writes these symbols with the type