Give LowerSubregs.cpp a top-level description.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@56596 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/CodeGen/LowerSubregs.cpp b/lib/CodeGen/LowerSubregs.cpp
index 00038d5c1c7101d24f5b7d8a2dc3874a75b026a9..f9cef4ccfb7880cb02166a6638fa766c371d2c66 100644 (file)
--- a/lib/CodeGen/LowerSubregs.cpp
+++ b/lib/CodeGen/LowerSubregs.cpp
@@ -6,6 +6,13 @@
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
+//
+// This file defines a MachineFunction pass which runs after register
+// allocation that turns subreg insert/extract instructions into register
+// copies, as needed. This ensures correct codegen even if the coalescer
+// isn't able to remove all subreg instructions.
+//
+//===----------------------------------------------------------------------===//
 
 #define DEBUG_TYPE "lowersubregs"
 #include "llvm/CodeGen/Passes.h"