Add some comments.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@106809 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/CodeGen/LLVMTargetMachine.cpp b/lib/CodeGen/LLVMTargetMachine.cpp
index 7f1a7c4..d437370 100644
--- a/lib/CodeGen/LLVMTargetMachine.cpp
+++ b/lib/CodeGen/LLVMTargetMachine.cpp
@@ -330,6 +330,13 @@
PM.add(createOptimizePHIsPass());
// Delete dead machine instructions regardless of optimization level.
+ //
+ // At -O0, fast-isel frequently creates dead instructions.
+ //
+ // With optimization, dead code should already be eliminated. However
+ // there is one known exception: lowered code for arguments that are only
+ // used by tail calls, where the tail calls reuse the incoming stack
+ // arguments directly (see t11 in test/CodeGen/X86/sibcall.ll).
PM.add(createDeadMachineInstructionElimPass());
printAndVerify(PM, "After codegen DCE pass",
/* allowDoubleDefs= */ true);