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 7f1a7c4f259e8029d019cbd56023466832a01009..d437370031deb0ee2582f0d776e4e16819d3c38f 100644 (file)
--- a/lib/CodeGen/LLVMTargetMachine.cpp
+++ b/lib/CodeGen/LLVMTargetMachine.cpp
@@ -330,6 +330,13 @@ bool LLVMTargetMachine::addCommonCodeGenPasses(PassManagerBase &PM,
     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);