[llvm] r269107 - ScheduleDAGInstrs: Comment on why subreg defs are not seen as uses; NFC

[Matthias Braun via llvm-commits]

Author: matze
Date: Tue May 10 15:11:58 2016
New Revision: 269107

URL: http://llvm.org/viewvc/llvm-project?rev=269107&view=rev
Log:
ScheduleDAGInstrs: Comment on why subreg defs are not seen as uses; NFC

Usually subregister definitions are consider uses of the remaining
lanes that did not get defined. Add a comment why the code in
ScheduleDAGInstrs does not add use dependencies regardless.

Modified:
    llvm/trunk/lib/CodeGen/ScheduleDAGInstrs.cpp

Modified: llvm/trunk/lib/CodeGen/ScheduleDAGInstrs.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/ScheduleDAGInstrs.cpp?rev=269107&r1=269106&r2=269107&view=diff
==============================================================================
--- llvm/trunk/lib/CodeGen/ScheduleDAGInstrs.cpp (original)
+++ llvm/trunk/lib/CodeGen/ScheduleDAGInstrs.cpp Tue May 10 15:11:58 2016
@@ -971,6 +971,10 @@ void ScheduleDAGInstrs::buildSchedGraph(
     // Now process all uses.
     for (unsigned j = 0, n = MI->getNumOperands(); j != n; ++j) {
       const MachineOperand &MO = MI->getOperand(j);
+      // Only look at use operands.
+      // We do not need to check for MO.readsReg() here because subsequent
+      // subregister defs will get output dependence edges and need no
+      // additional use dependencies.
       if (!MO.isReg() || !MO.isUse())
         continue;
       unsigned Reg = MO.getReg();