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.

llvm-svn: 269107

1 files changed, 4 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/ScheduleDAGInstrs.cpp b/llvm/lib/CodeGen/ScheduleDAGInstrs.cpp
index e9ab6a24f8a..d4c4c2c1ddf 100644
--- a/llvm/lib/CodeGen/ScheduleDAGInstrs.cpp
+++ b/llvm/lib/CodeGen/ScheduleDAGInstrs.cpp
@@ -971,6 +971,10 @@ void ScheduleDAGInstrs::buildSchedGraph(AliasAnalysis *AA,
     // 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();