LiveRangeCalc: Don't start liveranges of PHI instruction at the block begin.

Summary:
Letting them begin at the PHI instruction slightly simplifies the code
but more importantly avoids breaking the assumption that live ranges
starting at the block begin are also live at the end of the predecessor
blocks. The MachineVerifier checks that but was apparently never run in
the few instances where liveranges are calculated for machine-SSA
functions.

Reviewers: qcolombet

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D7779

llvm-svn: 230093

1 files changed, 2 insertions, 5 deletions

diff --git a/llvm/lib/CodeGen/LiveRangeCalc.cpp b/llvm/lib/CodeGen/LiveRangeCalc.cpp
index a07c3a35dc7..d804b39c597 100644
--- a/llvm/lib/CodeGen/LiveRangeCalc.cpp
+++ b/llvm/lib/CodeGen/LiveRangeCalc.cpp
@@ -43,11 +43,8 @@ void LiveRangeCalc::reset(const MachineFunction *mf,
 static void createDeadDef(SlotIndexes &Indexes, VNInfo::Allocator &Alloc,
                           LiveRange &LR, const MachineOperand &MO) {
     const MachineInstr *MI = MO.getParent();
-    SlotIndex DefIdx;
-    if (MI->isPHI())
-      DefIdx = Indexes.getMBBStartIdx(MI->getParent());
-    else
-      DefIdx = Indexes.getInstructionIndex(MI).getRegSlot(MO.isEarlyClobber());
+    SlotIndex DefIdx =
+        Indexes.getInstructionIndex(MI).getRegSlot(MO.isEarlyClobber());
 
     // Create the def in LR. This may find an existing def.
     LR.createDeadDef(DefIdx, Alloc);