[CodeGen] Fix forward scan in MachineBasicBlock::computeRegisterLiveness.

The scan was incorrectly skipping the first instruction, so a register
could appear to be dead when it was actually live. This eventually leads
to a machine verifier failure and miscompile in arm-ldst-opt.

Differential Revision: https://reviews.llvm.org/D54491

llvm-svn: 346821

1 files changed, 12 insertions, 15 deletions

diff --git a/llvm/lib/CodeGen/MachineBasicBlock.cpp b/llvm/lib/CodeGen/MachineBasicBlock.cpp
index 0b9ee462d1ae..03771bc5dae1 100644
--- a/llvm/lib/CodeGen/MachineBasicBlock.cpp
+++ b/llvm/lib/CodeGen/MachineBasicBlock.cpp
@@ -1380,24 +1380,21 @@ MachineBasicBlock::computeRegisterLiveness(const TargetRegisterInfo *TRI,
 
   // Try searching forwards from Before, looking for reads or defs.
   const_iterator I(Before);
-  // If this is the last insn in the block, don't search forwards.
-  if (I != end()) {
-    for (++I; I != end() && N > 0; ++I) {
-      if (I->isDebugInstr())
-        continue;
+  for (; I != end() && N > 0; ++I) {
+    if (I->isDebugInstr())
+      continue;
 
-      --N;
+    --N;
 
-      MachineOperandIteratorBase::PhysRegInfo Info =
-          ConstMIOperands(*I).analyzePhysReg(Reg, TRI);
+    MachineOperandIteratorBase::PhysRegInfo Info =
+        ConstMIOperands(*I).analyzePhysReg(Reg, TRI);
 
-      // Register is live when we read it here.
-      if (Info.Read)
-        return LQR_Live;
-      // Register is dead if we can fully overwrite or clobber it here.
-      if (Info.FullyDefined || Info.Clobbered)
-        return LQR_Dead;
-    }
+    // Register is live when we read it here.
+    if (Info.Read)
+      return LQR_Live;
+    // Register is dead if we can fully overwrite or clobber it here.
+    if (Info.FullyDefined || Info.Clobbered)
+      return LQR_Dead;
   }
 
   // If we reached the end, it is safe to clobber Reg at the end of a block of