CodeGen: Make computeRegisterLiveness search forward first

If there is an unused def, this would previously
report that the register was live. Check for uses
first so that it is reported as dead if never used.

llvm-svn: 341027

1 files changed, 35 insertions, 33 deletions

diff --git a/llvm/lib/CodeGen/MachineBasicBlock.cpp b/llvm/lib/CodeGen/MachineBasicBlock.cpp
index 62c360c5100..71fdb0f90f2 100644
--- a/llvm/lib/CodeGen/MachineBasicBlock.cpp
+++ b/llvm/lib/CodeGen/MachineBasicBlock.cpp
@@ -1375,8 +1375,42 @@ MachineBasicBlock::computeRegisterLiveness(const TargetRegisterInfo *TRI,
                                            unsigned Neighborhood) const {
   unsigned N = Neighborhood;
 
-  // Start by searching backwards from Before, looking for kills, reads or defs.
+  // 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, --N) {
+      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;
+    }
+  }
+
+  // If we reached the end, it is safe to clobber Reg at the end of a block of
+  // no successor has it live in.
+  if (I == end()) {
+    for (MachineBasicBlock *S : successors()) {
+      for (MCSubRegIterator SubReg(Reg, TRI, /*IncludeSelf*/true);
+           SubReg.isValid(); ++SubReg) {
+        if (S->isLiveIn(*SubReg))
+          return LQR_Live;
+      }
+    }
+
+    return LQR_Dead;
+  }
+
+
+  N = Neighborhood;
+
+  // Start by searching backwards from Before, looking for kills, reads or defs.
+  I = const_iterator(Before);
   // If this is the first insn in the block, don't search backwards.
   if (I != begin()) {
     do {
@@ -1420,38 +1454,6 @@ MachineBasicBlock::computeRegisterLiveness(const TargetRegisterInfo *TRI,
     return LQR_Dead;
   }
 
-  N = Neighborhood;
-
-  // Try searching forwards from Before, looking for reads or defs.
-  I = const_iterator(Before);
-  // If this is the last insn in the block, don't search forwards.
-  if (I != end()) {
-    for (++I; I != end() && N > 0; ++I, --N) {
-      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;
-    }
-  }
-
-  // If we reached the end, it is safe to clobber Reg at the end of a block of
-  // no successor has it live in.
-  if (I == end()) {
-    for (MachineBasicBlock *S : successors()) {
-      for (MCSubRegIterator SubReg(Reg, TRI, /*IncludeSelf*/true);
-           SubReg.isValid(); ++SubReg) {
-        if (S->isLiveIn(*SubReg))
-          return LQR_Live;
-      }
-    }
-
-    return LQR_Dead;
-  }
 
   // At this point we have no idea of the liveness of the register.
   return LQR_Unknown;