fix two problems with machine sinking:

1. Sinking would crash when the first instruction of a block was
   sunk due to iterator problems.
2. Instructions could be sunk to their current block, causing an
   infinite loop.

This fixes PR3968

llvm-svn: 68787

llvm/lib/CodeGen/MachineSink.cpp

@@ -111,20 +111,29 @@ bool MachineSinking::runOnMachineFunction(MachineFunction &MF) {
 }
 
 bool MachineSinking::ProcessBlock(MachineBasicBlock &MBB) {
-  bool MadeChange = false;
-  
   // Can't sink anything out of a block that has less than two successors.
-  if (MBB.succ_size() <= 1) return false;
+  if (MBB.succ_size() <= 1 || MBB.empty()) return false;
-  
+
+  bool MadeChange = false;
+
   // Walk the basic block bottom-up.  Remember if we saw a store.
-  bool SawStore = false;
+  MachineBasicBlock::iterator I = MBB.end();
-  for (MachineBasicBlock::iterator I = MBB.end(); I != MBB.begin(); ){
+  --I;
-    MachineBasicBlock::iterator LastIt = I;
+  bool ProcessedBegin, SawStore = false;
-    if (SinkInstruction(--I, SawStore)) {
+  do {
-      I = LastIt;
+    MachineInstr *MI = I;  // The instruction to sink.
-      ++NumSunk;
+    
-    }
+    // Predecrement I (if it's not begin) so that it isn't invalidated by
-  }
+    // sinking.
+    ProcessedBegin = I == MBB.begin();
+    if (!ProcessedBegin)
+      --I;
+    
+    if (SinkInstruction(MI, SawStore))
+      ++NumSunk, MadeChange = true;
+    
+    // If we just processed the first instruction in the block, we're done.
+  } while (!ProcessedBegin);
   
   return MadeChange;
 }
@@ -218,6 +227,11 @@ bool MachineSinking::SinkInstruction(MachineInstr *MI, bool &SawStore) {
   if (SuccToSinkTo->isLandingPad())
     return false;
   
+  // If is not possible to sink an instruction into its own block.  This can
+  // happen with loops.
+  if (MI->getParent() == SuccToSinkTo)
+    return false;
+  
   DEBUG(cerr << "Sink instr " << *MI);
   DEBUG(cerr << "to block " << *SuccToSinkTo);
   

llvm/test/CodeGen/Generic/2009-04-10-SinkCrash.ll

@@ -0,0 +1,16 @@
+; RUN: llvm-as < %s | llc
+
+define void @QRiterate(i32 %p.1, double %tmp.212) nounwind {
+entry:
+	br i1 false, label %shortcirc_next.1, label %exit.1.critedge
+
+shortcirc_next.1:		; preds = %shortcirc_next.1, %entry
+	%tmp.213 = fcmp une double %tmp.212, 0.000000e+00		; <i1> [#uses=1]
+	br i1 %tmp.213, label %shortcirc_next.1, label %exit.1
+
+exit.1.critedge:		; preds = %entry
+	ret void
+
+exit.1:		; preds = %shortcirc_next.1
+	ret void
+}