Avoid analyzing instructions in blocks not reachable from the entry block.

They are lots of trouble, and they don't matter. This fixes PR6559.

llvm-svn: 98103

llvm/lib/Analysis/ScalarEvolution.cpp

@@ -3101,9 +3101,16 @@ const SCEV *ScalarEvolution::createSCEV(Value *V) {
     return getUnknown(V);
 
   unsigned Opcode = Instruction::UserOp1;
-  if (Instruction *I = dyn_cast<Instruction>(V))
+  if (Instruction *I = dyn_cast<Instruction>(V)) {
     Opcode = I->getOpcode();
-  else if (ConstantExpr *CE = dyn_cast<ConstantExpr>(V))
+
+    // Don't attempt to analyze instructions in blocks that aren't
+    // reachable. Such instructions don't matter, and they aren't required
+    // to obey basic rules for definitions dominating uses which this
+    // analysis depends on.
+    if (!DT->isReachableFromEntry(I->getParent()))
+      return getUnknown(V);
+  } else if (ConstantExpr *CE = dyn_cast<ConstantExpr>(V))
     Opcode = CE->getOpcode();
   else if (ConstantInt *CI = dyn_cast<ConstantInt>(V))
     return getConstant(CI);

llvm/test/Analysis/ScalarEvolution/unreachable-code.ll

@@ -0,0 +1,13 @@
+; RUN: opt < %s -analyze -scalar-evolution | FileCheck %s
+
+; CHECK: %t = add i64 %t, 1
+; CHECK: -->  %t
+
+define void @foo() {
+entry:
+  ret void
+
+dead:
+  %t = add i64 %t, 1
+  ret void
+}