[IR] Make getFirstNonPHI return null if the BB is empty

getFirstNonPHI's documentation states that it returns null if there is
no non-PHI instruction.  However, it instead returns a pointer to the
end iterator.  The implementation of getFirstNonPHI claims that
dereferencing the iterator will result in an assertion failure but this
doesn't occur.  Instead, machinery like getFirstInsertionPt will attempt
to isa<> this invalid memory which results in unpredictable behavior.
Instead, make getFirst* return null if no such instruction exists.

llvm-svn: 241570

1 files changed, 22 insertions, 29 deletions

diff --git a/llvm/lib/IR/BasicBlock.cpp b/llvm/lib/IR/BasicBlock.cpp
index 77cb10d5b6b..0a0449434a7 100644
--- a/llvm/lib/IR/BasicBlock.cpp
+++ b/llvm/lib/IR/BasicBlock.cpp
@@ -163,47 +163,40 @@ CallInst *BasicBlock::getTerminatingMustTailCall() {
 }
 
 Instruction* BasicBlock::getFirstNonPHI() {
-  BasicBlock::iterator i = begin();
-  // All valid basic blocks should have a terminator,
-  // which is not a PHINode. If we have an invalid basic
-  // block we'll get an assertion failure when dereferencing
-  // a past-the-end iterator.
-  while (isa<PHINode>(i)) ++i;
-  return &*i;
+  for (Instruction &I : *this)
+    if (!isa<PHINode>(I))
+      return &I;
+  return nullptr;
 }
 
 Instruction* BasicBlock::getFirstNonPHIOrDbg() {
-  BasicBlock::iterator i = begin();
-  // All valid basic blocks should have a terminator,
-  // which is not a PHINode. If we have an invalid basic
-  // block we'll get an assertion failure when dereferencing
-  // a past-the-end iterator.
-  while (isa<PHINode>(i) || isa<DbgInfoIntrinsic>(i)) ++i;
-  return &*i;
+  for (Instruction &I : *this)
+    if (!isa<PHINode>(I) && !isa<DbgInfoIntrinsic>(I))
+      return &I;
+  return nullptr;
 }
 
 Instruction* BasicBlock::getFirstNonPHIOrDbgOrLifetime() {
-  // All valid basic blocks should have a terminator,
-  // which is not a PHINode. If we have an invalid basic
-  // block we'll get an assertion failure when dereferencing
-  // a past-the-end iterator.
-  BasicBlock::iterator i = begin();
-  for (;; ++i) {
-    if (isa<PHINode>(i) || isa<DbgInfoIntrinsic>(i))
+  for (Instruction &I : *this) {
+    if (isa<PHINode>(I) || isa<DbgInfoIntrinsic>(I))
       continue;
 
-    const IntrinsicInst *II = dyn_cast<IntrinsicInst>(i);
-    if (!II)
-      break;
-    if (II->getIntrinsicID() != Intrinsic::lifetime_start &&
-        II->getIntrinsicID() != Intrinsic::lifetime_end)
-      break;
+    if (auto *II = dyn_cast<IntrinsicInst>(&I))
+      if (II->getIntrinsicID() == Intrinsic::lifetime_start ||
+          II->getIntrinsicID() == Intrinsic::lifetime_end)
+        continue;
+
+    return &I;
   }
-  return &*i;
+  return nullptr;
 }
 
 BasicBlock::iterator BasicBlock::getFirstInsertionPt() {
-  iterator InsertPt = getFirstNonPHI();
+  Instruction *FirstNonPHI = getFirstNonPHI();
+  if (!FirstNonPHI)
+    return end();
+
+  iterator InsertPt = FirstNonPHI;
   if (isa<LandingPadInst>(InsertPt)) ++InsertPt;
   return InsertPt;
 }