From 0148a4c8541c3bd28aac54fdfd83b5c617912e3b Mon Sep 17 00:00:00 2001
From: David Majnemer <david.majnemer@gmail.com>
Date: Tue, 7 Jul 2015 18:49:41 +0000
Subject: [PATCH] Revert "Revert r241570, it caused PR24053"

This reverts commit r241602.  We had a latent bug in SCCP where we would
make a basic block empty and then proceed to ask questions about it's
terminator.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@241616 91177308-0d34-0410-b5e6-96231b3b80d8
---
 lib/IR/BasicBlock.cpp          | 51 +++++++++++++++-------------------
 lib/Transforms/Scalar/SCCP.cpp |  3 +-
 2 files changed, 23 insertions(+), 31 deletions(-)

diff --git a/lib/IR/BasicBlock.cpp b/lib/IR/BasicBlock.cpp
index 77cb10d5b6b..0a0449434a7 100644
--- a/lib/IR/BasicBlock.cpp
+++ b/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;
 }
diff --git a/lib/Transforms/Scalar/SCCP.cpp b/lib/Transforms/Scalar/SCCP.cpp
index 305175ff8f7..4d3a708fa20 100644
--- a/lib/Transforms/Scalar/SCCP.cpp
+++ b/lib/Transforms/Scalar/SCCP.cpp
@@ -1799,11 +1799,10 @@ bool IPSCCP::runOnModule(Module &M) {
         if (!TI->use_empty())
           TI->replaceAllUsesWith(UndefValue::get(TI->getType()));
         TI->eraseFromParent();
+        new UnreachableInst(M.getContext(), BB);
 
         if (&*BB != &F->front())
           BlocksToErase.push_back(BB);
-        else
-          new UnreachableInst(M.getContext(), BB);
         continue;
       }
 
-- 
2.34.1