Add helper func to get first non-alloca position

The LLVM performance tips suggest that allocas should be placed at the
beginning of the entry block. So far, llvm doesn’t provide any helper to
find that position.

Add BasicBlock::getFirstNonPHIOrDbgOrAlloca and IRBuilder::SetInsertPointPastAllocas(Function*)
that get an insert position after the (static) allocas at the start of a
function and use it in ShadowStackGCLowering.

Differential Revision: https://reviews.llvm.org/D132554

diff --git a/llvm/include/llvm/IR/BasicBlock.h b/llvm/include/llvm/IR/BasicBlock.h
index d487223..ff0ea61 100644 (file)
--- a/llvm/include/llvm/IR/BasicBlock.h
+++ b/llvm/include/llvm/IR/BasicBlock.h
@@ -196,6 +196,15 @@ public:
                                           ->getFirstInsertionPt().getNonConst();
   }
 
+  /// Returns an iterator to the first instruction in this block that is
+  /// not a PHINode, a debug intrinsic, a static alloca or any pseudo operation.
+  const_iterator getFirstNonPHIOrDbgOrAlloca() const;
+  iterator getFirstNonPHIOrDbgOrAlloca() {
+    return static_cast<const BasicBlock *>(this)
+        ->getFirstNonPHIOrDbgOrAlloca()
+        .getNonConst();
+  }
+
   /// Return a const iterator range over the instructions in the block, skipping
   /// any debug instructions. Skip any pseudo operations as well if \c
   /// SkipPseudoOp is true.

diff --git a/llvm/include/llvm/IR/IRBuilder.h b/llvm/include/llvm/IR/IRBuilder.h
index 3b2c699..8cf76d2 100644 (file)
--- a/llvm/include/llvm/IR/IRBuilder.h
+++ b/llvm/include/llvm/IR/IRBuilder.h
@@ -199,6 +199,14 @@ public:
       SetCurrentDebugLocation(IP->getDebugLoc());
   }
 
+  /// This specifies that created instructions should inserted at the beginning
+  /// end of the specified function, but after already existing static alloca
+  /// instructions that are at the start.
+  void SetInsertPointPastAllocas(Function *F) {
+    BB = &F->getEntryBlock();
+    InsertPt = BB->getFirstNonPHIOrDbgOrAlloca();
+  }
+
   /// Set location information used by debugging information.
   void SetCurrentDebugLocation(DebugLoc L) {
     AddOrRemoveMetadataToCopy(LLVMContext::MD_dbg, L.getAsMDNode());

diff --git a/llvm/lib/CodeGen/ShadowStackGCLowering.cpp b/llvm/lib/CodeGen/ShadowStackGCLowering.cpp
index 5f9ade1..711a2ac 100644 (file)
--- a/llvm/lib/CodeGen/ShadowStackGCLowering.cpp
+++ b/llvm/lib/CodeGen/ShadowStackGCLowering.cpp
@@ -320,9 +320,8 @@ bool ShadowStackGCLowering::runOnFunction(Function &F) {
   Instruction *StackEntry =
       AtEntry.CreateAlloca(ConcreteStackEntryTy, nullptr, "gc_frame");
 
-  while (isa<AllocaInst>(IP))
-    ++IP;
-  AtEntry.SetInsertPoint(IP->getParent(), IP);
+  AtEntry.SetInsertPointPastAllocas(&F);
+  IP = AtEntry.GetInsertPoint();
 
   // Initialize the map pointer and load the current head of the shadow stack.
   Instruction *CurrentHead =

diff --git a/llvm/lib/IR/BasicBlock.cpp b/llvm/lib/IR/BasicBlock.cpp
index f064ff5..687865d 100644 (file)
--- a/llvm/lib/IR/BasicBlock.cpp
+++ b/llvm/lib/IR/BasicBlock.cpp
@@ -253,6 +253,30 @@ BasicBlock::const_iterator BasicBlock::getFirstInsertionPt() const {
   return InsertPt;
 }
 
+BasicBlock::const_iterator BasicBlock::getFirstNonPHIOrDbgOrAlloca() const {
+  const Instruction *FirstNonPHI = getFirstNonPHI();
+  if (!FirstNonPHI)
+    return end();
+
+  const_iterator InsertPt = FirstNonPHI->getIterator();
+  if (InsertPt->isEHPad())
+    ++InsertPt;
+
+  if (isEntryBlock()) {
+    const_iterator End = end();
+    while (InsertPt != End &&
+           (isa<AllocaInst>(*InsertPt) || isa<DbgInfoIntrinsic>(*InsertPt) ||
+            isa<PseudoProbeInst>(*InsertPt))) {
+      if (const AllocaInst *AI = dyn_cast<AllocaInst>(&*InsertPt)) {
+        if (!AI->isStaticAlloca())
+          break;
+      }
+      ++InsertPt;
+    }
+  }
+  return InsertPt;
+}
+
 void BasicBlock::dropAllReferences() {
   for (Instruction &I : *this)
     I.dropAllReferences();

diff --git a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
index fc9c940..29fb40b 100644 (file)
--- a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -3937,9 +3937,8 @@ bool InstCombinerImpl::freezeOtherUses(FreezeInst &FI) {
   // replacement below is still necessary.
   Instruction *MoveBefore;
   if (isa<Argument>(Op)) {
-    MoveBefore = &FI.getFunction()->getEntryBlock().front();
-    while (isa<AllocaInst>(MoveBefore))
-      MoveBefore = MoveBefore->getNextNode();
+    MoveBefore =
+        &*FI.getFunction()->getEntryBlock().getFirstNonPHIOrDbgOrAlloca();
   } else {
     MoveBefore = cast<Instruction>(Op)->getInsertionPointAfterDef();
     if (!MoveBefore)