BasicBlock: add ctail, GetMergeInst, GetLoopMergeInst

source/opt/basic_block.cpp

@@ -17,6 +17,54 @@
 namespace spvtools {
 namespace ir {
 
+const Instruction* BasicBlock::GetMergeInst() const {
+  const Instruction* result = nullptr;
+  // If it exists, the merge instruction immediately precedes the
+  // terminator.
+  auto iter = ctail();
+  if (iter != cbegin()) {
+    --iter;
+    const auto opcode = iter->opcode();
+    if (opcode == SpvOpLoopMerge || opcode  == SpvOpSelectionMerge) {
+      result = &*iter;
+    }
+  }
+  return result;
+}
+
+Instruction* BasicBlock::GetMergeInst() {
+  Instruction* result = nullptr;
+  // If it exists, the merge instruction immediately precedes the
+  // terminator.
+  auto iter = tail();
+  if (iter != begin()) {
+    --iter;
+    const auto opcode = iter->opcode();
+    if (opcode == SpvOpLoopMerge || opcode  == SpvOpSelectionMerge) {
+      result = &*iter;
+    }
+  }
+  return result;
+}
+
+const Instruction* BasicBlock::GetLoopMergeInst() const {
+  if (auto* merge = GetMergeInst()) {
+    if (merge->opcode() == SpvOpLoopMerge) {
+      return merge;
+    }
+  }
+  return nullptr;
+}
+
+Instruction* BasicBlock::GetLoopMergeInst() {
+  if (auto* merge = GetMergeInst()) {
+    if (merge->opcode() == SpvOpLoopMerge) {
+      return merge;
+    }
+  }
+  return nullptr;
+}
+
 void BasicBlock::ForEachSuccessorLabel(
     const std::function<void(const uint32_t)>& f) {
   const auto br = &*insts_.back();

source/opt/basic_block.h

@@ -52,6 +52,15 @@ class BasicBlock {
   // The label starting this basic block.
   Instruction* GetLabelInst() { return label_.get(); }
 
+  // Returns the merge instruction in this basic block, if it exists.
+  // Otherwise return null.  May be used whenever tail() can be used.
+  const Instruction* GetMergeInst() const;
+  Instruction* GetMergeInst();
+  // Returns the OpLoopMerge instruciton in this basic block, if it exists.
+  // Otherwise return null.  May be used whenever tail() can be used.
+  const Instruction* GetLoopMergeInst() const;
+  Instruction* GetLoopMergeInst();
+
   // Returns the id of the label at the top of this block
   inline uint32_t id() const { return label_->result_id(); }
 
@@ -64,10 +73,18 @@ class BasicBlock {
     return const_iterator(&insts_, insts_.cend());
   }
 
+  // Returns an iterator pointing to the last instruction.  This may only
+  // be used if this block has an instruction other than the OpLabel
+  // that defines it.
   iterator tail() {
     assert(!insts_.empty());
     return iterator(&insts_, std::prev(insts_.end()));
   }
+  // Returns a const iterator, but othewrise similar to tail().
+  const_iterator ctail() const {
+    assert(!insts_.empty());
+    return const_iterator(&insts_, std::prev(insts_.cend()));
+  }
 
   // Runs the given function |f| on each instruction in this basic block, and
   // optionally on the debug line instructions that might precede them.