add unpredictable metadata type for control flow

This patch defines 'unpredictable' metadata. This metadata can be used to signal to the optimizer
or backend that a branch or switch is unpredictable, and therefore, it's probably better to not
split a compound predicate into multiple branches such as in CodeGenPrepare::splitBranchCondition().

This was discussed in:
https://llvm.org/bugs/show_bug.cgi?id=23827

Dependent patches to alter codegen and expose this in clang to follow.

Differential Revision; http://reviews.llvm.org/D12341

llvm-svn: 246688

llvm/docs/LangRef.rst

@@ -4163,6 +4163,16 @@ Examples:
     !2 = !{ i8 0, i8 2, i8 3, i8 6 }
     !3 = !{ i8 -2, i8 0, i8 3, i8 6 }
 
+'``unpredictable``' Metadata
+^^^^^^^^^^^^^^^^^^^^^
+
+``unpredictable`` metadata may be attached to any branch or switch
+instruction. It can be used to express the unpredictability of control
+flow. Similar to the llvm.expect intrinsic, it may be used to alter
+optimizations related to compare and branch instructions. The metadata
+is treated as a boolean value; if it exists, it signals that the branch
+or switch that it is attached to is completely unpredictable.
+
 '``llvm.loop``'
 ^^^^^^^^^^^^^^^
 

llvm/include/llvm/IR/IRBuilder.h

@@ -577,12 +577,15 @@ public:
   //===--------------------------------------------------------------------===//
 
 private:
-  /// \brief Helper to add branch weight metadata onto an instruction.
+  /// \brief Helper to add branch weight and unpredictable metadata onto an
+  /// instruction.
   /// \returns The annotated instruction.
   template <typename InstTy>
-  InstTy *addBranchWeights(InstTy *I, MDNode *Weights) {
+  InstTy *addBranchMetadata(InstTy *I, MDNode *Weights, MDNode *Unpredictable) {
     if (Weights)
       I->setMetadata(LLVMContext::MD_prof, Weights);
+    if (Unpredictable)
+      I->setMetadata(LLVMContext::MD_unpredictable, Unpredictable);
     return I;
   }
 
@@ -619,9 +622,10 @@ public:
   /// \brief Create a conditional 'br Cond, TrueDest, FalseDest'
   /// instruction.
   BranchInst *CreateCondBr(Value *Cond, BasicBlock *True, BasicBlock *False,
-                           MDNode *BranchWeights = nullptr) {
-    return Insert(addBranchWeights(BranchInst::Create(True, False, Cond),
-                                   BranchWeights));
+                           MDNode *BranchWeights = nullptr,
+                           MDNode *Unpredictable = nullptr) {
+    return Insert(addBranchMetadata(BranchInst::Create(True, False, Cond),
+                                    BranchWeights, Unpredictable));
   }
 
   /// \brief Create a switch instruction with the specified value, default dest,
@@ -629,8 +633,9 @@ public:
   /// allocation).
   SwitchInst *CreateSwitch(Value *V, BasicBlock *Dest, unsigned NumCases = 10,
                            MDNode *BranchWeights = nullptr) {
-    return Insert(addBranchWeights(SwitchInst::Create(V, Dest, NumCases),
-                                   BranchWeights));
+    // TODO: Add unpredictable metadata for a switch.
+    return Insert(addBranchMetadata(SwitchInst::Create(V, Dest, NumCases),
+                                    BranchWeights, nullptr));
   }
 
   /// \brief Create an indirect branch instruction with the specified address

llvm/include/llvm/IR/LLVMContext.h

@@ -61,7 +61,8 @@ public:
     MD_nonnull = 11, // "nonnull"
     MD_dereferenceable = 12, // "dereferenceable"
     MD_dereferenceable_or_null = 13, // "dereferenceable_or_null"
-    MD_make_implicit = 14 // "make.implicit"
+    MD_make_implicit = 14, // "make.implicit"
+    MD_unpredictable = 15 // "unpredictable"
   };
 
   /// getMDKindID - Return a unique non-zero ID for the specified metadata kind.

llvm/include/llvm/IR/MDBuilder.h

@@ -60,6 +60,9 @@ public:
   /// \brief Return metadata containing a number of branch weights.
   MDNode *createBranchWeights(ArrayRef<uint32_t> Weights);
 
+  /// Return metadata specifying that a branch or switch is unpredictable.
+  MDNode *createUnpredictable();
+
   /// Return metadata containing the entry count for a function.
   MDNode *createFunctionEntryCount(uint64_t Count);
 

llvm/lib/IR/LLVMContext.cpp

@@ -110,6 +110,12 @@ LLVMContext::LLVMContext() : pImpl(new LLVMContextImpl(*this)) {
   assert(MakeImplicitID == MD_make_implicit &&
          "make.implicit kind id drifted");
   (void)MakeImplicitID;
+
+  // Create the 'unpredictable' metadata kind.
+  unsigned UnpredictableID = getMDKindID("unpredictable");
+  assert(UnpredictableID == MD_unpredictable &&
+         "unpredictable kind id drifted");
+  (void)UnpredictableID;
 }
 LLVMContext::~LLVMContext() { delete pImpl; }
 

llvm/lib/IR/MDBuilder.cpp

@@ -53,6 +53,10 @@ MDNode *MDBuilder::createBranchWeights(ArrayRef<uint32_t> Weights) {
   return MDNode::get(Context, Vals);
 }
 
+MDNode *MDBuilder::createUnpredictable() {
+  return MDNode::get(Context, None);
+}
+
 MDNode *MDBuilder::createFunctionEntryCount(uint64_t Count) {
   SmallVector<Metadata *, 2> Vals(2);
   Vals[0] = createString("function_entry_count");