Add a new function attribute 'cold' to functions.

Other than recognizing the attribute, the patch does little else.
It changes the branch probability analyzer so that edges into
blocks postdominated by a cold function are given low weight.

Added analysis and code generation tests.  Added documentation for the
new attribute.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@182638 91177308-0d34-0410-b5e6-96231b3b80d8

3 files changed, 6 insertions, 0 deletions

diff --git a/include/llvm-c/Core.h b/include/llvm-c/Core.h
index 0920c5d0b0..409c0f15c5 100644
--- a/include/llvm-c/Core.h
+++ b/include/llvm-c/Core.h
@@ -166,6 +166,7 @@ typedef enum {
        and the path forward agreed upon.
     LLVMAddressSafety = 1ULL << 32,
     LLVMStackProtectStrongAttribute = 1ULL<<33
+    LLVMCold = 1ULL << 34
     */
 } LLVMAttribute;
 
diff --git a/include/llvm/Analysis/BranchProbabilityInfo.h b/include/llvm/Analysis/BranchProbabilityInfo.h
index 6c23f7c3ae..4ff7121728 100644
--- a/include/llvm/Analysis/BranchProbabilityInfo.h
+++ b/include/llvm/Analysis/BranchProbabilityInfo.h
@@ -131,11 +131,15 @@ private:
   /// \brief Track the set of blocks directly succeeded by a returning block.
   SmallPtrSet<BasicBlock *, 16> PostDominatedByUnreachable;
 
+  /// \brief Track the set of blocks that always lead to a cold call.
+  SmallPtrSet<BasicBlock *, 16> PostDominatedByColdCall;
+
   /// \brief Get sum of the block successors' weights.
   uint32_t getSumForBlock(const BasicBlock *BB) const;
 
   bool calcUnreachableHeuristics(BasicBlock *BB);
   bool calcMetadataWeights(BasicBlock *BB);
+  bool calcColdCallHeuristics(BasicBlock *BB);
   bool calcPointerHeuristics(BasicBlock *BB);
   bool calcLoopBranchHeuristics(BasicBlock *BB);
   bool calcZeroHeuristics(BasicBlock *BB);
diff --git a/include/llvm/IR/Attributes.h b/include/llvm/IR/Attributes.h
index 2c7da6485d..0d14709fe9 100644
--- a/include/llvm/IR/Attributes.h
+++ b/include/llvm/IR/Attributes.h
@@ -68,6 +68,7 @@ public:
                            ///< 0 means unaligned (different from align(1))
     AlwaysInline,          ///< inline=always
     ByVal,                 ///< Pass structure by value
+    Cold,                  ///< Marks function as being in a cold path.
     InlineHint,            ///< Source said inlining was desirable
     InReg,                 ///< Force argument to be passed in register
     MinSize,               ///< Function must be optimized for size first