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

1 files changed, 6 insertions, 0 deletions

diff --git a/llvm/lib/IR/LLVMContext.cpp b/llvm/lib/IR/LLVMContext.cpp
index af7609f..77fd3e1 100644
--- a/llvm/lib/IR/LLVMContext.cpp
+++ b/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; }