[SimplifyCFG] Don't speculatively execute BB if it's predictably not taken

If the branch isn't `unpredictable`, and it is predicted to *not* branch
to the block we are considering speculatively executing,
then it seems counter-productive to execute the code that is predicted not to be executed.

Reviewed By: spatel

Differential Revision: https://reviews.llvm.org/D106650

1 files changed, 14 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index a58568d..f2f1519 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -2372,6 +2372,20 @@ bool SimplifyCFGOpt::SpeculativelyExecuteBB(BranchInst *BI, BasicBlock *ThenBB,
   }
   assert(EndBB == BI->getSuccessor(!Invert) && "No edge from to end block");
 
+  // If the branch is non-unpredictable, and is predicted to *not* branch to
+  // the `then` block, then avoid speculating it.
+  if (!BI->getMetadata(LLVMContext::MD_unpredictable)) {
+    uint64_t TWeight, FWeight;
+    if (BI->extractProfMetadata(TWeight, FWeight) && (TWeight + FWeight) != 0) {
+      uint64_t EndWeight = Invert ? TWeight : FWeight;
+      BranchProbability BIEndProb =
+          BranchProbability::getBranchProbability(EndWeight, TWeight + FWeight);
+      BranchProbability Likely = TTI.getPredictableBranchThreshold();
+      if (BIEndProb >= Likely)
+        return false;
+    }
+  }
+
   // Keep a count of how many times instructions are used within ThenBB when
   // they are candidates for sinking into ThenBB. Specifically:
   // - They are defined in BB, and