The code that originally made me discover this is:

  if ((a & 0x1) == 0x1) {
    ..
  }

In this case we don't actually have any branch probability information and
should not assume to have any. LLVM transforms this into:

  %and = and i32 %a, 1
  %tobool = icmp eq i32 %and, 0

So, in this case, the result of a bitwise and is compared against 0,
but nevertheless, we should not assume to have probability
information.

llvm-svn: 234898

1 files changed, 8 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/BranchProbabilityInfo.cpp b/llvm/lib/Analysis/BranchProbabilityInfo.cpp
index 14800f4a28b..d7cee3afaf5 100644
--- a/llvm/lib/Analysis/BranchProbabilityInfo.cpp
+++ b/llvm/lib/Analysis/BranchProbabilityInfo.cpp
@@ -379,6 +379,14 @@ bool BranchProbabilityInfo::calcZeroHeuristics(BasicBlock *BB) {
   if (!CV)
     return false;
 
+  // If the LHS is the result of AND'ing a value with a single bit bitmask,
+  // we don't have information about probabilities.
+  if (Instruction *LHS = dyn_cast<Instruction>(CI->getOperand(0)))
+    if (LHS->getOpcode() == Instruction::And)
+      if (ConstantInt *AndRHS = dyn_cast<ConstantInt>(LHS->getOperand(1)))
+        if (AndRHS->getUniqueInteger().isPowerOf2())
+          return false;
+
   bool isProb;
   if (CV->isZero()) {
     switch (CI->getPredicate()) {