[AutoFDO] Fix handling of empty profiles

Summary:
If a profile has no samples for a function, then the function "entry count" is set to the value 0. Several places in the code test that if the Function::getEntryCount is defined at all. Here we change to treat a 0 entry count the same as undefined.

In particular, this fixes a problem in getLayoutSuccessorProbThreshold in MachineBlockPlacement.cpp where we use a different and inferior heuristic for laying out basic blocks.

Reviewers: danielcdh, dnovillo

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D23082

llvm-svn: 277849

1 files changed, 4 insertions, 1 deletions

diff --git a/llvm/lib/IR/Function.cpp b/llvm/lib/IR/Function.cpp
index 54638abe4c6..d7baa987b5c 100644
--- a/llvm/lib/IR/Function.cpp
+++ b/llvm/lib/IR/Function.cpp
@@ -1262,7 +1262,10 @@ Optional<uint64_t> Function::getEntryCount() const {
     if (MDString *MDS = dyn_cast<MDString>(MD->getOperand(0)))
       if (MDS->getString().equals("function_entry_count")) {
         ConstantInt *CI = mdconst::extract<ConstantInt>(MD->getOperand(1));
-        return CI->getValue().getZExtValue();
+        uint64_t Count = CI->getValue().getZExtValue();
+        if (Count == 0)
+          return None;
+        return Count;
       }
   return None;
 }