[LICM] Extend the MustExecute scope

CanProveNotTakenFirstIteration utility does not handle the case when
condition of the branch is a constant. Add its handling.

Reviewers: reames, anna, mkazantsev
Reviewed By: reames
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D46996

llvm-svn: 332695

1 files changed, 4 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/MustExecute.cpp b/llvm/lib/Analysis/MustExecute.cpp
index d8bc7fa19ae..7b1bb2037f3 100644
--- a/llvm/lib/Analysis/MustExecute.cpp
+++ b/llvm/lib/Analysis/MustExecute.cpp
@@ -70,6 +70,10 @@ static bool CanProveNotTakenFirstIteration(BasicBlock *ExitBlock,
   auto *BI = dyn_cast<BranchInst>(CondExitBlock->getTerminator());
   if (!BI || !BI->isConditional())
     return false;
+  // If condition is constant and false leads to ExitBlock then we always
+  // execute the true branch.
+  if (auto *Cond = dyn_cast<ConstantInt>(BI->getCondition()))
+    return BI->getSuccessor(Cond->getZExtValue() ? 1 : 0) == ExitBlock;
   auto *Cond = dyn_cast<CmpInst>(BI->getCondition());
   if (!Cond)
     return false;