[LICM] Hoist an invariant_start out of loops if there are no stores executed before it

Once the invariant_start is reached, we know that no instruction *after* it can modify the memory. So, if we can prove the location isn't read *between entry into the loop and the execution of the invariant_start*, we can execute the invariant_start before entering the loop.

Differential Revision: https://reviews.llvm.org/D51181

llvm-svn: 340617

1 files changed, 3 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/Scalar/LICM.cpp b/llvm/lib/Transforms/Scalar/LICM.cpp
index da750d00341..a642d2ef9ff 100644
--- a/llvm/lib/Transforms/Scalar/LICM.cpp
+++ b/llvm/lib/Transforms/Scalar/LICM.cpp
@@ -525,7 +525,9 @@ bool llvm::hoistRegion(DomTreeNode *N, AliasAnalysis *AA, LoopInfo *LI,
       }
 
       using namespace PatternMatch;
-      if (match(&I, m_Intrinsic<Intrinsic::experimental_guard>()) &&
+      if (((I.use_empty() &&
+            match(&I, m_Intrinsic<Intrinsic::invariant_start>())) ||
+           match(&I, m_Intrinsic<Intrinsic::experimental_guard>())) &&
           IsMustExecute && IsMemoryNotModified &&
           CurLoop->hasLoopInvariantOperands(&I)) {
         hoist(I, DT, CurLoop, SafetyInfo, ORE);