[EarlyCSE] Teach about CSE'ing over invariant.start intrinsics

Summary:
Teach EarlyCSE about invariant.start intrinsic. Specifically, we can perform
store-load, load-load forwarding over this call.

Reviewers: majnemer, reames, dberlin, sanjoy

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D23268

llvm-svn: 278153

1 files changed, 13 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Scalar/EarlyCSE.cpp b/llvm/lib/Transforms/Scalar/EarlyCSE.cpp
index 6376aae5cbd..0a8a7508c19 100644
--- a/llvm/lib/Transforms/Scalar/EarlyCSE.cpp
+++ b/llvm/lib/Transforms/Scalar/EarlyCSE.cpp
@@ -562,6 +562,19 @@ bool EarlyCSE::processNode(DomTreeNode *Node) {
       continue;
     }
 
+    // Skip invariant.start intrinsics since they only read memory, and we can
+    // forward values across it. Also, we dont need to consume the last store
+    // since the semantics of invariant.start allow us to perform DSE of the
+    // last store, if there was a store following invariant.start. Consider:
+    //
+    // store 30, i8* p
+    // invariant.start(p)
+    // store 40, i8* p
+    // We can DSE the store to 30, since the store 40 to invariant location p
+    // causes undefined behaviour.
+    if (match(Inst, m_Intrinsic<Intrinsic::invariant_start>()))
+      continue;
+
     if (match(Inst, m_Intrinsic<Intrinsic::experimental_guard>())) {
       if (auto *CondI =
               dyn_cast<Instruction>(cast<CallInst>(Inst)->getArgOperand(0))) {