[LICM] Hoist guards with invariant conditions

This patch teaches LICM to hoist guards from the loop if they are guaranteed to execute and
if there are no side effects that could prevent that.

Differential Revision: https://reviews.llvm.org/D50501
Reviewed By: reames

llvm-svn: 340256

1 files changed, 17 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Scalar/LICM.cpp b/llvm/lib/Transforms/Scalar/LICM.cpp
index ee187c1d601..aea09c76524 100644
--- a/llvm/lib/Transforms/Scalar/LICM.cpp
+++ b/llvm/lib/Transforms/Scalar/LICM.cpp
@@ -58,6 +58,7 @@
 #include "llvm/IR/IntrinsicInst.h"
 #include "llvm/IR/LLVMContext.h"
 #include "llvm/IR/Metadata.h"
+#include "llvm/IR/PatternMatch.h"
 #include "llvm/IR/PredIteratorCache.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/Debug.h"
@@ -465,6 +466,11 @@ bool llvm::hoistRegion(DomTreeNode *N, AliasAnalysis *AA, LoopInfo *LI,
     // that the next instruction visited is guaranteed to execute if the loop
     // is entered.
     bool IsMustExecute = CurLoop->getHeader() == BB;
+    // Keep track of whether the prefix instructions could have written memory.
+    // TODO: This and IsMustExecute may be done smarter if we keep track of all
+    // throwing and mem-writing operations in every block, e.g. using something
+    // similar to isGuaranteedToExecute.
+    bool IsMemoryNotModified = CurLoop->getHeader() == BB;
 
     for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E;) {
       Instruction &I = *II++;
@@ -523,8 +529,19 @@ bool llvm::hoistRegion(DomTreeNode *N, AliasAnalysis *AA, LoopInfo *LI,
         continue;
       }
 
+      using namespace PatternMatch;
+      if (match(&I, m_Intrinsic<Intrinsic::experimental_guard>()) &&
+          IsMustExecute && IsMemoryNotModified &&
+          CurLoop->hasLoopInvariantOperands(&I)) {
+        hoist(I, DT, CurLoop, SafetyInfo, ORE);
+        Changed = true;
+        continue;
+      }
+
       if (IsMustExecute)
         IsMustExecute = isGuaranteedToTransferExecutionToSuccessor(&I);
+      if (IsMemoryNotModified)
+        IsMemoryNotModified = !I.mayWriteToMemory();
     }
   }