[PATCH] D35264: [LICM] Teach LICM to hoist conditional loads

[Amara Emerson via Phabricator via llvm-commits]

aemerson created this revision.

If the pointer operand of a load has been accessed by a load/store in a dominating block of the loop then it's safe to hoist.


Repository:
  rL LLVM

https://reviews.llvm.org/D35264

Files:
  lib/Transforms/Scalar/LICM.cpp
  test/Transforms/LICM/hoisting.ll


Index: test/Transforms/LICM/hoisting.ll
===================================================================
--- test/Transforms/LICM/hoisting.ll
+++ test/Transforms/LICM/hoisting.ll
@@ -320,3 +320,37 @@
 loopexit:
   ret i32 %sum
 }
+
+
+declare i32 @foo3(i32) nounwind readonly
+
+;; It is ok and desirable to hoist this load %B due to the load of %A.
+define i32 @test_cond_load(i1 %c, i1 %c2, i32* noalias %ptr, i32* noalias %storeptr) {
+; CHECK-LABEL: @test_cond_load(
+; CHECK: %A = load i32, i32* %ptr
+; CHECK: br label %Loop.ph
+; CHECK: %B = load i32, i32* %ptr
+; CHECK: br label %Loop
+; CHECK: Loop:
+entry:
+	%A = load i32, i32* %ptr
+        br label %Loop.ph
+
+Loop.ph:
+        br label %Loop
+
+Loop:
+        br i1 %c2, label %if.then, label %Loop.cont
+if.then:
+	%B = load i32, i32* %ptr
+        %res = call i32 @foo3(i32 %B)
+        store i32 %res, i32* %storeptr
+        br label %Loop.cont
+
+Loop.cont:
+        %res.phi = phi i32 [0, %Loop], [%res, %if.then]
+	br i1 %c, label %Loop, label %Out
+Out:		; preds = %Loop
+	%C = sub i32 %A, %res.phi
+	ret i32 %C
+}
Index: lib/Transforms/Scalar/LICM.cpp
===================================================================
--- lib/Transforms/Scalar/LICM.cpp
+++ lib/Transforms/Scalar/LICM.cpp
@@ -903,6 +903,33 @@
   return true;
 }
 
+/// Checks if the given load's pointer is dereferenceable on all paths to the
+/// loop preheader.
+static bool isLoadPtrDereferenceable(LoadInst *LI, const DominatorTree *DT,
+                                     const Loop *CurLoop) {
+  if (LI->isVolatile() || LI->isAtomic())
+    return false;
+
+  BasicBlock *PH = CurLoop->getLoopPreheader();
+  Value *Ptr = LI->getPointerOperand();
+
+  for (auto *PUser : Ptr->users()) {
+    if (isa<LoadInst>(PUser) || isa<StoreInst>(PUser)) {
+      Instruction *LSI = cast<Instruction>(PUser);
+      // For a common case of the pointer originating from the loop, avoid a DT
+      // query in favour of a densemap lookup.
+      if (CurLoop->contains(LSI->getParent()) ||
+          !DT->dominates(LSI->getParent(), PH))
+        continue;
+      // Since the block dominates the preheader, we know that execution must
+      // have transferred from the user's block to its successors.
+      return true;
+    }
+  }
+  return false;
+}
+
+
 /// Only sink or hoist an instruction if it is not a trapping instruction,
 /// or if the instruction is known not to trap when moved to the preheader.
 /// or if it is a trapping instruction and is guaranteed to execute.
@@ -919,7 +946,12 @@
       isGuaranteedToExecute(Inst, DT, CurLoop, SafetyInfo);
 
   if (!GuaranteedToExecute) {
+    // Conditional loads may be hoisted if the address has already been accessed
+    // by a dominating block.
     auto *LI = dyn_cast<LoadInst>(&Inst);
+    if (LI && isLoadPtrDereferenceable(LI, DT, CurLoop))
+      return true;
+
     if (LI && CurLoop->isLoopInvariant(LI->getPointerOperand()))
       ORE->emit(OptimizationRemarkMissed(
                     DEBUG_TYPE, "LoadWithLoopInvariantAddressCondExecuted", LI)


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