[LoopVersioning] Expose loop versioning as a pass too

[anemet]

Summary

LoopVersioning is a transform utility that transform passes can use to
run-time disambiguate may-aliasing accesses. I'd like to also expose as
pass to allow it to be unit-tested.

I am planning to add support for non-aliasing annotation in
LoopVersioning and I'd like to be able to write tests directly using
this pass.

(After that feature is done, the pass could also be used to look for
optimization opportunities that are hidden behind incomplete alias
information at compile time.)

The pass drives LoopVersioning in its default way which is to fully
disambiguate may-aliasing accesses no matter how many checks are
required.

[sbaranga]

Hi Adam,

Thanks, I can see how this can be very useful.
Are you planning on using this for anything other than unit testing?

Otherwise, LGTM!

Thanks,
Silviu

lib/Transforms/Utils/LoopVersioning.cpp

166

nit:

vectorize -> version
distributing -> versioning

[sbaranga]

I have to retract my previous LGTM, I just spotted something that needs updating.

lib/Transforms/Utils/LoopVersioning.cpp

180

This needs to be (LAI.getNumRuntimePointerChecks() || !LAI.PSE.getUnionPredicate().isAlwaysTrue()).

Maybe it would also make sense to have a separate method in LAI for this logic (something like needsRuntimeChecks()?)

[anemet]

Thanks for the review, Silviu.

In D16612#340485, @sbaranga wrote:

Are you planning on using this for anything other than unit testing?

Not really other than for experimentation. I may propose a patch to schedule an off-by-default versioning early on in the pass pipeline. This could be tool for finding missed optimization due to incomplete alias information.

I also just wanted to mention the llvm-dev thread where this was discussed:

http://thread.gmane.org/gmane.comp.compilers.llvm.devel/86024

lib/Transforms/Utils/LoopVersioning.cpp

180

Ah, of course, thanks for catching this!

I didn't add a separate method because looking at the clients, it suggest that the pattern of usage is more like:

• check the number of memchecks, optionally by filtering the checks, fail if too many
• check the number of SCEV predicate checks, fail if too many
• if any was needed, version

[anemet]

Address Silviu's comments

[sbaranga]

Thanks, Adam! LGTM

-Silviu

include/llvm/InitializePasses.h

 void initializeDwarfEHPreparePass(PassRegistry&);
 void initializeFloat2IntPass(PassRegistry&);
 void initializeLoopDistributePass(PassRegistry&);
 void initializeSjLjEHPreparePass(PassRegistry&);
 void initializeDemandedBitsPass(PassRegistry&);
 void initializeFuncletLayoutPass(PassRegistry &);
 void initializeLoopLoadEliminationPass(PassRegistry&);
 void initializeFunctionImportPassPass(PassRegistry &);
+void initializeLoopVersioningPassPass(PassRegistry &);
 }
 
 #endif

include/llvm/Transforms/Scalar.h

 FunctionPass *createLoopDistributePass();
 
 //===----------------------------------------------------------------------===//
 //
 // LoopLoadElimination - Perform loop-aware load elimination.
 //
 FunctionPass *createLoopLoadEliminationPass();
 
+//===----------------------------------------------------------------------===//
+//
+// LoopVersioning - Perform loop multi-versioning.
+//
+FunctionPass *createLoopVersioningPass();
+
 } // End llvm namespace
 
 #endif

lib/Transforms/Scalar/Scalar.cpp

void llvm::initializeScalarOpts(PassRegistry &Registry) {
   initializeSpeculativeExecutionPass(Registry);
   initializeStraightLineStrengthReducePass(Registry);
   initializeLoadCombinePass(Registry);
   initializePlaceBackedgeSafepointsImplPass(Registry);
   initializePlaceSafepointsPass(Registry);
   initializeFloat2IntPass(Registry);
   initializeLoopDistributePass(Registry);
   initializeLoopLoadEliminationPass(Registry);
+  initializeLoopVersioningPassPass(Registry);
 }
 
 void LLVMInitializeScalarOpts(LLVMPassRegistryRef R) {
   initializeScalarOpts(*unwrap(R));
 }
 
 void LLVMAddAggressiveDCEPass(LLVMPassManagerRef PM) {
   unwrap(PM)->add(createAggressiveDCEPass());

lib/Transforms/Utils/LoopVersioning.cpp

if (!PN) {
         if (!VersionedLoop->contains(cast<Instruction>(User)->getParent()))
           User->replaceUsesOfWith(Inst, PN);
       PN->addIncoming(Inst, VersionedLoop->getExitingBlock());
     }
     // Add the new incoming value from the non-versioned loop.
     PN->addIncoming(NonVersionedLoopInst, NonVersionedLoop->getExitingBlock());
   }
 }
+
+namespace {
+/// \brief Also expose this is a pass.  Currently this is only used for
+/// unit-testing.  It adds all memchecks necessary to remove all may-aliasing
+/// array accesses from the loop.
+class LoopVersioningPass : public FunctionPass {
+public:
+  LoopVersioningPass() : FunctionPass(ID) {
+    initializeLoopVersioningPassPass(*PassRegistry::getPassRegistry());
+  }
+
+  bool runOnFunction(Function &F) override {
+    auto *LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
+    auto *LAA = &getAnalysis<LoopAccessAnalysis>();
+    auto *DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
+    auto *SE = &getAnalysis<ScalarEvolutionWrapperPass>().getSE();
+
+    // Build up a worklist of inner-loops to version. This is necessary as the
+    // act of versioning a loop creates new loops and can invalidate iterators
+    // across the loops.
+    SmallVector<Loop *, 8> Worklist;
+
+    for (Loop *TopLevelLoop : *LI)
+      for (Loop *L : depth_first(TopLevelLoop))
+        // We only handle inner-most loops.
+        if (L->empty())
+          Worklist.push_back(L);
+
+    // Now walk the identified inner loops.
+    bool Changed = false;
+    for (Loop *L : Worklist) {
+      const LoopAccessInfo &LAI = LAA->getInfo(L, ValueToValueMap());
+      if (LAI.getNumRuntimePointerChecks() ||
+          !LAI.PSE.getUnionPredicate().isAlwaysTrue()) {
+        LoopVersioning LVer(LAI, L, LI, DT, SE);
+        LVer.versionLoop();
+        Changed = true;
+      }
+    }
+
+    return Changed;
+  }
+
+  void getAnalysisUsage(AnalysisUsage &AU) const override {
+    AU.addRequired<LoopInfoWrapperPass>();
+    AU.addPreserved<LoopInfoWrapperPass>();
+    AU.addRequired<LoopAccessAnalysis>();
+    AU.addRequired<DominatorTreeWrapperPass>();
+    AU.addPreserved<DominatorTreeWrapperPass>();
+    AU.addRequired<ScalarEvolutionWrapperPass>();
+  }
+
+  static char ID;
+};
+}
+
+#define LVER_OPTION "loop-versioning"
+#define DEBUG_TYPE LVER_OPTION
+
+char LoopVersioningPass::ID;
+static const char LVer_name[] = "Loop Versioning";
+
+INITIALIZE_PASS_BEGIN(LoopVersioningPass, LVER_OPTION, LVer_name, false, false)
+INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
+INITIALIZE_PASS_DEPENDENCY(LoopAccessAnalysis)
+INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
+INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
+INITIALIZE_PASS_END(LoopVersioningPass, LVER_OPTION, LVer_name, false, false)
+
+namespace llvm {
+FunctionPass *createLoopVersioningPass() {
+  return new LoopVersioningPass();
+}
+}

test/Transforms/LoopVersioning/basic.ll

+; RUN: opt -basicaa -loop-versioning -S < %s | FileCheck %s
+
+target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
+
+; Version this loop with overlap checks between a, c and b, c.
+
+define void @f(i32* %a, i32* %b, i32* %c) {
+entry:
+  br label %for.body
+
+; CHECK: for.body.lver.check:
+; CHECK:   icmp
+; CHECK:   icmp
+; CHECK:   icmp
+; CHECK:   icmp
+; CHECK-NOT: icmp
+; CHECK:   br i1 %memcheck.conflict, label %for.body.ph.lver.orig, label %for.body.ph
+
+; CHECK: for.body.ph.lver.orig:
+; CHECK: for.body.lver.orig:
+; CHECK:   br i1 %exitcond.lver.orig, label %for.end, label %for.body.lver.orig
+; CHECK: for.body.ph:
+; CHECK: for.body:
+; CHECK:   br i1 %exitcond, label %for.end, label %for.body
+; CHECK: for.end:
+
+for.body:                                         ; preds = %for.body, %entry
+  %ind = phi i64 [ 0, %entry ], [ %add, %for.body ]
+
+  %arrayidxA = getelementptr inbounds i32, i32* %a, i64 %ind
+  %loadA = load i32, i32* %arrayidxA, align 4
+
+  %arrayidxB = getelementptr inbounds i32, i32* %b, i64 %ind
+  %loadB = load i32, i32* %arrayidxB, align 4
+
+  %mulC = mul i32 %loadA, %loadB
+
+  %arrayidxC = getelementptr inbounds i32, i32* %c, i64 %ind
+  store i32 %mulC, i32* %arrayidxC, align 4
+
+  %add = add nuw nsw i64 %ind, 1
+  %exitcond = icmp eq i64 %add, 20
+  br i1 %exitcond, label %for.end, label %for.body
+
+for.end:                                          ; preds = %for.body
+  ret void
+}