[DDG] Data Dependence Graph - Add query function for memory dependencies between two nodes

[bmahjour]

Summary

When working with the DDG it's useful to be able to query details of the memory dependencies between two nodes connected by a memory edge. The DDG does not hold a copy of the dependencies, but it contains a reference to a DependenceInfo object through which dependence information can be queried. This patch adds a query function to the DDG to obtain all the Dependence objects that exist between instructions of two nodes.

The same function has also been proposed under https://reviews.llvm.org/D73801, but since the review may take loner we'd like to add this query function as a separate revision here to unblock other uses of the DDG.

[Whitney]

llvm/unittests/Analysis/DDGTest.cpp

82

line 82 - 89 needed?

101

LI.front() should give you the first loop in a function.

102

should this be an assert instead?

[bmahjour]

Addressed clang-format issues and Whitney's comments.

[Whitney]

LGTM. Will approve tomorrow if no objection.

llvm/include/llvm/Analysis/DDG.h

public:
 
   /// Return the root node of the graph.
   NodeType &getRoot() const {
     assert(Root && "Root node is not available yet. Graph construction may "
                    "still be in progress\n");
     return *Root;
   }
 
+  /// Collect all the data dependency infos coming from any pair of memory
+  /// accesses from \p Src to \p Dst, and store them into \p Deps. Return true if
+  /// a dependence exists, and false otherwise.
+  bool getDependencies(const NodeType &Src, const NodeType &Dst,
+                       DependenceList &Deps) const;
 protected:
   // Name of the graph.
   std::string Name;
 
   // Store a copy of DependenceInfo in the graph, so that individual memory
   // dependencies don't need to be stored. Instead when the dependence is
   // queried it is recomputed using @DI.
   const DependenceInfo DI;
public:
   PreservedAnalyses run(Loop &L, LoopAnalysisManager &AM,
                         LoopStandardAnalysisResults &AR, LPMUpdater &U);
 
 private:
   raw_ostream &OS;
 };
 
 //===--------------------------------------------------------------------===//
+// DependenceGraphInfo Implementation
+//===--------------------------------------------------------------------===//
+
+template <typename NodeType>
+bool DependenceGraphInfo<NodeType>::getDependencies(
+    const NodeType &Src, const NodeType &Dst, DependenceList &Deps) const {
+  assert(Deps.empty() && "Expected empty output list at the start.");
+
+  // List of memory access instructions from src and dst nodes.
+  SmallVector<Instruction *, 8> SrcIList, DstIList;
+  auto isMemoryAccess = [](const Instruction *I) {
+    return I->mayReadOrWriteMemory();
+  };
+  Src.collectInstructions(isMemoryAccess, SrcIList);
+  Dst.collectInstructions(isMemoryAccess, DstIList);
+
+  for (auto *SrcI : SrcIList)
+    for (auto *DstI : DstIList)
+      if (auto Dep =
+              const_cast<DependenceInfo *>(&DI)->depends(SrcI, DstI, true))
+        Deps.push_back(std::move(Dep));
+
+  return !Deps.empty();
+}
+
+//===--------------------------------------------------------------------===//
 // GraphTraits specializations for the DDG
 //===--------------------------------------------------------------------===//
 
 /// non-const versions of the grapth trait specializations for DDG
 template <> struct GraphTraits<DDGNode *> {
   using NodeRef = DDGNode *;
 
   static DDGNode *DDGGetTargetNode(DGEdge<DDGNode, DDGEdge> *P) {

llvm/unittests/Analysis/CMakeLists.txt

add_llvm_unittest(AnalysisTests
   AssumeBundleQueriesTest.cpp
   BasicAliasAnalysisTest.cpp
   BlockFrequencyInfoTest.cpp
   BranchProbabilityInfoTest.cpp
   CallGraphTest.cpp
   CaptureTrackingTest.cpp
   CFGTest.cpp
   CGSCCPassManagerTest.cpp
+  DDGTest.cpp
   DivergenceAnalysisTest.cpp
   DomTreeUpdaterTest.cpp
   GlobalsModRefTest.cpp
   IVDescriptorsTest.cpp
   LazyCallGraphTest.cpp
   LoadsTest.cpp
   LoopInfoTest.cpp
   LoopNestTest.cpp

llvm/unittests/Analysis/DDGTest.cpp

+//===- DDGTest.cpp - DDGAnalysis unit tests -------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Analysis/DDG.h"
+#include "llvm/Analysis/AliasAnalysis.h"
+#include "llvm/Analysis/BasicAliasAnalysis.h"
+#include "llvm/Analysis/ScalarEvolution.h"
+#include "llvm/Analysis/TargetLibraryInfo.h"
+#include "llvm/AsmParser/Parser.h"
+#include "llvm/IR/Dominators.h"
+#include "llvm/Support/SourceMgr.h"
+#include "gtest/gtest.h"
+
+using namespace llvm;
+
+/// Build the DDG analysis for a loop and run the given test \p Test.
+static void runTest(
+    Module &M, StringRef FuncName,
+    function_ref<void(Function &F, LoopInfo &LI, DependenceInfo &DI, ScalarEvolution &SE)> Test) {
+  auto *F = M.getFunction(FuncName);
+  ASSERT_NE(F, nullptr) << "Could not find " << FuncName;
+
+  TargetLibraryInfoImpl TLII;
+  TargetLibraryInfo TLI(TLII);
+  AssumptionCache AC(*F);
+  DominatorTree DT(*F);
+  LoopInfo LI(DT);
+  ScalarEvolution SE(*F, TLI, AC, DT, LI);
+  AAResults AA(TLI);
+  DependenceInfo DI(F, &AA, &SE, &LI);
+  Test(*F, LI, DI, SE);
+}
+
+static std::unique_ptr<Module> makeLLVMModule(LLVMContext &Context,
+                                              const char *ModuleStr) {
+  SMDiagnostic Err;
+  return parseAssemblyString(ModuleStr, Err, Context);
+}
+
+TEST(DDGTest, getDependencies) {
+  const char *ModuleStr =
+      "target datalayout = \"e-m:e-i64:64-n32:64\"\n"
+      "target triple = \"powerpc64le-unknown-linux-gnu\"\n"
+      "\n"
+      "define dso_local void @foo(i32 signext %n, i32* noalias %A, i32* "
+      "noalias %B) {\n"
+      "entry:\n"
+      "   %cmp1 = icmp sgt i32 %n, 0\n"
+      "   br i1 %cmp1, label %for.body.preheader, label %for.end\n"
+      "\n"
+      "for.body.preheader:\n"
+      "   %wide.trip.count = zext i32 %n to i64\n"
+      "   br label %for.body\n"
+      " \n"
+      " for.body:\n"
+      "   %indvars.iv = phi i64 [ 0, %for.body.preheader ], [ "
+      "%indvars.iv.next, %for.body ]\n"
+      "   %arrayidx = getelementptr inbounds i32, i32* %A, i64 %indvars.iv\n"
+      "  %0 = trunc i64 %indvars.iv to i32\n"
+      "  store i32 %0, i32* %arrayidx, align 4, !tbaa !2\n"
+      "  %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1\n"
+      "  %arrayidx2 = getelementptr inbounds i32, i32* %A, i64 "
+      "%indvars.iv.next\n"
+      "  %1 = load i32, i32* %arrayidx2, align 4, !tbaa !2\n"
+      "  %add3 = add nsw i32 %1, 1\n"
+      "  %arrayidx5 = getelementptr inbounds i32, i32* %B, i64 %indvars.iv\n"
+      "  store i32 %add3, i32* %arrayidx5, align 4, !tbaa !2\n"
+      "  %exitcond = icmp ne i64 %indvars.iv.next, %wide.trip.count\n"
+      "  br i1 %exitcond, label %for.body, label %for.end.loopexit\n"
+      "\n"
+      "for.end.loopexit:\n"
+      "  br label %for.end\n"
+      "\n"
+      "for.end:\n"
+      "  ret void\n"
+      " }\n"
+      "!llvm.module.flags = !{!0}\n"
+      "!llvm.ident = !{!1}\n"
+      "!0 = !{i32 1, !\"wchar_size\", i32 4}\n"
+      "!1 = !{!\"clang version 11.0.0\"}\n"
+      "!2 = !{!3, !3, i64 0}\n"
+      "!3 = !{!\"int\", !4, i64 0}\n"
+      "!4 = !{!\"omnipotent char\", !5, i64 0}\n"
+      "!5 = !{!\"Simple C/C++ TBAA\"}\n";
+
+  LLVMContext Context;
+  std::unique_ptr<Module> M = makeLLVMModule(Context, ModuleStr);
+
+  runTest(*M, "foo", [&](Function &F, LoopInfo &LI, DependenceInfo &DI, ScalarEvolution &SE) {
+      Function::iterator FI = F.begin();
+      // Skip the basic blocks until we get a loop.
+      while (!LI.getLoopFor(&(*FI)))
+        ++FI;
+      BasicBlock *Header = &(*FI);      
+      assert(Header->getName() == "for.body");
+      Loop *L = LI.getLoopFor(Header);
+      EXPECT_NE(L, nullptr);
+
+      DataDependenceGraph DDG(*L, LI, DI);
+
+      // Collect all the nodes that have an outgoing memory edge
+      // while collecting all memory edges as well. There should
+      // only be one node with an outgoing memory edge and there
+      // should only be one memory edge in the entire graph.
+      std::vector<DDGNode *> DependenceSourceNodes;
+      std::vector<DDGEdge *> MemoryEdges;
+      for (DDGNode *N : DDG) {
+        for (DDGEdge *E : *N) {
+          bool SourceAdded = false;
+          if (E->isMemoryDependence()) {
+            MemoryEdges.push_back(E);
+            if (!SourceAdded) {
+              DependenceSourceNodes.push_back(N);
+              SourceAdded = true;
+            }
+          }
+        }
+      }
+
+      EXPECT_EQ(DependenceSourceNodes.size(), 1ull);
+      EXPECT_EQ(MemoryEdges.size(), 1ull);
+
+      DataDependenceGraph::DependenceList DL;
+      DDG.getDependencies(*DependenceSourceNodes.back(), MemoryEdges.back()->getTargetNode(), DL);
+
+      EXPECT_EQ(DL.size(), 1ull);
+      EXPECT_TRUE(DL.back()->isAnti());
+      EXPECT_EQ(DL.back()->getLevels(), 1u);
+      EXPECT_NE(DL.back()->getDistance(1), nullptr);
+      EXPECT_EQ(DL.back()->getDistance(1), SE.getOne(DL.back()->getDistance(1)->getType()));
+  });
+}