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

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.

Authored By: bmahjour

Reviewers: Meinersbur, Whitney, etiotto

Reviewed By: Whitney

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D80529

llvm/include/llvm/Analysis/DDG.h

@@ -284,6 +284,12 @@ public:
     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;
@@ -431,6 +437,32 @@ 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
 //===--------------------------------------------------------------------===//

llvm/unittests/Analysis/CMakeLists.txt

@@ -17,6 +17,7 @@ add_llvm_unittest(AnalysisTests
   CaptureTrackingTest.cpp
   CFGTest.cpp
   CGSCCPassManagerTest.cpp
+  DDGTest.cpp
   DivergenceAnalysisTest.cpp
   DomTreeUpdaterTest.cpp
   GlobalsModRefTest.cpp

llvm/unittests/Analysis/DDGTest.cpp

@@ -0,0 +1,128 @@
+//===- 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\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\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\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";
+
+  LLVMContext Context;
+  std::unique_ptr<Module> M = makeLLVMModule(Context, ModuleStr);
+
+  runTest(
+      *M, "foo",
+      [&](Function &F, LoopInfo &LI, DependenceInfo &DI, ScalarEvolution &SE) {
+        Loop *L = *LI.begin();
+        assert(L && "expected the loop to be identified.");
+
+        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()));
+      });
+}