[LoopInfo] Fix function getInductionVariable

The way function gets the induction variable is by judging whether
StepInst or IndVar in the phi statement is one of the operands of CMP.
But if the LatchCmpOp0/LatchCmpOp1 is a constant,  the subsequent
comparison may result in null == null, which is meaningless. This patch
fixes the typo.

Reviewed By: Whitney

Differential Revision: https://reviews.llvm.org/D112980

Author: duanbo.db

llvm/lib/Analysis/LoopInfo.cpp

@@ -301,15 +301,16 @@ PHINode *Loop::getInductionVariable(ScalarEvolution &SE) const {
   if (!CmpInst)
     return nullptr;
 
-  Instruction *LatchCmpOp0 = dyn_cast<Instruction>(CmpInst->getOperand(0));
-  Instruction *LatchCmpOp1 = dyn_cast<Instruction>(CmpInst->getOperand(1));
+  Value *LatchCmpOp0 = CmpInst->getOperand(0);
+  Value *LatchCmpOp1 = CmpInst->getOperand(1);
 
   for (PHINode &IndVar : Header->phis()) {
     InductionDescriptor IndDesc;
     if (!InductionDescriptor::isInductionPHI(&IndVar, this, &SE, IndDesc))
       continue;
 
-    Instruction *StepInst = IndDesc.getInductionBinOp();
+    BasicBlock *Latch = getLoopLatch();
+    Value *StepInst = IndVar.getIncomingValueForBlock(Latch);
 
     // case 1:
     // IndVar = phi[{InitialValue, preheader}, {StepInst, latch}]

llvm/unittests/Analysis/LoopInfoTest.cpp

@@ -1547,3 +1547,39 @@ TEST(LoopInfoTest, LoopUserBranch) {
     EXPECT_EQ(L->getLoopGuardBranch(), nullptr);
   });
 }
+
+TEST(LoopInfoTest, LoopInductionVariable) {
+  const char *ModuleStr =
+      "define i32 @foo(i32* %addr) {\n"
+      "entry:\n"
+      "  br label %for.body\n"
+      "for.body:\n"
+      "  %sum.08 = phi i32 [ 0, %entry ], [ %add, %for.body ]\n"
+      "  %addr.addr.06 = phi i32* [ %addr, %entry ], [ %incdec.ptr, %for.body "
+      "]\n"
+      "  %count.07 = phi i32 [ 6000, %entry ], [ %dec, %for.body ]\n"
+      "  %0 = load i32, i32* %addr.addr.06, align 4\n"
+      "  %add = add nsw i32 %0, %sum.08\n"
+      "  %dec = add nsw i32 %count.07, -1\n"
+      "  %incdec.ptr = getelementptr inbounds i32, i32* %addr.addr.06, i64 1\n"
+      "  %cmp = icmp ugt i32 %count.07, 1\n"
+      "  br i1 %cmp, label %for.body, label %for.end\n"
+      "for.end:\n"
+      "  %cmp1 = icmp eq i32 %add, -1\n"
+      "  %conv = zext i1 %cmp1 to i32\n"
+      "  ret i32 %conv\n"
+      "}\n";
+
+  // Parse the module.
+  LLVMContext Context;
+  std::unique_ptr<Module> M = makeLLVMModule(Context, ModuleStr);
+
+  runWithLoopInfoPlus(
+      *M, "foo", [&](Function &F, LoopInfo &LI, ScalarEvolution &SE) {
+        Function::iterator FI = F.begin();
+        BasicBlock *Header = &*(++FI);
+        Loop *L = LI.getLoopFor(Header);
+        EXPECT_NE(L, nullptr);
+        EXPECT_EQ(L->getInductionVariable(SE)->getName(), "count.07");
+      });
+}