[LoopUnroll] Add check to Latch's terminator in UnrollRuntimeLoopRema…

…inder

In this patch, I'm adding an extra check to the Latch's terminator in llvm::UnrollRuntimeLoopRemainder,
similar to how it is already done in the llvm::UnrollLoop.

The compiler would crash if this function is called with a malformed loop.

Patch by Rodrigo Caetano Rocha!

Differential Revision: https://reviews.llvm.org/D51486

llvm-svn: 342958

llvm/lib/Transforms/Utils/LoopUnrollRuntime.cpp

@@ -545,13 +545,27 @@ bool llvm::UnrollRuntimeLoopRemainder(Loop *L, unsigned Count,
   BasicBlock *Header = L->getHeader();
 
   BranchInst *LatchBR = cast<BranchInst>(Latch->getTerminator());
+
+  if (!LatchBR || LatchBR->isUnconditional()) {
+    // The loop-rotate pass can be helpful to avoid this in many cases.
+    LLVM_DEBUG(
+        dbgs()
+        << "Loop latch not terminated by a conditional branch.\n");
+    return false;
+  }
+
   unsigned ExitIndex = LatchBR->getSuccessor(0) == Header ? 1 : 0;
   BasicBlock *LatchExit = LatchBR->getSuccessor(ExitIndex);
-  // Cloning the loop basic blocks (`CloneLoopBlocks`) requires that one of the
-  // targets of the Latch be an exit block out of the loop. This needs
-  // to be guaranteed by the callers of UnrollRuntimeLoopRemainder.
-  assert(!L->contains(LatchExit) &&
-         "one of the loop latch successors should be the exit block!");
+
+  if (L->contains(LatchExit)) {
+    // Cloning the loop basic blocks (`CloneLoopBlocks`) requires that one of the
+    // targets of the Latch be an exit block out of the loop.
+    LLVM_DEBUG(
+        dbgs()
+        << "One of the loop latch successors must be the exit block.\n");
+    return false;
+  }
+
   // These are exit blocks other than the target of the latch exiting block.
   SmallVector<BasicBlock *, 4> OtherExits;
   bool isMultiExitUnrollingEnabled =

llvm/test/Transforms/LoopUnroll/runtime-loop-non-exiting-latch.ll

@@ -0,0 +1,27 @@
+; REQUIRES: asserts
+; RUN: opt < %s -S -loop-unroll -unroll-runtime=true -unroll-allow-remainder=true -unroll-count=4
+
+; Make sure that the runtime unroll does not break with a non-exiting latch.
+define i32 @test(i32* %a, i32* %b, i32* %c, i64 %n) {
+entry:
+  br label %while.cond
+
+while.cond:                                       ; preds = %while.body, %entry
+  %i.0 = phi i64 [ 0, %entry ], [ %inc, %while.body ]
+  %cmp = icmp slt i64 %i.0, %n
+  br i1 %cmp, label %while.body, label %while.end
+
+while.body:                                       ; preds = %while.cond
+  %arrayidx = getelementptr inbounds i32, i32* %b, i64 %i.0
+  %0 = load i32, i32* %arrayidx
+  %arrayidx1 = getelementptr inbounds i32, i32* %c, i64 %i.0
+  %1 = load i32, i32* %arrayidx1
+  %mul = mul nsw i32 %0, %1
+  %arrayidx2 = getelementptr inbounds i32, i32* %a, i64 %i.0
+  store i32 %mul, i32* %arrayidx2
+  %inc = add nsw i64 %i.0, 1
+  br label %while.cond
+
+while.end:                                        ; preds = %while.cond
+  ret i32 0
+}

llvm/unittests/Transforms/Utils/CMakeLists.txt

@@ -15,5 +15,6 @@ add_llvm_unittest(UtilsTests
   IntegerDivisionTest.cpp
   LocalTest.cpp
   SSAUpdaterBulkTest.cpp
+  UnrollLoopTest.cpp
   ValueMapperTest.cpp
   )

llvm/unittests/Transforms/Utils/UnrollLoopTest.cpp

@@ -0,0 +1,76 @@
+//===- UnrollLoopTest.cpp - Unit tests for UnrollLoop ---------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Transforms/Utils/UnrollLoop.h"
+#include "llvm/Analysis/AssumptionCache.h"
+#include "llvm/Analysis/LoopInfo.h"
+#include "llvm/Analysis/ScalarEvolution.h"
+#include "llvm/Analysis/TargetLibraryInfo.h"
+#include "llvm/AsmParser/Parser.h"
+#include "llvm/IR/BasicBlock.h"
+#include "llvm/IR/Dominators.h"
+#include "llvm/IR/LLVMContext.h"
+#include "llvm/Support/SourceMgr.h"
+#include "gtest/gtest.h"
+
+using namespace llvm;
+
+static std::unique_ptr<Module> parseIR(LLVMContext &C, const char *IR) {
+  SMDiagnostic Err;
+  std::unique_ptr<Module> Mod = parseAssemblyString(IR, Err, C);
+  if (!Mod)
+    Err.print("UnrollLoopTests", errs());
+  return Mod;
+}
+
+TEST(LoopUnrollRuntime, Latch) {
+  LLVMContext C;
+
+  std::unique_ptr<Module> M = parseIR(
+    C,
+    R"(define i32 @test(i32* %a, i32* %b, i32* %c, i64 %n) {
+entry:
+  br label %while.cond
+
+while.cond:                                       ; preds = %while.body, %entry
+  %i.0 = phi i64 [ 0, %entry ], [ %inc, %while.body ]
+  %cmp = icmp slt i64 %i.0, %n
+  br i1 %cmp, label %while.body, label %while.end
+
+while.body:                                       ; preds = %while.cond
+  %arrayidx = getelementptr inbounds i32, i32* %b, i64 %i.0
+  %0 = load i32, i32* %arrayidx
+  %arrayidx1 = getelementptr inbounds i32, i32* %c, i64 %i.0
+  %1 = load i32, i32* %arrayidx1
+  %mul = mul nsw i32 %0, %1
+  %arrayidx2 = getelementptr inbounds i32, i32* %a, i64 %i.0
+  store i32 %mul, i32* %arrayidx2
+  %inc = add nsw i64 %i.0, 1
+  br label %while.cond
+
+while.end:                                        ; preds = %while.cond
+  ret i32 0
+})"
+    );
+
+  auto *F = M->getFunction("test");
+  DominatorTree DT(*F);
+  LoopInfo LI(DT);
+  AssumptionCache AC(*F);
+  TargetLibraryInfoImpl TLII;
+  TargetLibraryInfo TLI(TLII);
+  ScalarEvolution SE(*F, TLI, AC, DT, LI);
+
+  Loop *L = *LI.begin();
+
+  bool PreserveLCSSA = L->isRecursivelyLCSSAForm(DT,LI);
+
+  bool ret = UnrollRuntimeLoopRemainder(L, 4, true, false, false, &LI, &SE, &DT, &AC, PreserveLCSSA);
+  EXPECT_FALSE(ret);
+}