Avoid a UB pointer overflow in the ArrayRef unit test

[vsk]

Summary

The intent of the test is to check that array lengths greater than
UINT_MAX work properly. Change the test to stress that scenario, without
triggering pointer overflow UB.

Caught by a WIP pointer overflow checker in clang.

[ab]

Huh, interesting.. Not a big deal, but can you avoid the #ifdef? Where was the overflow?

[vsk]

In D33149#755166, @ab wrote:

Huh, interesting.. Not a big deal, but can you avoid the #ifdef? Where was the overflow?

The overflow occurred in drop_front etc. The start of the array moved from 0x10000 to 2^64 - 2.

I think these tests need to add UINT_MAX or greater to a pointer to check that size_t-sized lengths work. For 32-bit machines, I don't think there's a way to add such a large offset to a pointer and not overflow.

[vsk]

@ab Ping, would you be ok with leaving the ifdef in?

[vsk]

Ahmed and I chatted about this. We discussed an alternative to this patch, which is to test with a length of max<ptroff_t>. This version of the test wouldn't require any ifdefs, and would still add some coverage for r271546.

llvm/trunk/unittests/ADT/ArrayRefTest.cpp

 //===- llvm/unittest/ADT/ArrayRefTest.cpp - ArrayRef unit tests -----------===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/Support/Allocator.h"
 #include "llvm/Support/raw_ostream.h"
 #include "gtest/gtest.h"
+#include <limits>
 #include <vector>
 using namespace llvm;
 
 // Check that the ArrayRef-of-pointer converting constructor only allows adding
 // cv qualifiers (not removing them, or otherwise changing the type)
 static_assert(
     std::is_convertible<ArrayRef<int *>, ArrayRef<const int *>>::value,
     "Adding const");
struct NonAssignable {
     void operator=(const NonAssignable &RHS) { assert(RHS.Ptr != nullptr); }
     bool operator==(const NonAssignable &RHS) const { return Ptr == RHS.Ptr; }
   } Array3Src[] = {"hello", "world"};
   ArrayRef<NonAssignable> Array3Copy = makeArrayRef(Array3Src).copy(Alloc);
   EXPECT_EQ(makeArrayRef(Array3Src), Array3Copy);
   EXPECT_NE(makeArrayRef(Array3Src).data(), Array3Copy.data());
 }
 
+TEST(ArrayRefTest, SizeTSizedOperations) {
+  ArrayRef<char> AR(nullptr, std::numeric_limits<ptrdiff_t>::max());
+
+  // Check that drop_back accepts size_t-sized numbers.
+  EXPECT_EQ(1U, AR.drop_back(AR.size() - 1).size());
+
+  // Check that drop_front accepts size_t-sized numbers.
+  EXPECT_EQ(1U, AR.drop_front(AR.size() - 1).size());
+
+  // Check that slice accepts size_t-sized numbers.
+  EXPECT_EQ(1U, AR.slice(AR.size() - 1).size());
+  EXPECT_EQ(AR.size() - 1, AR.slice(1, AR.size() - 1).size());
+}
+
 TEST(ArrayRefTest, DropBack) {
   static const int TheNumbers[] = {4, 8, 15, 16, 23, 42};
   ArrayRef<int> AR1(TheNumbers);
   ArrayRef<int> AR2(TheNumbers, AR1.size() - 1);
   EXPECT_TRUE(AR1.drop_back().equals(AR2));
-
-  // Check that drop_back accepts size_t-sized numbers.
-  ArrayRef<char> AR3((const char *)0x10000, SIZE_MAX - 0x10000);
-  EXPECT_EQ(1U, AR3.drop_back(AR3.size() - 1).size());
 }
 
 TEST(ArrayRefTest, DropFront) {
   static const int TheNumbers[] = {4, 8, 15, 16, 23, 42};
   ArrayRef<int> AR1(TheNumbers);
   ArrayRef<int> AR2(&TheNumbers[2], AR1.size() - 2);
   EXPECT_TRUE(AR1.drop_front(2).equals(AR2));
-
-  // Check that drop_front accepts size_t-sized numbers.
-  ArrayRef<char> AR3((const char *)0x10000, SIZE_MAX - 0x10000);
-  EXPECT_EQ(1U, AR3.drop_front(AR3.size() - 1).size());
 }
 
 TEST(ArrayRefTest, DropWhile) {
   static const int TheNumbers[] = {1, 3, 5, 8, 10, 11};
   ArrayRef<int> AR1(TheNumbers);
   ArrayRef<int> Expected = AR1.drop_front(3);
   EXPECT_EQ(Expected, AR1.drop_while([](const int &N) { return N % 2 == 1; }));
 
TEST(ArrayRefTest, Equals) {
   EXPECT_FALSE(AR1b.equals({2, 3, 4, 5, 6}));
   EXPECT_FALSE(AR1b.equals({3, 4, 5, 6, 7}));
 }
 
 TEST(ArrayRefTest, EmptyEquals) {
   EXPECT_TRUE(ArrayRef<unsigned>() == ArrayRef<unsigned>());
 }
 
-TEST(ArrayRefTest, Slice) {
-  // Check that slice accepts size_t-sized numbers.
-  ArrayRef<char> AR((const char *)0x10000, SIZE_MAX - 0x10000);
-  EXPECT_EQ(1U, AR.slice(AR.size() - 1).size());
-  EXPECT_EQ(AR.size() - 1, AR.slice(1, AR.size() - 1).size());
-}
-
 TEST(ArrayRefTest, ConstConvert) {
   int buf[4];
   for (int i = 0; i < 4; ++i)
     buf[i] = i;
 
   static int *A[] = {&buf[0], &buf[1], &buf[2], &buf[3]};
   ArrayRef<const int *> a((ArrayRef<int *>(A)));
   a = ArrayRef<int *>(A);