scudo-standalone: Add atomic_compare_exchange_weak

[1c3t3a]

Summary

This commit adds an atomic_compare_exchange_weak method
to the atomic helpers in scudo standalone.

[1c3t3a]

This extracts the changes around atomics from https://reviews.llvm.org/D126752?id=435038 and adds tests for it :)

[vitalybuka]

The patch is LGTM, but I would prefer you avoid it, and use existing strong version, unless you can prove meaningful performance difference with weak

[vitalybuka]

I assume we don't need this?

compiler-rt/lib/scudo/standalone/atomic_helpers.h

 template <typename T>
 inline bool atomic_compare_exchange_strong(volatile T *A, typename T::Type *Cmp,
                                            typename T::Type Xchg,
                                            memory_order MO) {
   return __atomic_compare_exchange(&A->ValDoNotUse, Cmp, &Xchg, false, MO,
                                    __ATOMIC_RELAXED);
 }
 
+template <typename T>
+inline bool atomic_compare_exchange_weak(volatile T *A, typename T::Type *Cmp,
+                                         typename T::Type Xchg,
+                                         memory_order MO) {
+  return __atomic_compare_exchange(&A->ValDoNotUse, Cmp, &Xchg, true, MO,
+                                   __ATOMIC_RELAXED);
+}
+
 // Clutter-reducing helpers.
 
 template <typename T>
 inline typename T::Type atomic_load_relaxed(const volatile T *A) {
   return atomic_load(A, memory_order_relaxed);
 }
 
 template <typename T>

compiler-rt/lib/scudo/standalone/tests/atomic_test.cpp

template <typename T> void checkAtomicCompareExchange() {
   Type OldVal = 42;
   Type NewVal = 24;
   Type V = OldVal;
   EXPECT_TRUE(atomic_compare_exchange_strong(reinterpret_cast<T *>(&V), &OldVal,
                                              NewVal, memory_order_relaxed));
   EXPECT_FALSE(atomic_compare_exchange_strong(
       reinterpret_cast<T *>(&V), &OldVal, NewVal, memory_order_relaxed));
   EXPECT_EQ(NewVal, OldVal);
+
+  OldVal = 42;
+  NewVal = 24;
+  V = OldVal;
+
+  EXPECT_TRUE(atomic_compare_exchange_weak(reinterpret_cast<T *>(&V), &OldVal,
+                                           NewVal, memory_order_relaxed));
+  EXPECT_FALSE(atomic_compare_exchange_weak(reinterpret_cast<T *>(&V), &OldVal,
+                                            NewVal, memory_order_relaxed));
+  EXPECT_EQ(NewVal, OldVal);
 }
 
 TEST(ScudoAtomicTest, AtomicCompareExchangeTest) {
   checkAtomicCompareExchange<atomic_u8>();
   checkAtomicCompareExchange<atomic_u16>();
   checkAtomicCompareExchange<atomic_u32>();
   checkAtomicCompareExchange<atomic_u64>();
   checkAtomicCompareExchange<atomic_uptr>();
 }
 
 } // namespace scudo