[analyzer] Fix crash when reasoning about C11 atomics (PR49422)

[vsavchenko]

Summary

rdar://75020762

[steakhal]

LGTM

clang/include/clang/StaticAnalyzer/Core/PathSensitive/BasicValueFactory.h

public:
   ASTContext &getContext() const { return Ctx; }
 
   const llvm::APSInt& getValue(const llvm::APSInt& X);
   const llvm::APSInt& getValue(const llvm::APInt& X, bool isUnsigned);
   const llvm::APSInt& getValue(uint64_t X, QualType T);
 
   /// Returns the type of the APSInt used to store values of the given QualType.
   APSIntType getAPSIntType(QualType T) const {
+    // For the purposes of the analysis and constraints, we treat atomics
+    // as their underlying types.
+    if (const AtomicType *AT = T->getAs<AtomicType>()) {
+      T = AT->getValueType();
+    }
+
     assert(T->isIntegralOrEnumerationType() || Loc::isLocType(T));
     return APSIntType(Ctx.getIntWidth(T),
                       !T->isSignedIntegerOrEnumerationType());
   }
 
   /// Convert - Create a new persistent APSInt with the same value as 'From'
   ///  but with the bitwidth and signedness of 'To'.
   const llvm::APSInt &Convert(const llvm::APSInt& To,

clang/test/Analysis/atomics.c

void test_atomic_compare_exchange_weak(struct RefCountedStruct *s) {
   uint32_t desired = 3;
   _Bool result = __c11_atomic_compare_exchange_weak((volatile _Atomic(uint32_t) *)&s->refCount, &expected, desired, memory_order_relaxed, memory_order_relaxed);
 
   // For now we expect both expected and refCount to be invalidated by the
   // call. In the future we should model more precisely.
   clang_analyzer_eval(s->refCount == 3); // expected-warning {{UNKNOWN}}
   clang_analyzer_eval(expected == 2); // expected-warning {{UNKNOWN}}
 }
+
+// PR49422
+void test_atomic_compare(int input) {
+  _Atomic(int) x = input;
+  if (x > 0) {
+    // no crash
+  }
+}