Fix deduction of __atomic_load's parameter types.

[EricWF]

Summary

atomic_load's allows it's first argument to be a pointer to a const type. However the second argument is an output parameter and must be a pointer to non-const.
This patch fixes the signature of atomic_load generated by clang so that it respects the above requirements.

[EricWF]

ping.

[EricWF]

ping.

[majnemer]

LGTM

lib/Sema/SemaChecking.cpp

     // C    __c11_atomic_load(A *, int)
     Load,
     // void __atomic_load(A *, CP, int)
+    LoadCopy,
+    // void __atomic_store(A *, CP, int)
     Copy,
     // C    __c11_atomic_add(A *, M, int)
     Arithmetic,

     // bool __atomic_compare_exchange(A *, C *, CP, bool, int, int)
     GNUCmpXchg
   } Form = Init;
-  const unsigned NumArgs[] = { 2, 2, 3, 3, 3, 4, 5, 6 };
-  const unsigned NumVals[] = { 1, 0, 1, 1, 1, 2, 2, 3 };
+  const unsigned NumArgs[] = { 2, 2, 3, 3, 3, 3, 4, 5, 6 };
+  const unsigned NumVals[] = { 1, 0, 1, 1, 1, 1, 2, 2, 3 };
   // where:
   //   C is an appropriate type,
   //   A is volatile _Atomic(C) for __c11 builtins and is C for GNU builtins,

     Form = Load;
     break;
 
-  case AtomicExpr::AO__c11_atomic_store:
   case AtomicExpr::AO__atomic_load:
+    Form = LoadCopy;
+    break;
+
+  case AtomicExpr::AO__c11_atomic_store:
   case AtomicExpr::AO__atomic_store:
   case AtomicExpr::AO__atomic_store_n:
     Form = Copy;

       return ExprError();
     }
     ValType = AtomTy->getAs<AtomicType>()->getValueType();
-  } else if (Form != Load && Op != AtomicExpr::AO__atomic_load) {
+  } else if (Form != Load && Form != LoadCopy) {
     if (ValType.isConstQualified()) {
       Diag(DRE->getLocStart(), diag::err_atomic_op_needs_non_const_pointer)
         << Ptr->getType() << Ptr->getSourceRange();

 
   // atomic_fetch_or takes a pointer to a volatile 'A'.  We shouldn't let the
   // volatile-ness of the pointee-type inject itself into the result or the
-  // other operands.
+  // other operands. Similarly atomic_load can take a pointer to a const 'A'.
   ValType.removeLocalVolatile();
+  ValType.removeLocalConst();
   QualType ResultType = ValType;
-  if (Form == Copy || Form == GNUXchg || Form == Init)
+  if (Form == Copy || Form == LoadCopy || Form == GNUXchg || Form == Init)
     ResultType = Context.VoidTy;
   else if (Form == C11CmpXchg || Form == GNUCmpXchg)
     ResultType = Context.BoolTy;

   if (!IsC11 && !IsN)
     ByValType = Ptr->getType();
 
-  // FIXME: __atomic_load allows the first argument to be a a pointer to const
-  // but not the second argument. We need to manually remove possible const
-  // qualifiers.
-
   // The first argument --- the pointer --- has a fixed type; we
   // deduce the types of the rest of the arguments accordingly.  Walk
   // the remaining arguments, converting them to the deduced value type.

   case Load:
     SubExprs.push_back(TheCall->getArg(1)); // Order
     break;
+  case LoadCopy:
   case Copy:
   case Arithmetic:
   case Xchg:

test/Sema/atomic-ops.c

   __atomic_load(I, *P, memory_order_relaxed, 42); // expected-error {{too many arguments}}
   (int)__atomic_load(I, I, memory_order_seq_cst); // expected-error {{operand of type 'void'}}
   __atomic_load(s1, s2, memory_order_acquire);
-  (void)__atomic_load(I, CI, memory_order_relaxed); // expected-warning {{passing 'const int *' to parameter of type 'int *' discards qualifiers}}
+  __atomic_load(CI, I, memory_order_relaxed);
+  __atomic_load(I, CI, memory_order_relaxed); // expected-warning {{passing 'const int *' to parameter of type 'int *' discards qualifiers}}
+  __atomic_load(CI, CI, memory_order_relaxed); // expected-warning {{passing 'const int *' to parameter of type 'int *' discards qualifiers}}
+
   __c11_atomic_store(i, 1, memory_order_seq_cst);
   __c11_atomic_store(p, 1, memory_order_seq_cst); // expected-warning {{incompatible integer to pointer conversion}}
   (int)__c11_atomic_store(d, 1, memory_order_seq_cst); // expected-error {{operand of type 'void'}}