[PATCH] D26196: Add support for non-zero null pointers

[Yaxun Liu via cfe-commits]

yaxunl marked 3 inline comments as done.
yaxunl added inline comments.


================
Comment at: lib/CodeGen/CGExprConstant.cpp:1340
+          return C;
+        return getNullPtr(PT, DestType);
       }
----------------
yaxunl wrote:
> rjmccall wrote:
> > efriedma wrote:
> > > Consider code like the following:
> > > 
> > >     int x = 0;
> > >     auto y1 = (__specialaddrspace int*)0;
> > >     auto y2 = (__specialaddrspace int*)((void)0, 0);
> > >     auto y3 = (__specialaddrspace int*)x;
> > > 
> > > How do you expect these three cases to behave?  (The first case involves a C null pointer constant, the second and third cases are different ways of writing a general int->ptr conversion.)
> > Yeah, I think you probably need to fix APValue to be unambiguous about whether the value is a formal null pointer (CK_NullToPointer) or just a cast of an integer (CK_IntegralToPointer).  It looks like PointerExprEvaluator will generate the exact same value for both.
> It seems the current implementation generates the correct IR.
> 
> I tried the following sample and I saw correct IR generated.
> 
> 
> ```
> private int* test_cast_0_to_ptr(void) {
>   return (private int*)0;
> }
> 
> private int* test_cast_int_to_ptr1(void) {
>   return (private int*)((void)0, 0);
> }
> 
> private int* test_cast_int_to_ptr2(void) {
>   int x = 0;
>   return (private int*)x;
> }
> 
> ```
> 
> The dumped AST is
> 
> 
> ```
> |-FunctionDecl 0x95fdc88 <ptr.cl:3:1, line:5:1> line:3:14 test_cast_0_to_ptr 'int *(void)'
> | `-CompoundStmt 0x95fdde8 <col:39, line:5:1>
> |   `-ReturnStmt 0x95fddd0 <line:4:3, col:24>
> |     `-CStyleCastExpr 0x95fdda8 <col:10, col:24> 'int *' <NullToPointer>
> |       `-IntegerLiteral 0x95fdd70 <col:24> 'int' 0
> |-FunctionDecl 0x95fdea0 <line:13:1, line:15:1> line:13:14 test_cast_int_to_ptr1 'int *(void)'
> | `-CompoundStmt 0x95fe098 <col:42, line:15:1>
> |   `-ReturnStmt 0x95fe080 <line:14:3, col:35>
> |     `-CStyleCastExpr 0x95fe058 <col:10, col:35> 'int *' <IntegralToPointer>
> |       `-ParenExpr 0x95fe038 <col:24, col:35> 'int'
> |         `-BinaryOperator 0x95fe010 <col:25, col:34> 'int' ','
> |           |-CStyleCastExpr 0x95fdf78 <col:25, col:31> 'void' <ToVoid>
> |           | `-IntegerLiteral 0x95fdf48 <col:31> 'int' 0
> |           `-IntegerLiteral 0x95fdfa0 <col:34> 'int' 0
> `-FunctionDecl 0x95fe150 <line:19:1, line:22:1> line:19:14 test_cast_int_to_ptr2 'int *(void)'
>   `-CompoundStmt 0x9620130 <col:42, line:22:1>
>     |-DeclStmt 0x9620080 <line:20:3, col:12>
>     | `-VarDecl 0x95fe210 <col:3, col:11> col:7 used x 'int' cinit
>     |   `-IntegerLiteral 0x9620060 <col:11> 'int' 0
>     `-ReturnStmt 0x9620118 <line:21:3, col:24>
>       `-CStyleCastExpr 0x96200f0 <col:10, col:24> 'int *' <IntegralToPointer>
> ```
> 
> Since only CK_NullToPointer is translated to null pointer through getNullPtr, CK_IntegralToPointer will result in either zero-valued pointer or inttoptr, the generated IR is correct.
Basically in the second and third case the destination type is not pointer, so they do not need to be emitted as null pointer. If a literal 0 is casted to a pointer type, then it should be emitted as a null pointer.


https://reviews.llvm.org/D26196