[PATCH] D28136: [OpenCL] Implement as_type operator as alias of __builtin_astype.

[Alexey Bader via Phabricator via cfe-commits]

bader added a comment.

> Why do you think this is a bug? It seems to follow standard behavior in C to promote char to int if required. Just like if you would have a C code:
> 
>   int as_int(int i);
>   void foo() {
>       char src = 1;
>       int dst = as_int(src);
>   } 
>    
> 
> This code would complie and the same exactly IR would be generated.

as_type is defined to be used for bit re-interpretation. (see 6.2.4.2 Reinterpreting Types Using as_type() and as_typen()). In this sense, it's exactly matches __bultin_astype built-in function.

Here are a few relevant OpenCL C language specification quotes from 6.2.4 section:

> All data types described in tables 6.1 and 6.2 (except bool, half and void) may be also reinterpreted as another data type of **the same size** using the as_type() operator for scalar data types and the as_typen() operator for vector data types.



> The usual type promotion for function arguments shall not be performed.



> It is an error to use as_type() or as_typen() operator to reinterpret data to a type of a different number of bytes.

So, aliasing as_type to __builtin_astype provides these checks, whereas we can't do it for overloadable as_type function declarations.

I also would like to address your original concerns:

> The main issue is after preprocessing the header the original function name is no longer available in diagnostics reported.

Actually diagnostics is able to provide a hint to exact code location in the header file where as_<type> is defined as macro, so user gets correct name of the operator in diagnostics as far as I know.

> The spec defines as_type as a builtin function and not a macro.

To be precise, spec defines as_type as an operator. So, the best way to implement it would be to add a language support of such operator, but AFAIK aliasing to __bulitin_astype via macro is sufficient enough for OpenCL use cases.

> Additionally your patch would allow as_type to be used with extra type (not only those defined in spec).

Not sure I get this. We defined limited set of as_* functions - only for types from tables 6.1 and 6.2 as specified by specification, so if OpenCL developer will try to call as_<type1>(type2), which is not defined in the header, compiler will report en error about calling undeclared function.

> Also I don't see the problem to implement as_type with just simply calling a builtin. It should be inlined later anyways.

Yes, but this solution will not give us error checking as pre-processor solution.

Does it make sense?


https://reviews.llvm.org/D28136