[cfe-dev] OpenCL address space and mangling

[Michele Scandale]

Hello Mon Ping,

I apologize for the mail length, but I hope to explain as clear as I can the 
points I think need to be discussed.

> Sorry of being late to this conversation. It doesn’t look consistent me. Address
> space numbers are not language constructs.   The language constructs are global
> and local.  Coming out of clang, I think it is more natural for the AS mangling
> and the type to match. In C++, clang will generate different names for
> structures which can be identical and uses those names consistently to mangle
> the function, e.g.,
>    %struct.foo = type { i32, i32 }
>    define void @_Z4testR3foo(%struct.foo* %foo)
>
> I view the address spaces coming out of clang represent how the target represent
> memory is a logical.  How a particular llvm maps them to physical memory is
> target dependent.  A backend may map them all the address spaces to the same
> physical memory or to different ones.  Due to this, I don’t think it make sense
> to distinguish between the two in clang for a particular target.

I agree, the fact that opencl address spaces are handled like other address 
spaces is a technical aspect. To have a common way I don't see a strict 
limitation in how address spaces are mangled (they can be numbers decided as 
convention in clang, or defined by targets, or whatever), but still mangling 
should preserve the differences that are present in the source language.

I want to remark IMO an important aspect:
"Pointer types may have an optional address space attribute defining the 
numbered address space where the pointed-to object resides. The default address 
space is number zero. The semantics of non-zero address spaces are 
target-specific." (http://llvm.org/docs/LangRef.html#pointer-type)

 From this description I understand that address spaces in the IR are physical 
address spaces. Because of this I consider wrong to use this property as is to 
represent inside the IR logical address space. Doing that would imply that each 
backend should be aware of language specific mapping: currently this is not the 
case and IMO it's a bad idea to have this.

But a derived information from the source language is still useful to perform 
optimization, both in the IR and later in the backend: the logical distinction 
of address spaces is still useful and IMO shoul be represented in the IR. Have 
both logical and physical address spaces information (it's not important to know 
is "AS1" means global or local, it's enough to know that 1 is differnt from 2) 
would be useful to have a better alias analysis also for those targets that 
physically have one unique address space. I consider that this can be solved 
independently from the mangling problem.

The answer to both question, I suggested to introduce another map in order to 
preserve the distinction between address spaces also for those targets that do 
not have physical distinct address spaces, like X86, and through this solve the 
problem related to the mangler.

As previously discussed, this is not the only viable solution, the mapping of 
logical address spaces to physical address spaces can be delayed till 
instruction selection: this would allow the frontend to lower this information 
in a target independent manner demanding a late IR pass the mapping task (this 
task would be language/target dependent, so basically who builds the pass 
pipeline must schedule this language dependent task that requires target 
informations). Still here may be useful to preserve the logical information of 
address spaces.
This kind of solution is feasible, but simply it does not seem the way chosen in 
clang to solve the problem.

My proposal was the one with the minimal impact on the codebase trying to 
maintain a desirable flexibility in order to build opencl toolchain compatible 
with the past.

Could you explain to me what you are proposing? How the mangler should be fixed? 
How address spaces are lowered in the IR? This lowering is target dependent or 
not? The mangling is also target dependent?

Thanks in advance.

Best regards,
-Michele