[cfe-dev] parallel C++

[Oleg Smolsky via cfe-dev]

On 2018-11-28 13:14, Edward Givelberg via cfe-dev wrote:
>
> [...]
> Naively, it seems to me that LLVM is a sequential VM, so perhaps its 
> architecture needs be extended.
> I am proposing an intermediate representation which encodes object 
> operations,
> let's call it IOR. IOR is translated to interconnect hardware 
> instructions, as well as LLVM's IR.
> I am proposing a dedicated back end to generate code for the 
> interconnect fabric.

Edward, it sounds to me like you are trying to reinvent Smalltalk. Its 
core is really about message passing and perhaps people have made 
attempts to make it parallel already.

On a more serious and specific note, I think you are ignoring the 
"abstract C machine" on which both C and C++ languages are built. 
Fundamentally, objects are laid out in memory (let's ignore the stack 
for now) and are built off primitive and user-defined types. These types 
are known (and stable) throughout the compilation process of a single 
program and so are the offsets of various fields that comprise the 
objects. All these objects (and often their sub-objects) can be read and 
written anywhere in a single-threaded program. Multi-threaded programs 
must be data-race-free, but essentially follow the same model.

The point I am trying to make is that the whole model is built on memory 
accesses that are eventually lowered to the ISA. There is no rigid 
protocol for invoking a member function or reading a member variable - 
things just happen in the program's address space. And then there is 
code optimizer. The memory accesses (expressed via LLVM IR, for example) 
go through various techniques that reduce and eliminate pointless 
work... at which point you have the target's ISA and absolutely no 
notion of a "method" or "object" (as a well-formed program cannot tell 
that the code has been re-arranged, reduced, reordered etc).

I suggest that you take a look at https://godbolt.org and see what the 
compiler emits with -O3 for a few short class/function templates as well 
as normal procedural code.

Oleg.