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digitalmars.D - A new look at rvalue references

bitwise and I had an interesting discussion about rvalue refs in 
the `auto ref` PR [1]. It revolved around whether the ABI [2] 
requires large structs to be passed as a pointer, as it happens 
to be the case with return values. The spec isn't clear on this, 
it merely says:

 Static arrays are passed as pointers to their first element.
I assumed the same would be true for large structs, because these and static arrays are treated alike as return values:
 For other sized structs and static arrays, the return value is 
 stored through a hidden pointer passed as an argument to the 
But bitwise's experiments showed that this isn't the case, and from what I can see, it doesn't even work according to spec for static arrays: The structs (and fixed-size arrays) are copied onto the stack by the caller, even for rvalues, and no pointer is passed to the callee. I'm aware that the spec is only for x86 32-bit, and that on e.g. Posix x86_64, D uses the Linux ABI, AFAIK. But assuming we could modify the ABI at will, we could have the following calling convention for large structs/arrays (say, anything that doesn't fit into two registers): - These arguments are always passed as a pointer to the callee. - If they are lvalues (and non-const), the caller has to make a copy and pass a pointer to this copy. - If they are rvalues (or const), the caller doesn't need to make a copy, but can pass a pointer to the original location. Wouldn't this make `ref` totally unnecessary as a performance trick, reducing it merely to its semantic meaning (i.e. mutation by the callee), and therefore also reduce the need for rvalue refs? [1] https://github.com/D-Programming-Language/dmd/pull/4717 [2] http://dlang.org/abi.html (section Calling Conventions)
Jul 11 2015