On 2022-05-04 12:49, David Laight wrote:

quoted

quoted

If you declare the union on the stack in the callers, and pass by value
- is the compiler not going to be clever enough to still DDRT?

Ah, OK, it should do the thing. I thought you wanted me to ditch the
union altogether.

Some architectures always pass struct/union by address.
Which is probably not what you had in mind.

Do you have any specific architecture in mind? I couldn't find any 
information that it happens anywhere, x86_64 ABI [1] (pages 20-21) 
aligns with my expectations, and my common sense can't explain why would 
some architectures do what you say.

In C, when the caller passes a struct as a parameter, the callee can 
freely modify it. If the compiler silently replaced it with a pointer, 
the callee would corrupt the caller's local variable, so such approach 
requires the caller to make an extra copy. Making an extra copy on the 
stack and passing a pointer doesn't make any sense to me if you can just 
make a copy on the stack (or to a register) and call it a parameter.

If you know any specific architecture supported by Linux that passes all 
unions by a pointer, could you please point me to it? Maybe I'm missing 
something in my logic, and a real-world example will explain things, but 
at the moment it sounds unrealistic to me.

Thanks,
Max

[1]: 
https://www.intel.com/content/dam/develop/external/us/en/documents/mpx-linux64-abi.pdf

	David

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