[M3devel] a trouble with passing records by value..

[Rodney M. Bates]

Jay K wrote:
>  > Date: Tue, 31 Aug 2010 21:24:07 -0500
>  > From: rodney_bates@
>  >
>  > If the writers of an ABI actually *required* that small 
> structs/records be
>  > passed in registers
>  
>  
> Yes, they have. The rules are hard for me to understand. It isn't just 
> based on size.
>  
>  
>  > then they have made a big blunder.
>  
>  
> No they have not. They know what they are doing.

When the ABI requires a compiler to do it wrong for the language semantics,
which it must then compensate for at a place that's outside the jurisdiction
of the ABI, the ABI has a design bug.  At least when the language in question
is one of those the ABI designers considered, which has to be the case
with C.

>  
>  
>  > Thus taking their address must always be possible,
>  
>  
> If they have to be in memory, then they will be put in memory, at or before
> the address is taken. That doesn't mean the parameters have to be passed 
> in memory.
>  
> 
>  > 1) Declare that language semantics trump ABI requirements and defy 
> the ABI.
>  
> Not necessary.
> Granted, we don't have to follow the ABI. The ABI is for 
> interoperability with C.
> I'm plenty willing to say you can't pass records by value between C and 
> Modula-3.
>  
> 
> 
>  > 2) Pass them in registers at the time of control transfer, but store them
>  > in memory in the prologue, then access them from memory thereafter.
>  
>  
> The backend does that as needed for C already.
>  
>  
> Data can live in multiple places.
> Just that there can be only be one mutable location at a time.
>  
> 
> Look at the code generated Tony asked about, add like
> printr("%p\n", &t);
>  
>  
> you'll see the registers get written to the stack.
> 
>  
>  - Jay