Change the arrch64 abi ...(Custom /Specific change)

[Jim Wilson]

On 04/04/2016 08:55 AM, Umesh Kalappa wrote:
> We are in process of changing the gcc compiler for aarch64 abi ,w.r.t
> varargs  function arguments handling.
>
> default(LP64) ,where 1,2,4 bytes  args are promoted to word size i.e 4
> bytes ,we need to change these behaviour to 8 bytes (double word).
>
> we are looking both hooks like  PROMOTE_MODE and
> TARGET_PROMOTE_FUNCTION_MODE to make the changes.

I think this would work.  You just need to promote all modes less than 8 
bytes to DImode, instead of the current code that promotes modes smaller 
than 4 bytes to SImode.  You would do this for the default LP64 type 
system, but not for the ILP32 type system.

This would affect all function arguments and locals, which may cause 
code size and/or performance issues.  You would have to check for that.

Also, this may prevent linking with any 3rd party code compiled by 
unmodified gcc, or code compiled with other compilers (e.g. LLVM), 
because changing TARGET_PROMOTE_FUNCTION_MODE can cause ABI changes. You 
may need to check that also.

Jim