Optimized Allocation of Argument registers

[Vladimir Makarov]

On 2014-11-17 8:13 AM, Ajit Kumar Agarwal wrote:
> Hello All:
>
> I was looking at the optimized usage and allocation to argument registers. There are two aspects to it as follows.
>
> 1. We need to specify the argument registers as followed by ABI in the target specific code. Based on the function
>   argument registers defined in the target dependent code the function argument registers are passed. If the
>   number of argument registers defined in the Architecture is large say 6/8 function argument registers.
> Most of the time in the benchmarks we don't pass so many arguments and the number of arguments passed
>   is quite less. Since we reserve the function arguments as specified in the target specific code for the given
> architecture, leads to unoptimized usage as this function argument registers will not be used in the function.
> Thus we need to steal some of the arguments registers and have the usage of those in the function depending
> on the support of the number of function argument registers. The stealing of function argument registers will
>   lead more number of registers available that are to be used in the function and leading to less spill and fetch.
>

The argument registers should be not reserved.  They should be present 
in RTL and RA allocator will figure out itself when it can use them. 
That is how other ports work.

> 2. The other aspect of the function argument registers is not spill and fetch the argument registers as they are
> live across the function call. But the liveness is limited to certain point of the called function after that point the
> function argument registers are not live and can be used inside the called function. Other aspect is if there is a
> shortage of registers than can the function argument registers should be used as spill candidate? Will this lead
> to the optimized code.
>
>

You can remove unnecessary code to save/restore arg registers around 
calls if you can figure out that they are not used in called functions. 
  There is already code for this written by Tom de Vries.  So you can 
use it.