Optimized Allocation of Argument registers

[Jeff Law]

On 11/17/14 06:13, 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.
>
> 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.
>
> Please let me know what do you think.
Typically GCC ports do not reserve the function argument/return 
registers, they are allocatable just like any other call clobbered register.

In essence arguments for function calls are set up at each call site by 
copying values out of pseudo registers, memory, constant 
initializations, etc to the appropriate outgoing argument register.  The 
allocator will, when possible and profitable try to assign the pseudo 
register to the appropriate argument register to eliminate the copies.

Similarly, GCC copies values out of the incoming argument registers and 
into pseudos at the start of a function.  The allocator, again when 
possible and profitable, will try to allocate the pseudos to the 
incoming argument registers to avoid the copy.

So in summary, there is no reason to reserve registers for argument 
passing in GCC and doing so would be wasteful.  Treat the argument 
registers as any other call clobbered register and allow the register 
allocator to make appropriate decisions based on liveness, expected 
uses, call-crossing liveness, related copies, etc etc.
jeff