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Re: Optimized Allocation of Argument registers


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


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