-----Original Message----- From: Jeff Law [mailto:l...@redhat.com] Sent: Monday, November 17, 2014 9:27 PM To: Ajit Kumar Agarwal; Vladimir Makarov; gcc Mailing List Cc: Vinod Kathail; Shail Aditya Gupta; Vidhumouli Hunsigida; Nagaraju Mekala Subject: 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. Thanks Jeff for the explanation and Clarifications. Thanks & Regards Ajit jeff
