[LLVMdev] TargetRegisterInfo and "infinite" register files

[Andrew Clinton]

On 05/17/2011 12:54 PM, Jakob Stoklund Olesen wrote:
> On May 17, 2011, at 9:18 AM, Andrew Clinton wrote:
>
>> I have faced this same problem in my backend, and I'm working around it by providing a large physical register set.  There are two problems with this:
>>
>> 1. There's a chance that the register allocator will run out of registers to assign, in which case the allocation will fail - making it necessary to retry with a larger register set
>> 2. The code generator consumes storage proportional to the number of registers that could be assigned
>>
>> I'd be interested in an improvement to the code generator that makes it possible to specify an infinite register set without the need to store the registers explicitly.
> As I explained to Justin, allocating against an infinite register file doesn't really do anything. It's a quadratic time no-op.
>
> What you can do instead is:
>
> 1) Just use virtual registers and skip register allocation, or
>
> 2) Allocate to a small register file, implement memory operand folding, and pretend that spill slots are registers.
>
> /jakob
>

Empirically, 1) is not true.  The linear scan register allocator appears 
to do a very good job of reusing registers that have been killed.  It 
also automatically inserts copies for operations that clobber register 
operands, and coalesces identity register moves.  2) is a good idea but 
when I implemented register allocation, I did not find a straightforward 
way to integrate spilled memory operands into my instruction set.

Andrew