[llvm-dev] Register spilling in caller saved backend

[Quentin Colombet via llvm-dev]

Hi Jacques,

Could you attach the MachineInstr before and after register allocation?

This is hard to give an answer without seeing the actual CFG and live-ranges.

Thanks,
Q.
> On Sep 1, 2015, at 8:52 AM, Jacques Pienaar via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> Hey,
> 
> I'm playing around with a backend with no callee saved registers and noticed more spilling than seems needed. I tried digging into the spilling code but with limited success. I also tried removing all the callee saved registers in the X86 backend and saw the same effect (basically making CSRs equal to CSR_NoRegs). So I seem to be misunderstanding something or missing something simple, and thought I'd ask.
> 
> Consider this program as an example:
> 
> volatile int x;
> 
> int __attribute__((noinline)) foo() { return x; }
> 
> int main() {
>   for (int i = 0; i < 10; ++i) {
>     foo();
>   }
> }
> 
> The resultant output code spills 'i' within the loop (I have to compile with -fno-unroll-loops else the loop and 'i' gets optimized away) while 'i' is assigned to a register that is used nowhere else in the generated program and I would consider 'foo' easy to analyse. At the call site of 'foo' the debugging output reports that the stack pointer and return value registers are "imp-use" and "imp-def" and no other registers are used.
> 
> Should the spilling have occurred? Or this due to some feature in the backend that is missing?
> 
> Thanks,
> 
> Jacques
> 
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