[llvm-dev] Register spilling in caller saved backend

[Jacques Pienaar via llvm-dev]

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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