[llvm-dev] Register spilling in caller saved backend

[Jacques Pienaar via llvm-dev]

On Tue, Sep 1, 2015 at 9:13 AM, Philip Reames <listmail at philipreames.com>
wrote:

>
>
> On 09/01/2015 08:52 AM, Jacques Pienaar via llvm-dev 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?
>
> What are you expectating here?  Are you expecting the backend to recognize
> that foo doesn't actually clobber any registers?  I don't believe we do
> that today.  Without callee saved registers, if the value 'i' needs to be
> preserved across the call, it will need to be spilled.  There's no register
> we can put it in without it being clobbered.
>

Yes that was what I was hoping for as it seems that all the information is
there to do that. So at the moment if there is a case such as this then the
backend should have some designated callee saved registers. That makes
sense if that information isn't being used at the moment then we can't know
if the register will be clobbered or not and have to assume it will.

Thanks
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