llvm dev - Jul 2015 - [LLVMdev] Are global variables candiates for register allocation?

I was wondering if global variables can be candidates for register
allocation. My use case is a global variable that is used in every
function in my program.

I'm wondering if it's better to pass it in, or let it stay as a global.

Passing it in will require a bit of work.

This came up in the past for GHC, and we recommended passing it as a
parameter everywhere, as it lets the register allocator spill it under high
register pressure.

GCC has support for allocating globals in GPRs and removing that GPR from
the allocatable set, but LLVM doesn't implement it so far as I know.

On Tue, Jun 30, 2015 at 5:43 PM, Hayden Livingston <halivingston at
gmail.com>
wrote:
> I was wondering if global variables can be candidates for register
> allocation. My use case is a global variable that is used in every
> function in my program.
>
> I'm wondering if it's better to pass it in, or let it stay as a
global.
>
> Passing it in will require a bit of work.
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
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Thanks, Reid. I'm not an optimization expert, but as a workaround,
can I do the following:

void myFunction()
{
   int local = global;
   .. use local ...
}

?

On Tue, Jun 30, 2015 at 6:53 PM, Reid Kleckner <rnk at google.com>
wrote:
> This came up in the past for GHC, and we recommended passing it as a
> parameter everywhere, as it lets the register allocator spill it under high
> register pressure.
>
> GCC has support for allocating globals in GPRs and removing that GPR from
> the allocatable set, but LLVM doesn't implement it so far as I know.
>
> On Tue, Jun 30, 2015 at 5:43 PM, Hayden Livingston <halivingston at
gmail.com>
> wrote:
>>
>> I was wondering if global variables can be candidates for register
>> allocation. My use case is a global variable that is used in every
>> function in my program.
>>
>> I'm wondering if it's better to pass it in, or let it stay as a
global.
>>
>> Passing it in will require a bit of work.
>> _______________________________________________
>> LLVM Developers mailing list
>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
>