Re: [PATCH bpf-next] bpf: abstract loop unrolling pragmas in BPF selftests

["Jose E. Marchesi" <jose.marchesi@oracle.com>]

> On Thu, 2024-02-08 at 16:35 +0100, Jose E. Marchesi wrote:
> [...]
>
>> If the compiler generates assembly code the same code for profile2.c for
>> before and after, that means that the loop does _not_ get unrolled when
>> profiler.inc.h is built with -O2 but without #pragma unroll.
>> 
>> But what if #pragma unroll is used?  If it unrolls then, that would mean
>> that the pragma does something more than -funroll-loops/-O2.
>> 
>> Sorry if I am not making sense.  Stuff like this confuses me to no end
>> ;)
>
> Sorry, I messed up while switching branches :(
> Here are the correct stats:
>
> | File            | insn # | insn # |
> |                 | before |  after |
> |-----------------+--------+--------|
> | profiler1.bpf.o |  16716 |   4813 |

This means:

- With both `#pragma unroll' and -O2 we get 16716 instructions.
- Without `#pragma unroll' and with -O2 we get 4813 instructions.

Weird.

> | profiler2.bpf.o |   2088 |   2050 |

- Without `#pragma unroll' and with -O2 we get 2088 instructions.
- With `#pragma loop unroll(disable)' and with -O2 we get 2050
  instructions.

Also surprising.

> | profiler3.bpf.o |   4465 |   1690 |