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

[Yonghong Song <yonghong.song@linux.dev>]

On 2/8/24 8:51 AM, Jose E. Marchesi wrote:
>> 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.

Thanks for the analysis. I can reproduce with vs. without '#pragma unroll' at -O2
level, the number of generated insns is indeed different, quite dramatically
as the above numbers. I will do some checking in compiler.

>
>> | 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 |