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

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

On 2/8/24 10:04 AM, Yonghong Song wrote:
>
> 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.

Okay, a quick checking compiler found that
   - with "#pragma unroll" means no profitability test and do full unroll as instructed
   - without "#pragma unroll" mean compiler will do profitability for full unroll,
     if compiler thinks full unroll is not profitable, there will be no unrolling.

So for gcc, even users saying '#pragma unroll', gcc still do profitability test?

>
>>
>>> | 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 |
>