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 |
