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 16:05 +0100, Jose E. Marchesi wrote:
>> > On Thu, 2024-02-08 at 13:55 +0100, Jose E. Marchesi wrote:
>> > [...]
>> > 
>> > > However, it would be good if some clang wizard could confirm what
>> > > impact, if any, #pragma unroll (aka #pragma clang loop unroll(enabled))
>> > > has over -O2, before ditching these pragmas from the selftests.
>> > 
>> > I compiled sefltests both with and without this patch,
>> > there are no differences in disassembly of generated BPF object files.
>> > (using current clang main).
>> > 
>> > [...]
>> 
>> Hmm, wouldn't that mean that the loops in profiler.inc.h never get
>> unrolled regardless of optimization level or pragma? (profiler2.c)
>> 
>
> No, the generated code is different between profiler{1,2,3}, e.g.:
>
> $ llvm-objdump -d before/profiler1.bpf.o | wc -l
> 5356
> $ llvm-objdump -d before/profiler2.bpf.o | wc -l
> 2329
> $ llvm-objdump -d before/profiler3.bpf.o | wc -l
> 1915
>
> What I meant, is that generated code for before/profiler1.bpf.o
> and after/profiler1.bpf.o is identical, etc.

Right.  But profiler2.c before and after the patch do:

--- Before:

profiler2.c:

// SPDX-License-Identifier: GPL-2.0
/* Copyright (c) 2020 Facebook */
#define barrier_var(var) /**/
/* undef #define UNROLL */
#define INLINE /**/
#include "profiler.inc.h"

profiler.inc.h:

#ifdef UNROLL
#pragma unroll
#endif
       for (WHATEVER) {
         [...]
       }

--- After:

profiler2.c:

// SPDX-License-Identifier: GPL-2.0
/* Copyright (c) 2020 Facebook */
#define barrier_var(var) /**/
#define NO_UNROLL
#define INLINE /**/
#include "profiler.inc.h"

profiler.inc.h:

#ifdef NO_UNROLL
#pragma clang loop unroll(disable)
#endif
	for (WHATEVER) {
          [...]
        }
---

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
;)