> On 26 Aug 2019, at 02:59, Denis Efremov <efre...@linux.com> wrote:
>
>
>
>> On 25.08.2019 19:37, Joe Perches wrote:
>>> On Sun, 2019-08-25 at 16:05 +0300, Denis Efremov wrote:
>>> This patch adds coccinelle script for detecting !likely and !unlikely
>>> usage. It's better to use unlikely instead of !likely and vice versa.
>>
>> Please explain _why_ is it better in the changelog.
>>
>
> In my naive understanding the negation (!) before the likely/unlikely
> could confuse the compiler
As a human I am confused. Is !likely(x) equivalent to x or !x?
Julia
> and the initial branch-prediction intent
> could be "falsified". I would say that either you need to move the
> negation under the bracket "!unlikely(cond) -> unlikely(!cond)" or
> you need to use likely instead "!unlikely(cond) -> likely(cond)".
> However, I'm not a compiler expert to state that this is a general
> rule. But, we've got 2 special macro for branch predicting, not one.
> There is also ftrace in-between. I will try to do some simple
> benchmarking.
>
>> btw: there are relatively few uses like this in the kernel.
>>
>> $ git grep -P '!\s*(?:un)?likely\s*\(' | wc -l
>> 40
>>
>> afaict: It may save 2 bytes of x86/64 object code.
>>
>> For instance:
>>
>> $ diff -urN kernel/tsacct.lst.old kernel/tsacct.lst.new|less
>> --- kernel/tsacct.lst.old 2019-08-25 09:21:39.936570183 -0700
>> +++ kernel/tsacct.lst.new 2019-08-25 09:22:20.774324886 -0700
>> @@ -24,158 +24,153 @@
>> 15: 48 89 fb mov %rdi,%rbx
>> u64 time, delta;
>>
>> - if (!likely(tsk->mm))
>> + if (unlikely(tsk->mm))
>> 18: 4c 8d ab 28 02 00 00 lea 0x228(%rbx),%r13
>> 1f: e8 00 00 00 00 callq 24 <__acct_update_integrals+0x24>
>> 20: R_X86_64_PLT32 __sanitizer_cov_trace_pc-0x4
>> 24: 4c 89 ef mov %r13,%rdi
>> 27: e8 00 00 00 00 callq 2c <__acct_update_integrals+0x2c>
>> 28: R_X86_64_PLT32 __asan_load8_noabort-0x4
>> - 2c: 4c 8b bb 28 02 00 00 mov 0x228(%rbx),%r15
>> - 33: 4d 85 ff test %r15,%r15
>> - 36: 74 34 je 6c <__acct_update_integrals+0x6c>
>> + 2c: 48 83 bb 28 02 00 00 cmpq $0x0,0x228(%rbx)
>> + 33: 00
>> + 34: 75 34 jne 6a <__acct_update_integrals+0x6a>
>> return;
>
> I think it's incorrect to say so in general. For example, on x86/64:
>
> $ make mrproper
> $ make allyesconfig
> $ make && mv vmlinux vmlinux-000
> $ make coccicheck MODE=patch COCCI=scripts/coccinelle/misc/unlikely.cocci |
> patch -p1
> $ make
> $ ./scripts/bloat-o-meter ./vmlinux-000 ./vmlinux
> add/remove: 0/0 grow/shrink: 3/4 up/down: 41/-35 (6)
> Function old new delta
> dpaa2_io_service_rearm 357 382 +25
> intel_pmu_hw_config 1277 1285 +8
> get_sigframe.isra.constprop 1657 1665 +8
> csum_partial_copy_from_user 605 603 -2
> wait_consider_task 3807 3797 -10
> __acct_update_integrals 384 373 -11
> pipe_to_sendpage 459 447 -12
> Total: Before=312759461, After=312759467, chg +0.00%
>
> It definitely influence the way the compiler optimizes the code.
>
>>
>> And here's a possible equivalent checkpatch test.
>> ---
>> scripts/checkpatch.pl | 18 ++++++++++++++++++
>> 1 file changed, 18 insertions(+)
>>
>> diff --git a/scripts/checkpatch.pl b/scripts/checkpatch.pl
>> index 287fe73688f0..364603ad1a47 100755
>> --- a/scripts/checkpatch.pl
>> +++ b/scripts/checkpatch.pl
>> @@ -6529,6 +6529,24 @@ sub process {
>> "Using $1 should generally have parentheses around the
>> comparison\n" . $herecurr);
>> }
>>
>> +# !(likely|unlikely)(condition) use should be (unlikely|likely)(condition)
>> + if ($perl_version_ok &&
>> + $line =~ /(\!\s*((?:un)?likely))\s*$balanced_parens/) {
>> + my $match = $1;
>> + my $type = $2;
>> + my $reverse;
>> + if ($type eq "likely") {
>> + $reverse = "unlikely";
>> + } else {
>> + $reverse = "likely";
>> + }
>> + if (WARN("LIKELY_MISUSE",
>> + "Prefer $reverse over $match\n" . $herecurr) &&
>> + $fix) {
>> + $fixed[$fixlinenr] =~ s/\Q$match\E\s*\(/$reverse(/;
>> + }
>> + }
>> +
>> # whine mightly about in_atomic
>> if ($line =~ /\bin_atomic\s*\(/) {
>> if ($realfile =~ m@^drivers/@) {
>>
>>
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