Messages in this thread | | | Subject | Re: [PATCH] scripts: coccinelle: check for !(un)?likely usage | From | Julia Lawall <> | Date | Mon, 26 Aug 2019 03:19:35 +0800 |
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> On 26 Aug 2019, at 02:59, Denis Efremov <efremov@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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