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.
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;
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/@) {
