On May 20 2007 18:12, Folkert van Heusden wrote:
>> >
>> > + if (unlikely(sig == SIGQUIT || sig == SIGILL || sig == SIGTRAP ||
>> > + sig == SIGABRT || sig == SIGBUS || sig == SIGFPE ||
>> > + sig == SIGSEGV || sig == SIGXCPU || sig == SIGXFSZ ||
>> > + sig == SIGSYS || sig == SIGSTKFLT))
>> > + {
>> > + printk(KERN_WARNING "Sig %d send to %d owned by %d.%d (%s)\n",
>> > + sig, t->pid, t->uid, t->gid, t->comm);
>> > + }
>> > +
>> > /*
>> > * fast-pathed signals for kernel-internal things like SIGSTOP
>> > * or SIGKILL.
>>
>> Would turning that into a switch() generate better code.
Yes, this time.
>Doubt it: in the worst case you still nee to check for each possibility.
>Furthermore a.f.a.i.k. with switch you cannot do 'unlinkely()'.
With if(), it generates a ton of "CMP, JE" instructions.
With switch(), I would assume gcc transforms it into using
a jump table (aka "JMP [table+sig]")
I tried it: with switch(), gcc transforms this into a
bitmap comparison ("MOV eax, 1; SHL eax, sig; TEST eax, 0x830109f8"),
which seems even cheaper than a jump table.
Jan
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