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Subject: Re: [tip:x86/asm] x86: Pack function addresses tightly as well
From: Linus Torvalds <torvalds@linux-foundation.org>
To: Andy Lutomirski <luto@amacapital.net>, Davidlohr Bueso <dave@stgolabs.net>,
        Peter Anvin <hpa@zytor.com>, Denys Vlasenko <dvlasenk@redhat.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Tim Chen <tim.c.chen@linux.intel.com>, Borislav Petkov <bp@alien8.de>,
        Peter Zijlstra <peterz@infradead.org>,
        "Chandramouleeswaran, Aswin" <aswin@hp.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Brian Gerst <brgerst@gmail.com>,
        Paul McKenney <paulmck@linux.vnet.ibm.com>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@kernel.org>,
        Jason Low <jason.low2@hp.com>
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On Fri, May 15, 2015 at 2:39 AM, tip-bot for Ingo Molnar
<tipbot@zytor.com> wrote:
>
> We can pack function addresses tightly as well:

So I really want to see performance numbers on a few
microarchitectures for this one in particular.

The kernel generally doesn't have loops (well, not the kinds of
high-rep loops that tend to be worth aligning), and I think the
general branch/loop alignment is likely fine. But the function
alignment doesn't tend to have the same kind of I$ advantages, it's
more lilely purely a size issue and not as interesting. Function
targets are also more likely to be not in the cache, I suspect, since
you don't have a loop priming it or a short forward jump that just got
the cacheline anyway. And then *not* aligning the function would
actually tend to make it *less* dense in the I$.

Put another way: I suspect this is more likely to hurt, and less
likely to help than the others.

Size matters, but size matters mainly from an I$ standpoint, not from
some absolute 'big is bad" issue. Also, even when size matters,
performance matters too. I do want performance numbers. Is this
measurable?

                         Linus