linux-kernel - Re: [tip:x86/asm] x86: Pack function addresses tightly as well

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Message-ID: <CA+55aFywCXk083w78cQGbRKh-ERLtE8v9PZhqxaHcyHJxSNsFQ@mail.gmail.com>
Date:	Fri, 15 May 2015 11:36:47 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andy Lutomirski <luto@...capital.net>,
	Davidlohr Bueso <dave@...olabs.net>,
	Peter Anvin <hpa@...or.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Borislav Petkov <bp@...en8.de>,
	Peter Zijlstra <peterz@...radead.org>,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Brian Gerst <brgerst@...il.com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>, Jason Low <jason.low2@...com>
Cc:	"linux-tip-commits@...r.kernel.org" 
	<linux-tip-commits@...r.kernel.org>
Subject: Re: [tip:x86/asm] x86: Pack function addresses tightly as well

On Fri, May 15, 2015 at 2:39 AM, tip-bot for Ingo Molnar
<tipbot@...or.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
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