linux-kernel - Re: [PATCH] x86: Align jump targets to 1 byte boundaries

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Message-ID: <5527CD92.1080901@zytor.com>
Date:	Fri, 10 Apr 2015 06:18:10 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Denys Vlasenko <dvlasenk@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Jason Low <jason.low2@...com>,
	Peter Zijlstra <peterz@...radead.org>,
	Davidlohr Bueso <dave@...olabs.net>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Aswin Chandramouleeswaran <aswin@...com>,
	LKML <linux-kernel@...r.kernel.org>,
	Borislav Petkov <bp@...en8.de>,
	Andy Lutomirski <luto@...capital.net>,
	Brian Gerst <brgerst@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH] x86: Align jump targets to 1 byte boundaries

On 04/10/2015 05:50 AM, Denys Vlasenko wrote:
> 
> However, I'm an -Os guy. Expect -O2 people to disagree :)
> 

The problem with -Os is that the compiler will make *any* tradeoffs to
save a byte.  It is really designed to squeeze as much code into a
fixed-size chunk, e.g. a ROM, as possible.

We have asked for an -Okernel mode from the gcc folks forever.  It
basically would mean "-Os except when really dumb."

As far as the 16-byte alignment, my understanding is not that it is
related to the I$ but rather is the decoder datum.

	-hpa


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