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From: Ingo Molnar <mingo@kernel.org>
To: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>,
	Borislav Petkov <bp@alien8.de>, "H. Peter Anvin" <hpa@zytor.com>,
	Andy Lutomirski <luto@amacapital.net>,
	Frederic Weisbecker <fweisbec@gmail.com>,
	Will Drewry <wad@chromium.org>, Kees Cook <keescook@chromium.org>,
	x86@kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] x86/asm/entry/32: simplify pushes of zeroed pt_regs->REGs
Date: Sat, 12 Mar 2016 16:45:02 +0100	[thread overview]
Message-ID: <20160312154502.GD17873@gmail.com> (raw)
In-Reply-To: <1457715214-7432-1-git-send-email-dvlasenk@redhat.com>


* Denys Vlasenko <dvlasenk@redhat.com> wrote:

> Use of a temporary R8 register here seems to be unnecessary.
> 
> "push %r8" is a two-byte insn (it needs REX prefix to specify R8),
> "push $0" is two-byte too. It seems just using the latter would be
> no worse.
> 
> Thus, code had an unnecessary "xorq %r8,%r8" insn.
> It probably costs nothing in execution time here since we are probably
> limited by store bandwidth at this point, but still.

Note that the 3 fewer instruction in the image also shrink the code by 16 bytes:

arch/x86/entry/entry_64_compat.o:

   text    data     bss     dec     hex filename
    380       0       0     380     17c entry_64_compat.o.before
    364       0       0     364     16c entry_64_compat.o.after

because (at least in this defconfig build) one of these functions shrunk below a 
16-byte boundary. So cache footprint got denser.

Thanks,

	Ingo

      parent reply	other threads:[~2016-03-12 15:45 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-03-11 16:53 [PATCH] x86/asm/entry/32: simplify pushes of zeroed pt_regs->REGs Denys Vlasenko
2016-03-12 15:38 ` Ingo Molnar
2016-03-12 17:53   ` Denys Vlasenko
2016-03-12 18:05     ` Andy Lutomirski
2016-03-12 21:41       ` Denys Vlasenko
2016-03-12 15:45 ` Ingo Molnar [this message]

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