linux-kernel - Re: [PATCH] x86/asm/entry/32: simplify pushes of zeroed pt

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Date:	Sat, 12 Mar 2016 16:45:02 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Denys Vlasenko <dvlasenk@...hat.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>,
	Borislav Petkov <bp@...en8.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andy Lutomirski <luto@...capital.net>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/asm/entry/32: simplify pushes of zeroed pt_regs->REGs


* Denys Vlasenko <dvlasenk@...hat.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