linux-kernel - Re: [PATCH 3/4] x86/asm/entry/64: use smaller insns

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Message-ID: <20150326092712.GA27751@pd.tnic>
Date:	Thu, 26 Mar 2015 10:27:12 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Andy Lutomirski <luto@...capital.net>,
	Oleg Nesterov <oleg@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/4] x86/asm/entry/64: use smaller insns

On Wed, Mar 25, 2015 at 05:05:50PM -0700, Linus Torvalds wrote:
> so 'as' is clearly just stupid. It already takes the size of the
> constant into account and generates different instructions. Why not
> for the common 32-bit case too?

I think the destination register mandates which insn to use:

        mov $0x12345678, %rdi
        mov $0x12345678, %edi

...

  15:   48 c7 c7 78 56 34 12    mov    $0x12345678,%rdi
  1c:   bf 78 56 34 12          mov    $0x12345678,%edi

and 'as' is perhaps not insolent enough to go and change it when seeing
the 32-bit constant.

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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