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

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Date:	Wed, 25 Mar 2015 17:05:50 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Denys Vlasenko <dvlasenk@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Borislav Petkov <bp@...en8.de>,
	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 4:56 PM, H. Peter Anvin <hpa@...or.com> wrote:
> No, movabs is yet another instruction (with a 64-bit absolute address.) But movq can mean 10 or 7 bytes...

I mentioned movabs because that is literally what as generates at
least for me (or then objdump is confused):

  [torvalds@i7 ~]$ as -v
  GNU assembler version 2.24 (x86_64-redhat-linux) using BFD version
version 2.24

  [torvalds@i7 ~]$ cat t.s
  main:
        movq $0x12, %rdi
        movq $0x1234, %rdi
        movq $0x123456, %rdi
        movq $0x12345678, %rdi
        movq $0x123456789ab, %rdi

  [torvalds@i7 ~]$ as t.s
  [torvalds@i7 ~]$ objdump -d a.out
  ...
     0: 48 c7 c7 12 00 00 00 mov    $0x12,%rdi
     7: 48 c7 c7 34 12 00 00 mov    $0x1234,%rdi
     e: 48 c7 c7 56 34 12 00 mov    $0x123456,%rdi
    15: 48 c7 c7 78 56 34 12 mov    $0x12345678,%rdi
    1c: 48 bf ab 89 67 45 23 movabs $0x123456789ab,%rdi
    23: 01 00 00

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?

Oh well.

                       Linus
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