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Subject: Re: [PATCH 3/4] x86/asm/entry/64: use smaller insns
From: "H. Peter Anvin" <hpa@zytor.com>
Date: Wed, 25 Mar 2015 17:12:20 -0700
To: Linus Torvalds <torvalds@linux-foundation.org>
CC: Denys Vlasenko <dvlasenk@redhat.com>, Ingo Molnar <mingo@kernel.org>,
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        Alexei Starovoitov <ast@plumgrid.com>, Will Drewry <wad@chromium.org>,
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Ok, "movabs" is crazier than I thought.

On March 25, 2015 5:05:50 PM PDT, Linus Torvalds <torvalds@linux-foundation.org> wrote:
>On Wed, Mar 25, 2015 at 4:56 PM, H. Peter Anvin <hpa@zytor.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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