lkml.org 
[lkml]   [2015]   [Mar]   [25]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    SubjectRe: [PATCH 3/4] x86/asm/entry/64: use smaller insns
    From
    Date
    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

    --
    Sent from my mobile phone. Please pardon brevity and lack of formatting.


    \
     
     \ /
      Last update: 2015-03-26 01:21    [W:3.713 / U:0.008 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site