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ReturnInfinity / BareMetal-OS-legacy

BareMetal is a 64-bit OS for x86-64 based computers. The OS is written entirely in Assembly while applications can be written in Assembly, C/C++, and Rust.
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Use switch tables instead of long strings of cmp/je combos #117

Open faissaloo opened 7 years ago

faissaloo commented 7 years ago

Instead of doing stuff like: 

os_system_config:
    cmp rdx, 0
    je os_system_config_timecounter
    cmp rdx, 1
    je os_system_config_argc
    cmp rdx, 2
    je os_system_config_argv
    cmp rdx, 3
    je os_system_config_networkcallback_get
    cmp rdx, 4
    je os_system_config_networkcallback_set
    cmp rdx, 5
    je os_system_config_clockcallback_get
    cmp rdx, 6
    je os_system_config_clockcallback_set
    cmp rdx, 20
    je os_system_config_video_base
    cmp rdx, 21
    je os_system_config_video_x
    cmp rdx, 22
    je os_system_config_video_y
    cmp rdx, 23
    je os_system_config_video_bpp
    cmp rdx, 30
    je os_system_config_mac
    ret

You could do: 

os_system_functions dq   os_system_config_timecounter, os_system_config_argc, os_system_config_argv, os_system_config_networkcallback_get, os_system_config_networkcallback_set, os_system_config_clockcallback_get, os_system_config_clockcallback_get, os_system_config_clockcallback_set, os_system_config_no_function, os_system_config_no_function, os_system_config_no_function, os_system_config_no_function, os_system_config_no_function, os_system_config_no_function, os_system_config_no_function, os_system_config_no_function, os_system_config_no_function, os_system_config_no_function, os_system_config_no_function, os_system_config_no_function, os_system_config_no_function, os_system_config_no_function, os_system_config_video_x, os_system_config_video_y, os_system_config_video_bpp, os_system_config_no_function, os_system_config_no_function, os_system_config_no_function, os_system_config_no_function, os_system_config_no_function, os_system_config_no_function, os_system_config_mac

os_system_config:
    jmp qword [os_system_functions+rdx*4]
    os_system_config_no_function:
    ret

And I believe it would be much faster, I also believe this is what GCC does (or at least, something similar) for switch statements.

effbiae commented 7 years ago

i got curious on this question of multiple cmps vs lookup table.

i found that cmps win for rdx<7 and then lookup win for the rest. funny - i remember back in school, a lecturer said it's faster to do linear search on n<=7 - i wonder if it's still true.

here's the gas code i used on linux - you can call lookup() and cmps()

$ cat j.s .intel_syntax noprefix .globl cmps,lookup .data a:;b:;c:;d:;e:;f:;g:;h:;i:;j:;k:;l:; ret cmps: cmp rdi,0 je a cmp rdi,1 je b cmp rdi,2 je c cmp rdi,3 je d cmp rdi,4 je e cmp rdi,5 je f cmp rdi,6 je g cmp rdi,7 je h cmp rdi,22 je i cmp rdi,23 je j cmp rdi,24 je k cmp rdi,31 je l ret .data;.align 8 funs: .quad a,b,c,d,e,f,g,h,z,z,z,z,z,z,z,z,z,z,z,z,z,z,i,j,k,z,z,z,z,z,z,l,z lookup: jmp qword [funs+rdi*8] z: ret

On 7 October 2016 at 20:44, Faissal Bensefia notifications@github.com wrote:

Instead of doing stuff like:

os_system_config: cmp rdx, 0 je os_system_config_timecounter cmp rdx, 1 je os_system_config_argc cmp rdx, 2 je os_system_config_argv cmp rdx, 3 je os_system_config_networkcallback_get cmp rdx, 4 je os_system_config_networkcallback_set cmp rdx, 5 je os_system_config_clockcallback_get cmp rdx, 6 je os_system_config_clockcallback_set cmp rdx, 20 je os_system_config_video_base cmp rdx, 21 je os_system_config_video_x cmp rdx, 22 je os_system_config_video_y cmp rdx, 23 je os_system_config_video_bpp cmp rdx, 30 je os_system_config_mac ret

You could do:

os_system_functions dq os_system_config_timecounter, os_system_config_argc, os_system_config_argv, os_system_config_networkcallback_get, os_system_config_networkcallback_set, os_system_config_clockcallback_get, os_system_config_clockcallback_get, os_system_config_clockcallback_set, os_system_config_no_function, os_system_config_no_function, os_system_config_no_function, os_system_config_no_function, os_system_config_no_function, os_system_config_no_function, os_system_config_no_function, os_system_config_no_function, os_system_config_no_function, os_system_config_no_function, os_system_config_no_function, os_system_config_no_function, os_system_config_no_function, os_system_config_no_function, os_system_config_video_x, os_system_config_video_y, os_system_config_video_bpp, os_system_config_no_function, os_system_config_no_function, os_system_config_no_function, os_system_config_no_function, os_system_config_no_function, os_system_config_no_function, os_system_config_mac

os_system_config: jmp qword [os_system_functions+rdx*4] os_system_config_no_function: ret

And I believe it would be much faster, I also believe this is what GCC does (or at least, something similar) for switch statements.

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IanSeyler commented 7 years ago

Awesome idea!! I'll give it a test on my end.