The mov [rip-0x117cfdf], eax instruction is indeed peculiar. It actually is
polling a safe point. When ctrl-c is pressed, the running thread is ordered
to stop as quickly as possible. It can't just halt anywhere but must
continue to execute until it comes to a point where it is safe to halt -- a
safe point. Meanwhile, the VM has set the memory page at address rip-0x117cfdf
to write protected which makes it so the instruction triggers a page
fault. You can see how it is handled in the memory_signal_handler_impl()
function in errors.cpp This page explains safe points in more detail:
http://psy-lob-saw.blogspot.se/2014/03/where-is-my-safepoint.html
r14 contains the datastack top pointer. It is incremented by 0x10 which is
16 bytes or 2 cells. Your code was supposed to put two items on the stack,
but then the optimizer did a great job and noticed that the values didn't
have to be put on the stack but could be kept in registers. But it didn't
optimize away the stack increment/decrement. Possibly, the optimizer in
0.96 or 0.97 did a better job at optimizing code like that.
2017-10-12 23:32 GMT+02:00 Alexander Ilin <ajs...@yandex.ru>:
>
> Hello!
>
> I'm trying to make sense of some disassembled code. Could someone help
> me out here?
>
> ```
> IN: scratchpad [ 256 <iota> [ drop ] each ] disassemble
> 0000001a8dbcdfc0: 89053a30e8fe mov [rip-0x117cfc6], eax !
> What's this?
> 0000001a8dbcdfc6: b800010000 mov eax, 0x100
> 0000001a8dbcdfcb: 31db xor ebx, ebx
> 0000001a8dbcdfcd: 4983c610 add r14, 0x10
> 0000001a8dbcdfd1: e909000000 jmp 0x1a8dbcdfdf (( gensym ) + 0x1f)
> 0000001a8dbcdfd6: 48ffc3 inc rbx
> 0000001a8dbcdfd9: 89052130e8fe mov [rip-0x117cfdf], eax !
> What's this?
> 0000001a8dbcdfdf: 4839c3 cmp rbx, rax
> 0000001a8dbcdfe2: 0f8ceeffffff jl dword 0x1a8dbcdfd6 (( gensym ) + 0x16)
> 0000001a8dbcdfe8: 4983ee10 sub r14, 0x10
> 0000001a8dbcdfec: 89050e30e8fe mov [rip-0x117cff2], eax !
> What's this?
> 0000001a8dbcdff2: c3 ret
> 0000001a8dbcdff3: 0000 add [rax], al
> 0000001a8dbcdff5: 0000 add [rax], al
> 0000001a8dbcdff7: 0000 add [rax], al
> 0000001a8dbcdff9: 0000 add [rax], al
> 0000001a8dbcdffb: 0000 add [rax], al
> 0000001a8dbcdffd: 0000 add [rax], al
> 0000001a8dbcdfff: 00 invalid
> ```
>
> The lines like `mov [rip-0x117cfdf], eax` I read as "copy the contents
> of the EAX register to a memory location pointed to by a memory address
> relative to the current Instruction Pointer register (rIP)". Am I right? If
> so, why is that being done inside the loop, and twice at the procedure
> entry and exit?
>
> Also, why is the register r14 incremented by 0x10 on entry, and
> decremented on exit? What is r14 used for?
>
> ---=====---
> Александр
>
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--
mvh/best regards Björn Lindqvist
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