Re: [PATCH v2] x86/asm: Use asm_inline() instead of asm() in __untagged_addr()

[Ingo Molnar]

* Uros Bizjak <ubizjak@xxxxxxxxx> wrote:

> Use asm_inline() to instruct the compiler that the size of asm()
> is the minimum size of one instruction, ignoring how many instructions
> the compiler thinks it is. ALTERNATIVE macro that expands to several
> pseudo directives causes instruction length estimate to count
> more than 20 instructions.
> 
> bloat-o-meter reports minimal code size increase
> (x86_64 defconfig with CONFIG_ADDRESS_MASKING, gcc-14.2.1):
> 
>   add/remove: 2/2 grow/shrink: 5/1 up/down: 2365/-1995 (370)
> 
> 	Function                          old     new   delta
> 	-----------------------------------------------------
> 	do_get_mempolicy                    -    1449   +1449
> 	copy_nodes_to_user                  -     226    +226
> 	__x64_sys_get_mempolicy            35     213    +178
> 	syscall_user_dispatch_set_config  157     332    +175
> 	__ia32_sys_get_mempolicy           31     206    +175
> 	set_syscall_user_dispatch          29     181    +152
> 	__do_sys_mremap                  2073    2083     +10
> 	sp_insert                         133     117     -16
> 	task_set_syscall_user_dispatch    172       -    -172
> 	kernel_get_mempolicy             1807       -   -1807
> 
>   Total: Before=21423151, After=21423521, chg +0.00%
> 
> The code size increase is due to the compiler inlining
> more functions that inline untagged_addr(), e.g:
> 
> task_set_syscall_user_dispatch() is now fully inlined in
> set_syscall_user_dispatch():
> 
> 	000000000010b7e0 <set_syscall_user_dispatch>:
> 	  10b7e0:	f3 0f 1e fa          	endbr64
> 	  10b7e4:	49 89 c8             	mov    %rcx,%r8
> 	  10b7e7:	48 89 d1             	mov    %rdx,%rcx
> 	  10b7ea:	48 89 f2             	mov    %rsi,%rdx
> 	  10b7ed:	48 89 fe             	mov    %rdi,%rsi
> 	  10b7f0:	65 48 8b 3d 00 00 00 	mov    %gs:0x0(%rip),%rdi
> 	  10b7f7:	00
> 	  10b7f8:	e9 03 fe ff ff       	jmp    10b600 <task_set_syscall_user_dispatch>

So this was a tail-call optimization that jumped to a standalone 
<task_set_syscall_user_dispatch>, right? So now we'll avoid the 
tail-jump and maybe a bit of the register parameter shuffling? Which 
would explain the bloatometer delta of -172 for 
task_set_syscall_user_dispatch?

Could you also cite the first relevant bits of <task_set_syscall_user_dispatch>?

I don't seem to be able to reproduce this inlining decision, my version 
of GCC is:

  gcc version 14.2.0 (Ubuntu 14.2.0-4ubuntu2) 

which is one patch version older than your 14.2.1.

I tried GCC 11, 12, 13 as well, but none did this tail optimization, 
all appear to be inlining <task_set_syscall_user_dispatch> into 
<set_syscall_user_dispatch>. What am I missing?

Another question, where do the size increases in these functions come 
from:

>       __x64_sys_get_mempolicy            35     213    +178
>       syscall_user_dispatch_set_config  157     332    +175
>       __ia32_sys_get_mempolicy           31     206    +175
>       set_syscall_user_dispatch          29     181    +152

(I have to ask, because I have trouble reproducing with my toolchain so 
I cannot look at this myself.)

Thanks,

	Ingo