David Laight [off-list ref] wrote:

quoted

iov_iter_init                            inc 0x27 -> 0x31 +0xa

Are you hitting the gcc bug that loads the constant from memory?

I'm not sure what that looks like.  For your perusal, here's a disassembly of
the use-switch-on-enum variant:

   0xffffffff8177726c <+0>:     cmp    $0x1,%esi
   0xffffffff8177726f <+3>:     jbe    0xffffffff81777273 <iov_iter_init+7>
   0xffffffff81777271 <+5>:     ud2
   0xffffffff81777273 <+7>:     test   %esi,%esi
   0xffffffff81777275 <+9>:     movw   $0x1,(%rdi)
   0xffffffff8177727a <+14>:    setne  0x3(%rdi)
   0xffffffff8177727e <+18>:    xor    %eax,%eax
   0xffffffff81777280 <+20>:    movb   $0x0,0x2(%rdi)
   0xffffffff81777284 <+24>:    movb   $0x1,0x4(%rdi)
   0xffffffff81777288 <+28>:    mov    %rax,0x8(%rdi)
   0xffffffff8177728c <+32>:    mov    %rdx,0x10(%rdi)
   0xffffffff81777290 <+36>:    mov    %r8,0x18(%rdi)
   0xffffffff81777294 <+40>:    mov    %rcx,0x20(%rdi)
   0xffffffff81777298 <+44>:    jmp    0xffffffff81d728a0 <__x86_return_thunk>

versus the use-bitmap variant:

   0xffffffff81777311 <+0>:     cmp    $0x1,%esi
   0xffffffff81777314 <+3>:     jbe    0xffffffff81777318 <iov_iter_init+7>
   0xffffffff81777316 <+5>:     ud2
   0xffffffff81777318 <+7>:     test   %esi,%esi
   0xffffffff8177731a <+9>:     movb   $0x2,(%rdi)
   0xffffffff8177731d <+12>:    setne  0x1(%rdi)
   0xffffffff81777321 <+16>:    xor    %eax,%eax
   0xffffffff81777323 <+18>:    mov    %rdx,0x10(%rdi)
   0xffffffff81777327 <+22>:    mov    %rax,0x8(%rdi)
   0xffffffff8177732b <+26>:    mov    %r8,0x18(%rdi)
   0xffffffff8177732f <+30>:    mov    %rcx,0x20(%rdi)
   0xffffffff81777333 <+34>:    jmp    0xffffffff81d72960 <__x86_return_thunk>

It seems to be that the former is loading byte constants individually, whereas
Linus combined all those fields into a single byte and eliminated one of them.

David