David Laight <David.Laight@xxxxxxxxxx> wrote: > > 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
