From: David Howells > Sent: Friday, August 18, 2023 5:49 PM > > 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. I think you need to re-order the structure. The top set writes to bytes 0..4 with: > 0xffffffff81777275 <+9>: movw $0x1,(%rdi) > 0xffffffff8177727a <+14>: setne 0x3(%rdi) > 0xffffffff81777280 <+20>: movb $0x0,0x2(%rdi) > 0xffffffff81777284 <+24>: movb $0x1,0x4(%rdi) Note that the 'setne' writes into the middle of the constants. The lower writes bytes 0..1 with: > 0xffffffff8177731a <+9>: movb $0x2,(%rdi) > 0xffffffff8177731d <+12>: setne 0x1(%rdi) I think that if you move the 'conditional' value to offset 4 you'll get fewer writes. Probably a 32bit load into %eax and then a write. I don't think gcc likes generating 16bit immediates. In some tests I did it loaded a 32bit value into %eax and then wrote the low bits. So the code is much the same (on x86) for 2 or 4 bytes of constants. I'm sure you can use the 'data-16' prefix with an immediate. I'm not sure why you have two non-zero values when Linus only had one though. OTOH you don't want to be writing 3 bytes of constants. Also gcc won't generate: movl $0xaabbccdd,%eax setne %al // overwriting the dd movl %eax,(%rdi) and I suspect the partial write (to %al) will be a stall. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
