On Wed, Apr 14, 2021 at 6:08 PM David Laight <david.lai...@aculab.com> wrote: > > From: Eric Dumazet > > Sent: 14 April 2021 17:00 > ... > > > Repeated unsafe_get_user() calls are crying out for an optimisation. > > > You get something like: > > > failed = 0; > > > copy(); > > > if (failed) goto error; > > > copy(); > > > if (failed) goto error; > > > Where 'failed' is set by the fault handler. > > > > > > This could be optimised to: > > > failed = 0; > > > copy(); > > > copy(); > > > if (failed) goto error; > > > Even if it faults on every invalid address it probably > > > doesn't matter - no one cares about that path. > > > > > > On which arch are you looking at ? > > > > On x86_64 at least, code generation is just perfect. > > Not even a conditional jmp, it is all handled by exceptions (if any) > > > > stac > > copy(); > > copy(); > > clac > > > > > > <out_of_line> > > efault_end: do error recovery. > > It will be x86_64. > I'm definitely seeing repeated tests of (IIRC) %rdx. > > It may well be because the compiler isn't very new. > Will be an Ubuntu build of 9.3.0. > Does that support 'asm goto with outputs' - which > may be the difference. >
Yep, probably. I am using some recent clang version. > David > > - > Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 > 1PT, UK > Registration No: 1397386 (Wales)