Messages in this thread | | | From | David Laight <> | Subject | RE: [PATCH v2 3/3] rseq: optimise rseq_get_rseq_cs() and clear_rseq_cs() | Date | Wed, 14 Apr 2021 16:08:46 +0000 |
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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.
David
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