On Tue, Sep 17, 2013 at 4:10 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> Linus suggested using asm goto to get rid of the typical SETcc + TEST
> instruction pair -- which also clobbers an extra register -- for our
> typical modify_and_test() functions.
Thinking about this, we actually have another place in x86 low-level
code where "asm goto" makes a lot of sense: exception handling for
__put_user_asm().
I'd love to use it for __get_user_asm() too, but it doesn't work there
because "asm goto" cannot have outputs (and a get_user obviously needs
an output - the value it gets). But for put_user(), it seems to be a
very good match.
The attached patch is ENTIRELY untested, but I did check some of the
generated assembly language. And the output is absolutely beautiful,
because now gcc sees the error case directly, so the straight-line
code is just he single "mov" instruction, no tests, no nothing. The
exception case will just jump to the local label directly.
Of course, the STAC/CLAC noise is there, and we really should try to
come up with a better model for that (so that the code that uses
__put_user() because it wants to do many of them in one go after
having done one access_ok() check) but that's a separate issue.
hpa, comments? Are you looking at perhaps moving the stac/clac
instructions out? With this, "filldir()" ends up lookng something like
...
data32 xchg %ax,%ax # stac
mov %rcx,0x8(%rax)
data32 xchg %ax,%ax # clac
mov 0x10(%rbx),%r13
data32 xchg %ax,%ax # stac
mov %r8,0x0(%r13)
data32 xchg %ax,%ax # clac
data32 xchg %ax,%ax # stac
mov %r12w,0x10(%r13)
data32 xchg %ax,%ax # clac
...
which is a bit sad, since the code really is almost perfect aside from
the tons of extra nops/stac/clac instructions...
Linus
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