Linux-kernel mailing list archive 1997-49,: Re: copy-bit macro

# Re: copy-bit macro

Theodore Y. Ts'o (tytso@MIT.EDU)
Fri, 12 Dec 1997 09:17:52 -0500

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From: Stefan Monnier <monnier+lists/linux/kernel/news/@TEQUILA.SYSTEMSZ.CS.YALE.EDU>
Date: 11 Dec 1997 17:59:10 -0500

"Theodore Y. Ts'o" <tytso@MIT.EDU> writes:
> (1 << OUT_FLAG_NUMBER) : 0") both involve an assembly language
> conditional jump instruction, which is expensive, because it involves
> pipeline stalls, etc.

Has it been measured ? I know that branch prediction is not 100%, but
I'm just trying to make sure we're not trying to solve a non-problem (if the
prediction works well, then the jump costs 0 (zero) cycles).

Branch prediction works in conditional testing when branching out of
loops. In the cases where you'd use copy_bit(), it isn't in a loop
condition, and yes it matters.

- Ted

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