Re: [PATCH] no need to check for NULL before calling kfree() -fs/ext2/

[linux-os]

On Sat, 26 Mar 2005, Arjan van de Ven wrote:

> On Fri, 2005-03-25 at 17:29 -0500, linux-os wrote:
> > Isn't it expensive of CPU time to call kfree() even though the
> > pointer may have already been freed?
>
> nope
>
> a call instruction is effectively half a cycle or less, the branch

Wrong!

> predictor of the cpu can predict perfectly where the next instruction is
> from. The extra if() you do in front is a different matter, that can
> easily cost 100 cycles+. (And those are redundant cycles because kfree
> will do the if again anyway). So what you propose is to spend 100+
> cycles to save half a cycle. Not a good tradeoff ;)

Wrong!

Pure unmitigated bull-shit. I measure (with hardware devices)
the execution time of real code in modern CPUs. I do this for
a living so you don't have to stand in line for a couple of
hours to have your baggage scanned at the airport.

Always, always, a call will be more expensive than a branch
on condition. It's impossible to be otherwise. A call requires
that the return address be written to memory (the stack),
using register indirection (the stack-pointer).

If somebody said; "I think that the code will look better
and the few cycles lost will not be a consequence with modern
CPUs...", then there is a point. But coming up with this
disingenuous bullshit is something else.

Cheers,
Dick Johnson
Penguin : Linux version 2.6.11 on an i686 machine (5537.79 BogoMips).
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                 98.36% of all statistics are fiction.
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