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Re: [PATCH] df: new option: --total (-c) to produce grand total (in the

From: Paolo Bonzini
Subject: Re: [PATCH] df: new option: --total (-c) to produce grand total (in the same way as du)
Date: Mon, 08 Sep 2008 10:39:15 +0200
User-agent: Thunderbird (Macintosh/20080707)

>> I disagree. The & => && change inserts a conditional branch into the control
>> flow, with the potential to save a single memory access. I count ca. 2 CPU
>> cycles for a memory access and ca. 8 CPU cycles for a conditional jump,
>> therefore I would say that the change slows down the program a bit.
> Surely you are not quibbling about a few extra cycles in the run-time
> of *df*.  This is no hot code path.  I suppose you're merely raising a
> theoretical counterpoint to my absolute statement.
> The benefit of using "&&" over "&" in those cases is precisely to obviate
> that tedious code inspection.  That benefit far outweighs the minuscule
> (and probably not measurable) cost of a few extra CPU cycles in df.

I expect that good compilers (not GCC right now) will convert && to & if
it is profitable to remove a conditional branch (e.g. in "a = b && c"
but not necessarily in "while (b && c)").


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