netdev - RE: [PATCH net-next] drivers/net: Remove boolean comparisons to true/false

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Message-ID: <1328879062.2443.16.camel@edumazet-laptop>
Date:	Fri, 10 Feb 2012 14:04:22 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Laight <David.Laight@...LAB.COM>
Cc:	Joe Perches <joe@...ches.com>, David Miller <davem@...emloft.net>,
	jeffrey.t.kirsher@...el.com, netdev@...r.kernel.org,
	linux-wireless <linux-wireless@...r.kernel.org>
Subject: RE: [PATCH net-next] drivers/net: Remove boolean comparisons to
 true/false

Le vendredi 10 février 2012 à 09:32 +0000, David Laight a écrit :

> The generated code is particularly horrid for boolean arithmetic.
> IIRC:
>    bool_var &= bool_var_1;
> typically requires a sequence of compare and branch instructions.
> 

I suggest you try to upgrade your old compiler and/or always check your
assertions before posting on netdev such claims.

	bool_var &= bool_var_1;

generates a single AND instruction, no compare, no branch.
It would be a failure from compiler optimizer if that was the case.

And (bool_var ? 1 : 0) or (!!bool_var) are NOP for a compiler as well.

(Only one movzbl instruction to convert a byte to a word if context
requires this)

Only difference for bool or unsigned int vars is type promotion, and
fact that compiler knows that 'bool' only can be 0 or 1, so allows more
optimisations than plain "unsigned int".

Really, bools are not evil if well used.

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