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

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Date:	Fri, 10 Feb 2012 09:32:39 -0000
From:	"David Laight" <David.Laight@...LAB.COM>
To:	"Joe Perches" <joe@...ches.com>,
	"David Miller" <davem@...emloft.net>
Cc:	<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

 
> -	u32 func_encode = func |
> -			((is_Pf == true ? 1 : 0) <<
IGU_FID_ENCODE_IS_PF_SHIFT);
> +	u32 func_encode = func | (is_Pf ? 1 : 0) <<
IGU_FID_ENCODE_IS_PF_SHIFT;

This sort of thing is why I personally don't like 'bool' at all.
If 'is_Pf' were an integer type that is known to only contain 0 or 1
then the code can just be:
	u32 func_encode = func | is_Pf << IGU_FID_ENCODE_IS_PF_SHIFT;
although that is still valid when is_Pf is bool, the compiler
is required to generate code on every access that converts
all non-zero values to 1 - so the generated code is likely
to be 'is_Pf ? 1 : 0'.

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

	David


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