Re: [RFC][PATCH] A generic boolean

From: Jeff Garzik
Date: Wed Jul 19 2006 - 20:12:31 EST

ricknu-0@xxxxxxxxxxxxxx wrote:
Citerar Jeff Garzik <jeff@xxxxxxxxxx>:
Also, you don't want to force 'unsigned char' on code, because often code prefers a machine integer to something smaller than a machine integer.

But isn't a bit smaller than a byte? Sorry, do not understand what you mean.

For all processors, it is generally preferred to have integer operations performed on a "machine integer." A machine integer is the natural data type of the processor. If it's a 32-bit processor, the natural data type for the ALU is a 32-bit int. If it's a 64-bit processor, the natural data type for the ALU is a 64-bit int. [though, for some 64-bit processors, a 32-bit int may be best for the situation anyway]

As such, the compiler and/or CPU must do more work, if an operation such as a bit test is performed on a data type other than a machine int.

Consider for example ARM or Alpha architectures, which may not have instructions 8-bit unsigned char integers. The integers have to be _converted_ to a machine integer, before the operation is performed.

It is for this reason that you often see boolean implemented as 'int' rather than 'unsigned char'. 'int' is much more "natural", when you consider all the architectures Linux must support.

The best solution is to typedef 'bool' to the compiler's internal boolean data type, and then update code to use 'bool'. Then all these issues magically go away, because you never have to care what the compiler's underlying boolean data type is.


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