Re: [RFC][PATCH] A generic boolean
From: Jeff Garzik
Date: Wed Jul 19 2006 - 20:12:31 EST
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.
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/