Re: [PATCH 12/25] slub: make ->reserved unsigned int

[Alexey Dobriyan <adobriyan@gmail.com>]

On Tue, Mar 06, 2018 at 12:43:26PM -0600, Christopher Lameter wrote:
> On Mon, 5 Mar 2018, Alexey Dobriyan wrote:
> 
> > ->reserved is either 0 or sizeof(struct rcu_head), can't be negative.
> 
> Thus it should be size_t? ;-)

:-)

Christoph, using "unsigned int" should be default for kernel really.

As was noted earlier it doesn't matter for constants as x86_64 clears
upper half of a register. But it matters for sizes which aren't known
at compile time.

I've looked at a lot of places where size_t is used.
There is a certain degree of "type correctness" when people try to keep
type as much as possible. It works until first multiplication.

	int n;
	size_t len = sizeof(struct foo0) + n * sizeof(struct foo);

Most likely MOVSX or CDQE will be generated which is not the case
if everything is "unsigned int".

Generally, on x86_64,

	uint32_t > uint64_t > uint16_t
	uint8_t	 >

uint64_t adds REX prefix.
uint16_t additionally adds 66 prefix

uint8_t doesn't add anything but it is suboptimal on embedded archs
which emit "& 0xff" and thus should be used only for trimming memory
usage.

Additionally,

	unsigned int > int

as it is easy for compiler to lose track of value range and generate
size extensions.

There is only one exception, namely, when pointers are mixed with
integers:

	int n;
	void *p = p0 + n;

Quite often, gcc generates bigger code when types are made unsigned.
I don't quite understand how it thinks, but overall code will be smaller
if every signed type is made into unsigned.