On 12.06.24 09:01, Wei Yang wrote:
On Tue, Jun 11, 2024 at 11:20:00AM +0200, David Hildenbrand wrote:
On 11.06.24 02:56, Wei Yang wrote:
On Mon, Jun 10, 2024 at 10:22:49AM +0200, David Hildenbrand wrote:
On 10.06.24 05:40, Oscar Salvador wrote:
On Fri, Jun 07, 2024 at 10:37:11AM +0200, David Hildenbrand wrote:
It looks rather weird that totalhigh_pages() returns an
"unsigned long" but nr_free_highpages() returns an "unsigned int".
Let's return an "unsigned long" from nr_free_highpages() to be
consistent.
While at it, use a plain "0" instead of a "0UL" in the !CONFIG_HIGHMEM
totalhigh_pages() implementation, to make these look alike as well.
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
...
-static inline unsigned int nr_free_highpages(void) { return 0; }
-static inline unsigned long totalhigh_pages(void) { return 0UL; }
+static inline unsigned long nr_free_highpages(void) { return 0; }
+static inline unsigned long totalhigh_pages(void) { return 0; }
Although I doubt it has any consequences, I would just leave them both with UL,
so the return type is consistent with what we are returning.
These suffixes are only required when using constants that would not fit
into the native (int) type, or converting from that native (int) type to
something else automatically by the compiler would mess things up (for example,
undesired sign extension). For 0 that is certainly impossible :)
That's also the reason why in include/linux we now have:
t14s: ~/git/linux/include/linux $ git grep "return 0UL;"
skbuff.h: return 0UL;
uaccess.h:static inline unsigned long user_access_save(void) { return 0UL; }
t14s: ~/git/linux/include/linux $ git grep "0UL;"
bitmap.h: *dst = ~0UL;
dax.h: return ~0UL;
mtd/map.h: r.x[i] = ~0UL;
netfilter.h: return ((ul1[0] ^ ul2[0]) | (ul1[1] ^ ul2[1])) == 0UL;
skbuff.h: return 0UL;
uaccess.h:static inline unsigned long user_access_save(void) { return 0UL; }
... compared to a long list if "unsigned long" functions that simply "return 0;"
Seems this is the current status.
Then my question is do we have a guide line for this? Or 0 is the special
case? Sounds positive value has no sign extension problem. If we need to
return 1, we suppose to use 1 or 1UL? I found myself confused.
I grepped "return 1" and do find some cases without UL:
backing-dev.h: wb_stat_error() return 1 for unsigned long.
pgtable.h: pte_batch_hint() return 1 for unsigned int.
So the guide line is for positive value, it is not necessary to use UL?
I think when returning simple values (0/1/-1), we really don't need these
suffices at all. The standard says "The type of an integer constant is the
first of the corresponding list in which its value can be represented.". I
thought it would always use an "int", but that is not the case.
So, if we use "-1", the compiler will use an "int", and sign extension to
"unsigned" long will do the right thing.
Simple test:
-1 results in: 0xffffffffffffffff
-1U results in: 0xffffffff
-1UL results in: 0xffffffffffffffff
0xffffffff results in: 0xffffffff
0xffffffffU results in: 0xffffffff
0xffffffffUL results in: 0xffffffff
~0xffffffff results in: 0x0
~0xffffffffU results in: 0x0
~0xffffffffUL results in: 0xffffffff00000000
0xffffffffffffffff results in: 0xffffffffffffffff
0xffffffffffffffffU results in: 0xffffffffffffffff
I expect this to be 0xffffffff. Why this extend it to a UL?
Apparently, the "U" only restricts the set of types to "unsigned ones".
https://en.cppreference.com/w/cpp/language/integer_literal
So the compiler will use the first "unsigned" type that can hold that value.
--
Cheers,
David / dhildenb
