Attribute alloc__size use and clang 5.0.1 vs. gcc7 (e.g.): __builtin_object_size(p,1) and __builtin_object_size(p,3) disagreements result

[Mark Millard]

[Noting a typo in the program source, and
so in the output text: the 2nd occurance of: "my_calloc_alt0
should have been: "my_calloc_alt1
. Hand edited corrections below for clarity.]

On 2018-Jan-20, at 3:27 PM, Mark Millard <marklmi26-fbsd at yahoo.com> wrote:

> [Bugzilla 225197 indirectly lead to this.
> Avoiding continuing there.]
> 
> I decided to compare some alternate uses of
> __attribute__((alloc_size(. . .))) compiled
> and run under clang 5.0.1 and gcc7. I did not
> get what I expected based on prior discussion
> material.
> 
> This is an FYI since I do not know how important
> the distinctions that I found are.
> 
> Here is the quick program:
> 
> # more alloc_size_attr_test.c 
> #include <stdlib.h>
> #include <stdio.h>
> 
> __attribute__((alloc_size(1,2)))
> void* my_calloc_alt0(size_t n, size_t s)
> {
>   void* p = calloc(n,s);
>   printf("calloc __builtin_object_size 0,1,2,3: %ld, %ld, %ld, %ld\n"
>         ,(long) __builtin_object_size(p, 0)
>         ,(long) __builtin_object_size(p, 1)
>         ,(long) __builtin_object_size(p, 2)
>         ,(long) __builtin_object_size(p, 3)
>         );
>   return p;
> }
> 
> __attribute__((alloc_size(1))) __attribute__((alloc_size(2)))
> void* my_calloc_alt1(size_t n, size_t s)
> {
>   void* p = calloc(n,s);
>   printf("calloc __builtin_object_size 0,1,2,3: %ld, %ld, %ld, %ld\n"
>         ,(long) __builtin_object_size(p, 0)
>         ,(long) __builtin_object_size(p, 1)
>         ,(long) __builtin_object_size(p, 2)
>         ,(long) __builtin_object_size(p, 3)
>         );
>   return p;
> }
> 
> int main()
> {
>   void* p = my_calloc_alt0(2,7);
>   printf("my_calloc_alt0 __builtin_object_size 0,1,2,3: %ld, %ld, %ld, %ld\n"
>         ,(long) __builtin_object_size(p, 0)
>         ,(long) __builtin_object_size(p, 1)
>         ,(long) __builtin_object_size(p, 2)
>         ,(long) __builtin_object_size(p, 3)
>         );
>   void* q = my_calloc_alt1(2,7);
>   printf("my_calloc_alt0 __builtin_object_size 0,1,2,3: %ld, %ld, %ld, %ld\n"

The above line should have been:

printf("my_calloc_alt1 __builtin_object_size 0,1,2,3: %ld, %ld, %ld, %ld\n"

>         ,(long) __builtin_object_size(q, 0)
>         ,(long) __builtin_object_size(q, 1)
>         ,(long) __builtin_object_size(q, 2)
>         ,(long) __builtin_object_size(q, 3)
>         );
> }
> 
> # uname -apKU
> FreeBSD FBSDFSSD 12.0-CURRENT FreeBSD 12.0-CURRENT  r327485M  amd64 amd64 1200054 1200054
> 
> The system-clang 5.0.1 result was:
> 
> # clang -O2 alloc_size_attr_test.c

The later outputs are edited for clarity:

> # ./a.out
> calloc __builtin_object_size 0,1,2,3: 14, 14, 14, 0
> my_calloc_alt0 __builtin_object_size 0,1,2,3: 14, 14, 14, 0
> calloc __builtin_object_size 0,1,2,3: 14, 14, 14, 0
my_calloc_alt1 __builtin_object_size 0,1,2,3: 14, 14, 14, 0
> 
> The lang/gcc7 result was:
> 
> # gcc7 -O2 alloc_size_attr_test.c
> 
> # ./a.out
> calloc __builtin_object_size 0,1,2,3: -1, -1, 0, 0
> my_calloc_alt0 __builtin_object_size 0,1,2,3: 14, 14, 14, 14
> calloc __builtin_object_size 0,1,2,3: -1, -1, 0, 0
my_calloc_alt1 __builtin_object_size 0,1,2,3: 14, 7, 14, 14
> 
> I'll ignore that gcc does not provide actual sizes
> via __builtin_object_size for calloc use.
> 
> Pairing the other lines for easy comparison, with
> some notes mixed in:
> 
> __attribute__((alloc_size(1,2))) style:
> my_calloc_alt0 __builtin_object_size 0,1,2,3: 14, 14, 14, 0  (system clang)
> my_calloc_alt0 __builtin_object_size 0,1,2,3: 14, 14, 14, 14 (gcc7)
> 
> __attribute__((alloc_size(1))) __attribute__((alloc_size(2))) style:
my_calloc_alt1 __builtin_object_size 0,1,2,3: 14, 14, 14, 0  (system clang)
my_calloc_alt1 __builtin_object_size 0,1,2,3: 14, 7, 14, 14  (gcc7)
> 
> Thus. . .
> 
> For __attribute__((alloc_size(1))) __attribute__((alloc_size(2))):
> __builtin_object_size(p,1) is not equivalent (clang vs. gcc7)
> 
> For both of the alloc_size usage styles:
> __builtin_object_size(p,3) is not equivalent (clang vs. gcc7)
> 
> This means that the two style of alloc_size use are not
> equivalent across some major compilers/toolchains.
> 
> But I do not know if either of the differences is a problem or
> not.
> 
> 
> Note: without a sufficient -O<?> all the figures can be
> the mix of -1's and 0's.


===
Mark Millard
marklmi at yahoo.com
( markmi at dsl-only.net is
going away in 2018-Feb, late)