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]

[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"
         ,(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

# ./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_alt0 __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_alt0 __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_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, 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)