Below is a small example C source showing the clang 15+ armv7
problem that leads to the unbounded looping in later code in
the lang/gcc12+ builds: a data structure is mis-initialized,
breaking its invariant properties used by the later code
structure.
# more partition.c
// Minor varation of part of some gcc source code!
// For system-clang 15: cc -g -O2 partition.c ; ./a.out
// For devel/llvm16: clang16 -g -O2 partition.c ; ./a.out
#include <stdio.h>
#define NUM_ELEMENTS 32
struct partition_elem
{
struct partition_elem* next;
int class_element;
unsigned class_count;
};
typedef struct partition_def
{
int num_elements;
struct partition_elem elements[NUM_ELEMENTS];
} *partition;
struct partition_def partition_storage;
partition
partition_new (int num_elements)
{
int e;
if (NUM_ELEMENTS < num_elements) num_elements = NUM_ELEMENTS;
partition part= &partition_storage;
part->num_elements = num_elements;
for (e = 0; e < num_elements; ++e)
{
part->elements[e].class_element = e;
part->elements[e].next = &(part->elements[e]);
part->elements[e].class_count = 1;
}
for (e = 0; e < num_elements; ++e)
printf("%d: %p : next?:
%p\n",e,(void*)&part->elements[e],(void*)part->elements[e].next);
return part;
}
int main(void)
{
partition part;
part= partition_new(NUM_ELEMENTS);
return !part;
}
In the output below, note the blocks of 4 "next"
values that do not change. Each should match the
earlier hexadecimal value on the same line: point
back to same element of the array. 3 of 4 do not.
# cc -g -O2 partition.c
# ./a.out
0: 0x40a84 : next?: 0x40a84
1: 0x40a90 : next?: 0x40a84
2: 0x40a9c : next?: 0x40a84
3: 0x40aa8 : next?: 0x40a84
4: 0x40ab4 : next?: 0x40ab4
5: 0x40ac0 : next?: 0x40ab4
6: 0x40acc : next?: 0x40ab4
7: 0x40ad8 : next?: 0x40ab4
8: 0x40ae4 : next?: 0x40ae4
9: 0x40af0 : next?: 0x40ae4
10: 0x40afc : next?: 0x40ae4
11: 0x40b08 : next?: 0x40ae4
12: 0x40b14 : next?: 0x40b14
13: 0x40b20 : next?: 0x40b14
14: 0x40b2c : next?: 0x40b14
15: 0x40b38 : next?: 0x40b14
16: 0x40b44 : next?: 0x40b44
17: 0x40b50 : next?: 0x40b44
18: 0x40b5c : next?: 0x40b44
19: 0x40b68 : next?: 0x40b44
20: 0x40b74 : next?: 0x40b74
21: 0x40b80 : next?: 0x40b74
22: 0x40b8c : next?: 0x40b74
23: 0x40b98 : next?: 0x40b74
24: 0x40ba4 : next?: 0x40ba4
25: 0x40bb0 : next?: 0x40ba4
26: 0x40bbc : next?: 0x40ba4
27: 0x40bc8 : next?: 0x40ba4
28: 0x40bd4 : next?: 0x40bd4
29: 0x40be0 : next?: 0x40bd4
30: 0x40bec : next?: 0x40bd4
31: 0x40bf8 : next?: 0x40bd4
Turns out that the -O2 is important: no other that I
tried got the problem, including -O3 not getting the
problem. lang/gcc12+ builds happen to use -O2 , at
least in my environment.
-g is not required for the problem.
===
Mark Millard
marklmi at yahoo.com
