https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90962
Bug ID: 90962
Summary: Array bound over optimization
Product: gcc
Version: 7.3.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: patrick.pelissier at gmail dot com
Target Milestone: ---
For the following program, GCC optimizes away the loop that affects the array:
#include <stdlib.h>
#include <stdint.h>
struct node_s {
unsigned int size;
struct m_s *tab[1];
};
typedef struct m_s {
unsigned int type;
union {
struct node_s n;
} val[1];
} *m_t;
extern m_t node_c(uint8_t, unsigned int_t);
#define SET_AT(_x,_n,_y) ((_x)->val[0].n.tab[(_n)] = (_y))
m_t add_vc (unsigned int size, const m_t *tab)
{
if (size == 0)
return NULL;
m_t y = node_c (2, size);
for (unsigned int i = 0; i < size; i++) /* here */
SET_AT (y, i, tab[i]);
return y;
}
The following asm is generated for x86-64 when building it with gcc -O2 -S
t.c:
add_vc:
.LFB10:
.cfi_startproc
testl %edi, %edi
je .L3
pushq %rbx
.cfi_def_cfa_offset 16
.cfi_offset 3, -16
movq %rsi, %rbx
movl %edi, %esi
movl $2, %edi
call node_c
movq (%rbx), %rdx
movq %rdx, 16(%rax) // Only tab[0] is set
popq %rbx
.cfi_def_cfa_offset 8
ret
.p2align 4,,10
.p2align 3
.L3:
.cfi_restore 3
xorl %eax, %eax
ret
The loop is removed with GCC 7.3, GCC 8.2 and GCC 9.1: only tab[0] is set.
The loop is not removed and the program behaves as expected with GCC 4.9, GCC
6.3
