[Bug c/109071] New: -Warray-bounds warning when array index checked via inline

["kees at outflux dot net" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109071

            Bug ID: 109071
           Summary: -Warray-bounds warning when array index checked via
                    inline
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: kees at outflux dot net
  Target Milestone: ---

Created attachment 54611
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=54611&action=edit
PoC for -Warray-bounds false positive

The Linux kernel is seeing -Warray-bounds warnings when array indexes are being
checked via inlines. This appears to be in the overly noisy/false positive
territory, but I don't actually know what's going on.

The upstream report is here:
https://lore.kernel.org/lkml/20230306220947.1982272-1-trix@redhat.com/

Originally I thought this was another -fsanitizer=shift issue, but after
reducing the test-case, it seems to be related to inlining or some other aspect
of optimization passes.

If the "assign" function is open-coded in the caller, the warning goes away.

If the index checks are moved before the "assign" calls, the warning goes away.

If there is only 1 call to "assign", the warning goes away.

Fundamentally there should be no warning at all since the value of "index" is
entirely unknown _except_ when it makes the call to "warn".

$ cat test.c
extern void warn(void);

#define MAX_ENTRIES     4

static inline void assign(int val, int *regs, int index)
{
        if (index >= MAX_ENTRIES)
                warn();
        *regs = val;
}

struct nums {
        int vals[MAX_ENTRIES];
};

void sparx5_psfp_sg_set(int *ptr, struct nums *sg, int index)
{
        int *val;

        val = &sg->vals[index];

        assign(0,    ptr, index);
        assign(*val, ptr, index);
}

$ gcc -Wall -O2  -c -o test.o test.c
test.c: In function 'sparx5_psfp_sg_set':
test.c:20:24: warning: array subscript 4 is above array bounds of 'int[4]'
[-Warray-bounds=]
   20 |         val = &sg->vals[index];
      |                ~~~~~~~~^~~~~~~
test.c:13:13: note: while referencing 'vals'
   13 |         int vals[MAX_ENTRIES];
      |             ^~~~