[Bug c/104828] New: Wrong out-of-bounds array access warning on literal pointers

[goswin-v-b at web dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104828

            Bug ID: 104828
           Summary: Wrong out-of-bounds array access warning on literal
                    pointers
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: goswin-v-b at web dot de
  Target Milestone: ---

Trying to access a pointer cast from an integer literal gives a out of bounds
warning:

--------------------------------------------------------------------------
#define UART0_BASE      0x3F201000
void putc(char c) {
  volatile unsigned int *UART0_DR = (volatile unsigned int *)(UART0_BASE);
  volatile unsigned int *UART0_FR = (volatile unsigned int *)(UART0_BASE +
0x18);
  while (*UART0_FR & (1 << 5) ) { }
  *UART0_DR = c;
}
--------------------------------------------------------------------------

<source>:5:3: warning: array subscript 0 is outside array bounds of 'volatile
unsigned int [0]' [-Warray-bounds]
    5 |   *UART0_DR = c;
      |   ^~~~~~~~~

The error goes away if the pointer is global or static.

The error remains if the pointer is returned from a function with alloc_size
attribute:

--------------------------------------------------------------------------
#include <unistd.h>
#include <cstdint>

#define UART0_BASE      0x3F201000

volatile uint32_t * make(uintptr_t addr, size_t size = 4) __attribute__
((alloc_size (2)));
volatile uint32_t * make(uintptr_t addr, size_t size) {
    (void)size;
    return (volatile uint32_t *)addr;
}

void putc(char c) {
  volatile uint32_t *UART0_DR = make(UART0_BASE);
  volatile uint32_t *UART0_FR = make(UART0_BASE + 0x18);
  while (*UART0_FR & (1 << 5) ) { }
  *UART0_DR = c;
}
--------------------------------------------------------------------------

<source>:16:3: warning: array subscript 0 is outside array bounds of 'volatile
uint32_t [0]' [-Warray-bounds]
   16 |   *UART0_DR = c;
      |   ^~~~~~~~~

The warning goes away if the "make" helper is extern and can't be inlined.

Gcc 11.2 and before do not give this warning.