[Bug target/29932] avr-gcc wrongly optimizes bit sets/resets for IO register 0x20

["mlitwack at employees dot org" <gcc-bugzilla@gcc.gnu.org>]

------- Comment #2 from mlitwack at employees dot org  2007-03-31 15:45 -------
Subject: Re:  avr-gcc wrongly optimizes bit sets/resets
        for IO register 0x20

Hi Eric,

Indeed, the test program works for me too under GCC 4.1.2.  However, I
am developing on the ATtiny24 and 44, which isn't supported in GCC 4.1.x
(-mmcu=attiny24 and friends).  That's why I was using the 4.2
pre-release.

The test program still generates bad code under a more recent
pre-release (gcc-core-4.2.0-20070316) (I do check periodically).
Although not a critical bug since there are plenty of ways to work
around it, I was hoping to head it off now before 4.2.0 made it to
release.

It might be worth noting that the more generic -mmcu=avr2 also causes
the pre-release to generate bad code for I/O register 0x20.

Below is more detail on what's happening.

Thanks,

-mark



On Fri, 2007-03-30 at 19:58 +0000, eweddington at cso dot atmel dot com
wrote:
> 
> ------- Comment #1 from eweddington at cso dot atmel dot com  2007-03-30 20:58 -------
> The test program works for me for AVR GCC 4.1.1. (WinAVR distro 20070122)
> 
> 


----------------------------------------------

File gcc-sbi-bug.c:

typedef unsigned char uint8_t;
#define _MMIO_BYTE(mem_addr) (*(volatile uint8_t *)(mem_addr))
#define _SFR_IO8(io_addr) _MMIO_BYTE((io_addr) + 0x20)

#define PCMSK1  _SFR_IO8(0x20)

main()
{
  PCMSK1 |= 0x01;
}

----------------------------------------------

Code generated by gcc-core-4.2.0-20070316
  (avr-gcc -Os -S gcc-sbi-bug.c -mmcu=attiny44):

        .file   "gcc-sbi-bug.c"
__SREG__ = 0x3f
__SP_H__ = 0x3e
__SP_L__ = 0x3d
__tmp_reg__ = 0
__zero_reg__ = 1
        .global __do_copy_data
        .global __do_clear_bss
        .text
.global main
        .type   main, @function
main:
/* prologue: frame size=0 */
        ldi r28,lo8(__stack - 0)
        ldi r29,hi8(__stack - 0)
        out __SP_H__,r29
        out __SP_L__,r28
/* prologue end (size=4) */
        sbi 64-0x20,0                         <----- WRONG!
/* epilogue: frame size=0 */
        rjmp exit
/* epilogue end (size=1) */
/* function main size 6 (1) */
        .size   main, .-main
/* File "gcc-sbi-bug.c": code    6 = 0x0006 (   1), prologues   4,
epilogues   1 */

----------------------------------------------

Code generated by gcc-4.1.2:
  (avr-gcc -Os -S gcc-sbi-bug.c):

        .file   "gcc-sbi-bug.c"
        .arch avr2
__SREG__ = 0x3f
__SP_H__ = 0x3e
__SP_L__ = 0x3d
__tmp_reg__ = 0
__zero_reg__ = 1
        .global __do_copy_data
        .global __do_clear_bss
        .text
.global main
        .type   main, @function
main:
/* prologue: frame size=0 */
        ldi r28,lo8(__stack - 0)
        ldi r29,hi8(__stack - 0)
        out __SP_H__,r29
        out __SP_L__,r28
/* prologue end (size=4) */
        in r24,64-0x20                    <----- OK
        ori r24,lo8(1)                    <-----
        out 64-0x20,r24                   <-----
/* epilogue: frame size=0 */
        rjmp exit
/* epilogue end (size=1) */
/* function main size 8 (3) */
        .size   main, .-main
/* File "gcc-sbi-bug.c": code    8 = 0x0008 (   3), prologues   4,
epilogues   1 */

----------------------------------------------


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29932