[Bug c/39635] New: integer wrong code bug on AVR

[Bug c/39635] New: integer wrong code bug on AVR

[regehr at cs dot utah dot edu]

See below.  This bug was found using Avrora bug is reproducible in AVR studio. 

The problem exists in HEAD but not apparently in the 4.3 series.

The generated code looks to be wrong at -O2 and -Os.

regehr@john-home:~$ avr-gcc -v
Using built-in specs.
Target: avr
Configured with: ../configure --prefix=/home/regehr/avrgcc440/install
--target=avr --enable-languages=c,c++ --disable-nls --disable-libssp :
(reconfigured) ../configure --prefix=/home/regehr/avrgcc440/install
--target=avr --enable-languages=c,c++ --disable-nls --disable-libssp :
(reconfigured) ../configure --prefix=/home/regehr/avrgcc440/install
--target=avr --enable-languages=c,c++ --disable-nls --disable-libssp
Thread model: single
gcc version 4.5.0 20090403 (experimental) (GCC) 
regehr@john-home:~$ avr-gcc -O1 -mmcu=atmega128 small.c -o small.elf 
regehr@john-home:~$ java avrora.Main -colors=false -monitors=break small.elf
Avrora [Beta 1.7.107] - (c) 2003-2007 UCLA Compilers Group

Loading small.elf...[OK: 0.060 seconds]
=={ Simulation events }=======================================================
Node          Time   Event
------------------------------------------------------------------------------
   0           178  break instruction @ 0x0130, r30:r31 = 0x0001
==============================================================================
Simulated time: 179 cycles
Time for simulation: 0.061 seconds
Total throughput: 0.0029344263 mhz
regehr@john-home:~$ avr-gcc -O2 -mmcu=atmega128 small.c -o small.elf 
regehr@john-home:~$ java avrora.Main -colors=false -monitors=break small.elf
Avrora [Beta 1.7.107] - (c) 2003-2007 UCLA Compilers Group

Loading small.elf...[OK: 0.057 seconds]
=={ Simulation events }=======================================================
Node          Time   Event
------------------------------------------------------------------------------
   0           166  break instruction @ 0x0122, r30:r31 = 0x0C00
==============================================================================
Simulated time: 167 cycles
Time for simulation: 0.061 seconds
Total throughput: 0.002737705 mhz
regehr@john-home:~$ cat small.c
#include <stdint.h>
#include <limits.h>

#define safe_mod_macro_uint8_t_u_u(ui1,ui2) \
        ((((uint8_t)(ui2)) == ((uint8_t)0)) \
                        ? ((uint8_t)(ui1)) \
                        : (((uint8_t)(ui1)) % ((uint8_t)(ui2))))

static uint8_t
safe_mod_func_uint8_t_u_u (uint8_t _ui1, uint8_t _ui2)
{
  return safe_mod_macro_uint8_t_u_u(_ui1,_ui2);
}

#define safe_lshift_macro_uint16_t_u_u(left,right) \
         (((((unsigned int)(right)) >= sizeof(uint16_t)*CHAR_BIT) \
                         || (((uint16_t)(left)) > ((UINT16_MAX) >> ((unsigned
int)(right))))) \
                        ? ((uint16_t)(left)) \
                        : (((uint16_t)(left)) << ((unsigned int)(right))))

static uint16_t
safe_lshift_func_uint16_t_u_u(uint16_t _left, unsigned int _right)
{
  return safe_lshift_macro_uint16_t_u_u(_left,_right);
}

int8_t func_7 (uint8_t p_8);
int8_t func_7 (uint8_t p_8)
{
  if (safe_mod_func_uint8_t_u_u (0xC, safe_lshift_func_uint16_t_u_u (p_8,
p_8)))
    {
      return 0;
    }
  return 1;
}

static inline void platform_main_end(int8_t crc);
static inline void platform_main_end(int8_t crc)
{
  asm volatile("cli" "\n\t"
               "mov r30, %A0" "\n\t"
               "mov r31, %B0" "\n\t"
               "break"
               :
               : "r" (crc)
               : "memory"
               );
}

int main (void)
{
  platform_main_end (func_7(1));
  return 0;
}


-- 
           Summary: integer wrong code bug on AVR
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: regehr at cs dot utah dot edu
 GCC build triplet: i486-linux-gnu
  GCC host triplet: i486-linux-gnu
GCC target triplet: avr


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39635

[Bug c/39635] integer wrong code bug on AVR

[regehr at cs dot utah dot edu]

------- Comment #1 from regehr at cs dot utah dot edu  2009-04-04 05:44 -------
To be clear: func_7() should return 1, but does not at -O2 and -Os.

In the Avrora output the result of the function looks like this: 

  r30:r31 = 0x0001

or 

  r30:r31 = 0x0C00


-- 

regehr at cs dot utah dot edu changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|integer wrong code bug on   |integer wrong code bug on
                   |AVR                         |AVR


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39635

[Bug c/39635] [avr] integer wrong code bug

[wvangulik at xs4all dot nl]

------- Comment #2 from wvangulik at xs4all dot nl  2009-04-07 09:51 -------
The problem is it uses R22 as loop counter and as the shift counter. I could
not (yet) reproduce the problem in a smaller example. 

This could be the same problem as:
http://lists.gnu.org/archive/html/avr-gcc-list/2009-03/msg00203.html
or 
http://lists.gnu.org/archive/html/avr-gcc-list/2009-03/msg00158.html

 ; (insn 16 15 17 main.c:24 (set (reg/v:HI 22 r22 [orig:42 _left ] [42])
 ;         (ashift:HI (reg/v:HI 22 r22 [orig:42 _left ] [42])
 ;             (reg:QI 22 r22 [orig:54 _right.2 ] [54]))) 54 {ashlhi3}
(expr_list:REG_DEAD (reg:QI 18 r18 [orig:54 _right.2 ] [54])
 ;         (expr_list:REG_UNUSED (reg:QI 23 r23)
 ;             (nil))))
        rjmp 2f  ;  16  ashlhi3/1       [length = 6]
1:      lsl r22
        rol r23
2:      dec r22
        brpl 1b

An other interresting point is the rtl stating reg 23 is unused. Unfortunatly
the avr backend can not remove the useless shift. But that is another issue.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39635

[Bug c/39635] [avr] integer wrong code bug

[wvangulik at xs4all dot nl]

------- Comment #3 from wvangulik at xs4all dot nl  2009-04-07 13:26 -------
(In reply to comment #2)
> The problem is it uses R22 as loop counter and as the shift counter. I could
> not (yet) reproduce the problem in a smaller example. 

This is my minimal test case:

static uint16_t
safe_lshift_func_uint16_t_u_u(uint16_t _left, unsigned int _right)
{
  return _left << _right;
}

char func_7 (uint8_t p_8)
{
  if(safe_lshift_func_uint16_t_u_u (p_8, p_8))
    {
      return 0;
    }
  return 1;
}

Not using the function but shifting it directly solves the problem in this
case. It seems to me this problem is triggered when the function is inlined.

Note that this also fails for 4.1.2, 4.2.2, 4.3.0 and 4.3.2. (WinAvr20070525,
20071221, 20080610, 20081205)

> This could be the same problem as:
> http://lists.gnu.org/archive/html/avr-gcc-list/2009-03/msg00203.html
> or 
> http://lists.gnu.org/archive/html/avr-gcc-list/2009-03/msg00158.html
> 
>  ; (insn 16 15 17 main.c:24 (set (reg/v:HI 22 r22 [orig:42 _left ] [42])
>  ;         (ashift:HI (reg/v:HI 22 r22 [orig:42 _left ] [42])
>  ;             (reg:QI 22 r22 [orig:54 _right.2 ] [54]))) 54 {ashlhi3}
> (expr_list:REG_DEAD (reg:QI 18 r18 [orig:54 _right.2 ] [54])
>  ;         (expr_list:REG_UNUSED (reg:QI 23 r23)
>  ;             (nil))))
>         rjmp 2f  ;  16  ashlhi3/1       [length = 6]
> 1:      lsl r22
>         rol r23
> 2:      dec r22
>         brpl 1b
> 
> An other interresting point is the rtl stating reg 23 is unused. Unfortunatly
> the avr backend can not remove the useless shift. But that is another issue.
> 


-- 

wvangulik at xs4all dot nl changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |wvangulik at xs4all dot nl


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39635

[Bug target/39635] [avr] integer wrong code bug

[eric dot weddington at atmel dot com]

------- Comment #4 from eric dot weddington at atmel dot com  2009-04-07 15:18 -------
This looks like a duplicate of bug #39386.

*** This bug has been marked as a duplicate of 39386 ***


-- 

eric dot weddington at atmel dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
          Component|c                           |target
         Resolution|                            |DUPLICATE


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39635

[Bug target/39635] [avr] integer wrong code bug

[hutchinsonandy at aim dot com]

------- Comment #5 from hutchinsonandy at aim dot com  2009-09-13 16:14 -------
It looks like most of AVR shift/rotates are messed up.
For the case we where we have non constant shifts, the peephole may grab a
scratch register. In this case it look like it grabs one that is free
afterwards and not before. Hence overlap issue

The rotate split pattern problem is different as noted in message links. .  In
this case it is not apparent why the split is used for only different
source/destination registers. If the split were constrained so that src=dest
the overlap would be much easily to handle and it would seemingly produce
better code for the common case where src=dest.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39635