GCC Bugzilla Bug 34916

This test case fails with latest gcc 4.3.0. The test assumes 32 bit integers,
but even after changing the constants to long, it produces incorrect results.

With -O2 we get this:

int f(unsigned number_of_digits_to_use)
{
 if (number_of_digits_to_use > 1294)
206:        65 e0               ldi        r22, 0x05        ; 5
208:        8f 30               cpi        r24, 0x0F        ; 15
20a:        96 07               cpc        r25, r22
20c:        c0 f4               brcc        .+48             ; 0x23e <f+0x38>
   return 0;
 return ((number_of_digits_to_use * 3321928L) / 1000000L + 1) /16;
20e:        bc 01               movw        r22, r24
210:        80 e0               ldi        r24, 0x00        ; 0
212:        90 e0               ldi        r25, 0x00        ; 0
214:        0e 94 6c 06         call        0xcd8        ; 0xcd8 <__mulsi3>
[...]

It "forgets" to load r18:r19:r20:r21 with 3321928 before calling __mulsi3.

Created an attachment (id=14991) [edit]
Combine pass RTL dump file

Created an attachment (id=14992) [edit]
dce pass RTL dump file (bfore combine)

Posted two RTL dump file of smaller testcase:


long f2(long number_of_digits_to_use)
{

  return ( number_of_digits_to_use * 11L ) ;
}

This faile GCC head 4.3.0 of 13/12/2007

Combine decide load of constant is not needed. Seem to happen after combine
tries combining load with multiply (ie multiply by constant). 

Assembler of short testcase. Constant load (11L) missing

  16                    .Ltext0:
  17                    .global f2
  19                    f2:
  20                    .LFB2:
  21                    .LM1:
  22                    .LVL0:
  23                    /* prologue: function */
  24                    /* frame size = 0 */
  25                    .LM2:
  26 0000 0E94 0000             call __mulsi3
  27                    .LVL1:
  28                    /* epilogue start */
  29                    .LM3:
  30 0004 0895                  ret
  31                    .LFE2:
  57                    .Letext0:
DEFINED SYMBOLS

The WRONG CODE is still present on 4.3.0 20080121  HEAD. 
This is a regresssion from 4.2 (A big one too!)

4.2.2 20071221 (Winavr) OK
4.3.0 20071213 FAILS
4.3.0 HEAD 20080121  FAILS

vr-gcc -c -mmcu=atmega128  -g -w  -Q -O2 -DSTACK_SIZE=400 -da  -DNO_TRAMPOLINES
-fno-show-column  -DSIGNAL_SUPPRESS -lm -Wa,-adhlns=xpr27364.lst -lm 
-std=gnu99 xpr27364.c -o xpr27364.o

I just found out another data point: this happens when you pass
-mmcu=atmega128, but not if you pass -mmcu=attiny13 (for instance).

Regressions should have a target milestone

Update milestone after 4.3.0 release.

This appear to be same bug where  combine is erroneously assuming all DF
register references are to different instructions. So it tries combining
instructions with themselves and stuff gets lost.

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35519

The above testcase still fails with gcc version 4.4.0 20080305.

However, with patch from PR35519 it produces correct code:

  23                    /* prologue: function */
  24                    /* frame size = 0 */
  25                    .LM2:
  26 0000 2BE0                  ldi r18,lo8(11)
  27 0002 30E0                  ldi r19,hi8(11)
  28 0004 40E0                  ldi r20,hlo8(11)
  29 0006 50E0                  ldi r21,hhi8(11)
  30 0008 0E94 0000             call __mulsi3
  31                    .LVL1:
  32                    /* epilogue start */
  33                    .LM3:
  34 000c 0895                  ret

Subject: Bug 34916

Author: hutchinsonandy
Date: Fri Apr  4 23:45:46 2008
New Revision: 133920

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=133920
Log:
PR rtl-optimization/34916
PR middle-end/35519
* combine.c (create_log_links): Do not create duplicate LOG_LINKS
between instruction pairs

Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/combine.c

Andy, since this was a 4.3 regression is there any way we can back port this
and commit it on the 4.3 branch?

Subject: Re:  [4.3/4.4 Regression] gcc.c-torture/execute/pr27364.c
 fails with -O1, -O2 and -Os

I believe the rules allow for this after a suitable grace period.

Remind me towards end of week and I will post for approval.

Andy



-----Original Message-----
From: eric dot weddington at atmel dot com <gcc-bugzilla@gcc.gnu.org>
To: hutchinsonandy@aim.com
Sent: Tue, 8 Apr 2008 11:16 am
Subject: [Bug target/34916] [4.3/4.4 Regression] 
gcc.c-torture/execute/pr27364.c fails with -O1, -O2 and -Os




------- Comment #10 from eric dot weddington at atmel dot com  
2008-04-08 15:16
-------
Andy, since this was a 4.3 regression is there any way we can back port 
this
and commit it on the 4.3 branch?


--


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=34916

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Subject: Bug 34916

Author: hutchinsonandy
Date: Wed Apr  9 22:50:42 2008
New Revision: 134152

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=134152
Log:
2008-04-09 Andy Hutchinson <hutchinsonandy@aim.com>

        PR rtl-optimization/34916
        PR middle-end/35519
        * combine.c (create_log_links): Do not create duplicate LOG_LINKS
        between instruction pairs

Modified:
    branches/gcc-4_3-branch/gcc/ChangeLog
    branches/gcc-4_3-branch/gcc/combine.c

Fixed 4.3 and 4.4