[Bug c++/22167] New: Strange optimization bug when using -Os

[Bug c++/22167] New: Strange optimization bug when using -Os

[dahowell at directv dot com]

The attached file osbugs.cpp will print a "BUG!!!!" message when compiled with 
the -Os switch with g++ 3.4.4 on powerpc.  This is due to the optimizer 
generating incorrect code, by apparently incorrectly determining that S::p is 
not being changed by the method S::init() before S::~S() is called.

I found this bug originally in the 3.3.5 version of the compiler, and it was 
causing a strange crash down in the basic_string code, but I've been able 
strip the code down to what I think is about the minimum needed to reproduce 
the optimization failure.  No pre-processor directives are in the attached 
source file.

The bug has been reproduced in mips-linux 3.3.5, and i386-linux 3.3.2 and 
3.3.5.  The bug does not appear in i386-linux 3.4.4.

Sample output, showing that this bug occurs across several different 
distributions of GCC:

powerpc, 3.4.4:

$ g++-3.4 --version | head -n1
g++-3.4 (GCC) 3.4.4 20050314 (prerelease) (Debian 3.4.3-13)
$ g++-3.4 -O2 osbugs.cpp -o osbugs
$ ./osbugs
p is 0x10011050 and p2 is 0x10011050 (should be non-zero and equal)
$ g++-3.4 -Os osbugs.cpp -o osbugs
$ ./osbugs
p is 0x0 and p2 is 0x0 (should be non-zero and equal)
BUG!!!! p is 0x10011050 and p2 is 0x0

i386, 3.3.5:

$ g++ --version | head -n1
g++ (GCC) 3.3.5 (Debian 1:3.3.5-13)
$ g++ -O2 osbugs.cpp -o osbugs
$ ./osbugs
p is 0x804a008 and p2 is 0x804a008 (should be non-zero and equal)
$ g++ -Os osbugs.cpp -o osbugs
$ ./osbugs
p is 0x0 and p2 is 0x0 (should be non-zero and equal)
BUG!!!! p is 0x804a008 and p2 is 0x0

i386, 3.3.2:

$ g++ --version | head -n1
g++ (GCC) 3.3.2 (Red Hat Linux 3.3.2-1)
$ g++ -O2 osbugs.cpp -o osbugs
$ ./osbugs
p is 0x8f0f008 and p2 is 0x8f0f008 (should be non-zero and equal)
$ g++ -Os osbugs.cpp -o osbugs
$ ./osbugs
p is 0x0 and p2 is 0x0 (should be non-zero and equal)
BUG!!!! p is 0x8497008 and p2 is 0x0

mips, 3.3.5:

$ mips-linux-g++ --version | head -n1
mips-linux-g++ (GCC) 3.3.5
$ mips-linux-g++ -O2 osbugs.cpp -o osbugs

[mipsbox]$ ./osbugs
p is 0x10000240 and p2 is 0x10000240 (should be non-zero and equal)

$ mips-linux-g++ -Os osbugs.cpp -o osbugs

[mipsbox]$ ./osbugs
p is 0x10000240 and p2 is 0x0 (should be non-zero and equal)
BUG!!!! p is 0x10000240 and p2 is 0x0

-- 
           Summary: Strange optimization bug when using -Os
           Product: gcc
           Version: 3.4.4
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: dahowell at directv dot com
                CC: gcc-bugs at gcc dot gnu dot org
 GCC build triplet: powerpc-linux
  GCC host triplet: powerpc-linux
GCC target triplet: powerpc-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22167

[Bug c++/22167] Strange optimization bug when using -Os

[dahowell at directv dot com]

------- Additional Comments From dahowell at directv dot com  2005-06-23 23:48 -------
Created an attachment (id=9138)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=9138&action=view)
Short test file which demonstrates optimization failuer

This is pretty much what was left of basic_string.h after I removed all the
code which was not needed to cause the bug to happen.  For the curious, S was
derived from the basic_string template, A was derived from the allocator
object, and p used to be the _M_rep pointer to the basic_string::_Rep
structure.  S::init was once basic_string::append.

The function do_nothing was once the allocator's dispose method, but as you can
see it now truly should do nothing.  But it doesn't quite do nothing, it causes
the optimizer to fail when optimizing for size.  The seemingly pointless
static_cast<A> also is needed to cause the bug.  Inlining either S::init or
do_nothing will also make the bug magically disappear.

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22167

[Bug c++/22167] Strange optimization bug when using -Os

[dahowell at directv dot com]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
   Attachment #9138|Short test file which       |Short test file which
        description|demonstrates optimization   |demonstrates optimization
                   |failuer                     |failure


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22167

[Bug rtl-optimization/22167] Strange optimization bug when using -Os

[pinskia at gcc dot gnu dot org]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|c++                         |rtl-optimization
           Keywords|                            |wrong-code


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22167

[Bug rtl-optimization/22167] [3.4/4.0?/4.1? regression] Strange optimization bug when using -Os

[reichelt at gcc dot gnu dot org]

------- Additional Comments From reichelt at gcc dot gnu dot org  2005-06-29 14:40 -------
I can confirm the bug on mips-sgi-irix6.5 (GCC 3.3.x, 3.4.x).
I can't test whether the bug is also present on gcc 4.0.0 and later.

The same bug also appears on i686-pc-linux-gnu, but only with GCC 3.3.x.


-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|                            |1
      Known to fail|                            |3.3 3.4.4
      Known to work|                            |3.2.3
   Last reconfirmed|0000-00-00 00:00:00         |2005-06-29 14:40:54
               date|                            |
            Summary|Strange optimization bug    |[3.4/4.0?/4.1? regression]
                   |when using -Os              |Strange optimization bug
                   |                            |when using -Os
   Target Milestone|---                         |3.4.5


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22167

[Bug rtl-optimization/22167] [3.4 regression] Strange optimization bug when using -Os

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-06-29 15:14 -------
It works for me on powerpc-darwin with 4.0.0, 4.1.0 but does not work with 3.3.3 and 3.4.0, so 
removing the 4.0, 4.1 regression markers.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
      Known to work|3.2.3                       |3.2.3 4.0.0 4.1.0
            Summary|[3.4/4.0?/4.1? regression]  |[3.4 regression] Strange
                   |Strange optimization bug    |optimization bug when using
                   |when using -Os              |-Os


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22167

[Bug rtl-optimization/22167] [3.4 regression] Strange optimization bug when using -Os

[rsandifo at gcc dot gnu dot org]

------- Additional Comments From rsandifo at gcc dot gnu dot org  2005-07-14 13:49 -------
I think this is a bug in gcse.c:hoist_code.  It appears to date back
to when the code was first added.

The function is supposed to be doing the following:

  for each block BB
    for each expr E
      HOISTABLE := 0
A:    if (E is very busy at the end of BB
          && E is transparent at the end of BB)
        for each block BB' that is dominated by BB
          if (BB' uses E
              && a computation of E at the end of BB would reach that use)
            HOISTABLE := HOISTABLE + 1
      HOIST_EXPRS[E] := HOISTABLE > 1
    if there exists an E such that HOIST_EXPRS[E] > 0
      for each expr E
B:      if HOIST_EXPRS[E]
	  for each block BB' that is dominated by BB
	    if (BB' uses E
	        && a computation of E at the end of BB would reach that use)
	      replace the use with the reaching reg

Unfortunately, the implementation of condition B is checking the wrong
bitmap (hoist_vbeout instead of hoist_exprs).  In other words, the code
is effectively doing:

B:      if E is very busy at the end of BB

Thus hoist_code() would do nothing for a function that has no candidate
expressions with two or more suitable uses.  However, if a function
_does_ have such an expression, hoist_code() would hoist all candidate
expressions, even if they have only one use.  And it would effectively
drop the all-important "E is transparent at the end of BB" condition
when doing this.

So what happens on MIPS (and presumably powerpc) is this: we have two
candidate expressions that are of interest:

  1. (mem s.p)
  2. (high "p is 0x%x and p2 ...")

Both expressions are very busy on the output from block 0, which ends
with the call to S::init().  And both expressions have more than one
suitable use.  However, expression 1 is not entered in HOIST_EXPRS
because it is not transparent at the end of block 0.  We'd have to
insert the load _before_ the call to S::init(), and that would change
its value.  Expression 2 _is_ transparent at the end of block 0 and
so HOIST_EXPRS[2] is correctly set to true.

So we have one expression that we want to hoist -- expression 2 -- and
we therefore enter the second loop.  This then ignores the fact that
expression 1 is not transparent at the end of block 0 and hoists it
anyway.

Although I haven't checked, I think the reason this doesn't show up
on i686 is because there's no equivalent of (high ...) that would
cause the second loop to be executed.

Richard


-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
         AssignedTo|unassigned at gcc dot gnu   |rsandifo at gcc dot gnu dot
                   |dot org                     |org
             Status|NEW                         |ASSIGNED
   Last reconfirmed|2005-06-29 14:40:54         |2005-07-14 13:49:01
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22167

[Bug rtl-optimization/22167] [3.4, 4.1 regression] Strange optimization bug when using -Os

[rsandifo at gcc dot gnu dot org]

------- Additional Comments From rsandifo at gcc dot gnu dot org  2005-07-14 17:55 -------
The reduced testcase that I've just attached fails for 4.1 as
well as 3.4 on mips64-elf.  I haven't yet tried 4.0.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
      Known to fail|3.3 3.4.4                   |3.3 3.4.4 4.1.0
      Known to work|3.2.3 4.0.0 4.1.0           |3.2.3 4.0.0
            Summary|[3.4 regression] Strange    |[3.4, 4.1 regression]
                   |optimization bug when using |Strange optimization bug
                   |-Os                         |when using -Os


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22167

[Bug rtl-optimization/22167] [3.4/4.0/4.1 regression] Strange optimization bug when using -Os

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-07-14 18:57 -------
The reduced testcase also fails on ppc-darwin with 4.0.0.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
      Known to fail|3.3 3.4.4 4.1.0             |3.3 3.4.4 4.1.0 4.0.0
      Known to work|3.2.3 4.0.0                 |3.2.3
            Summary|[3.4/4.1 regression] Strange|[3.4/4.0/4.1 regression]
                   |optimization bug when using |Strange optimization bug
                   |-Os                         |when using -Os


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22167

[Bug rtl-optimization/22167] [3.4/4.0/4.1 regression] Strange optimization bug when using -Os

[cvs-commit at gcc dot gnu dot org]

------- Additional Comments From cvs-commit at gcc dot gnu dot org  2005-07-21 06:51 -------
Subject: Bug 22167

CVSROOT:	/cvs/gcc
Module name:	gcc
Changes by:	rsandifo@gcc.gnu.org	2005-07-21 06:51:26

Modified files:
	gcc            : ChangeLog gcse.c 
	gcc/testsuite  : ChangeLog 
Added files:
	gcc/testsuite/g++.dg/opt: pr22167.C 

Log message:
	PR rtl-optimization/22167
	* gcse.c (hoist_code): Fix hoist_exprs[] check.

Patches:
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/ChangeLog.diff?cvsroot=gcc&r1=2.9495&r2=2.9496
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/gcse.c.diff?cvsroot=gcc&r1=1.346&r2=1.347
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/testsuite/ChangeLog.diff?cvsroot=gcc&r1=1.5790&r2=1.5791
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/testsuite/g++.dg/opt/pr22167.C.diff?cvsroot=gcc&r1=NONE&r2=1.1



-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22167

[Bug rtl-optimization/22167] [3.4/4.0/4.1 regression] Strange optimization bug when using -Os

[cvs-commit at gcc dot gnu dot org]

------- Additional Comments From cvs-commit at gcc dot gnu dot org  2005-07-21 06:53 -------
Subject: Bug 22167

CVSROOT:	/cvs/gcc
Module name:	gcc
Branch: 	gcc-4_0-branch
Changes by:	rsandifo@gcc.gnu.org	2005-07-21 06:53:29

Modified files:
	gcc            : ChangeLog gcse.c 
	gcc/testsuite  : ChangeLog 
Added files:
	gcc/testsuite/g++.dg/opt: pr22167.C 

Log message:
	PR rtl-optimization/22167
	* gcse.c (hoist_code): Fix hoist_exprs[] check.

Patches:
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/ChangeLog.diff?cvsroot=gcc&only_with_tag=gcc-4_0-branch&r1=2.7592.2.324&r2=2.7592.2.325
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/gcse.c.diff?cvsroot=gcc&only_with_tag=gcc-4_0-branch&r1=1.335&r2=1.335.2.1
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/testsuite/ChangeLog.diff?cvsroot=gcc&only_with_tag=gcc-4_0-branch&r1=1.5084.2.290&r2=1.5084.2.291
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/testsuite/g++.dg/opt/pr22167.C.diff?cvsroot=gcc&only_with_tag=gcc-4_0-branch&r1=NONE&r2=1.1.2.1



-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22167

[Bug rtl-optimization/22167] [3.4/4.0/4.1 regression] Strange optimization bug when using -Os

[cvs-commit at gcc dot gnu dot org]

------- Additional Comments From cvs-commit at gcc dot gnu dot org  2005-07-21 06:56 -------
Subject: Bug 22167

CVSROOT:	/cvs/gcc
Module name:	gcc
Branch: 	gcc-3_4-branch
Changes by:	rsandifo@gcc.gnu.org	2005-07-21 06:56:24

Modified files:
	gcc            : ChangeLog gcse.c 
	gcc/testsuite  : ChangeLog 
Added files:
	gcc/testsuite/g++.dg/opt: pr22167.C 

Log message:
	PR rtl-optimization/22167
	* gcse.c (hoist_code): Fix hoist_exprs[] check.

Patches:
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/ChangeLog.diff?cvsroot=gcc&only_with_tag=gcc-3_4-branch&r1=2.2326.2.882&r2=2.2326.2.883
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/gcse.c.diff?cvsroot=gcc&only_with_tag=gcc-3_4-branch&r1=1.288.2.9&r2=1.288.2.10
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/testsuite/ChangeLog.diff?cvsroot=gcc&only_with_tag=gcc-3_4-branch&r1=1.3389.2.406&r2=1.3389.2.407
http://gcc.gnu.org/cgi-bin/cvsweb.cgi/gcc/gcc/testsuite/g++.dg/opt/pr22167.C.diff?cvsroot=gcc&only_with_tag=gcc-3_4-branch&r1=NONE&r2=1.1.4.1



-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22167

[Bug rtl-optimization/22167] [3.4/4.0/4.1 regression] Strange optimization bug when using -Os

[rsandifo at gcc dot gnu dot org]

------- Additional Comments From rsandifo at gcc dot gnu dot org  2005-07-21 06:58 -------
Patch committed.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|                            |FIXED


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=22167