[Bug c++/36861] New: code generation regression with -O3

[Bug c++/36861] New: code generation regression with -O3

[lothar at tradescape dot biz]

gcc 4.3.1 generates code with -O3 that is more than 10 times slower than with
-O0. gcc 4.2.3 does not show this behavior. I am going to attach the test case
that I used to produce these numbers:

with gcc 4.2.3:
( time ./avltest-nn >/dev/null; time ./avltest-no >/dev/null; time ./avltest-on
>/dev/null; time ./avltest-oo >/dev/null; )

real    0m1.112s
user    0m1.028s
sys     0m0.012s

real    0m0.078s
user    0m0.060s
sys     0m0.008s

real    0m0.940s
user    0m0.916s
sys     0m0.008s

real    0m0.088s
user    0m0.072s
sys     0m0.004s


with gcc 4.3.1:
( time ./avltest-nn >/dev/null; time ./avltest-no >/dev/null; time ./avltest-on
>/dev/null; time ./avltest-oo >/dev/null; )

real    0m0.974s
user    0m0.948s
sys     0m0.004s

real    0m12.936s
user    0m12.893s
sys     0m0.016s

real    0m1.128s
user    0m1.012s
sys     0m0.000s

real    0m0.104s
user    0m0.076s
sys     0m0.004s

Here is the information about the compilers used:

$ /opt2/linux/ix86/bin/g++-4.2.3 -v
Using built-in specs.
Target: x86_64-pc-linux-gnu
Configured with: ../gcc-4.2.3/configure --program-suffix=-4.2.3
--enable-__cxa_atexit --enable-languages=c,c++,java --prefix=/opt2/linux/ix86
--target=x86_64-pc-linux-gnu --with-sysroot=/opt2/linux/ix86/gcc-sysroot
--enable-version-specific-runtime-libs --enable-clocale=gnu
Thread model: posix
gcc version 4.2.3

$ /opt2/linux/ix86/bin/g++-4.3.1 -v
Using built-in specs.
Target: x86_64-pc-linux-gnu
Configured with: ../gcc-4.3.1/configure --enable-targets=all --enable-multilib
--enable-__cxa_atexit --enable-languages=c,c++,java
--enable-version-specific-runtime-libs --disable-nls --enable-clocale=gnu
--program-suffix=-4.3.1 --prefix=/opt2/linux/ix86 --target=i686-pc-linux-gnu
--target=x86_64-pc-linux-gnu --with-sysroot=/opt2/linux/ix86/gcc-sysroot
Thread model: posix
gcc version 4.3.1 (GCC)


-- 
           Summary: code generation regression with -O3
           Product: gcc
           Version: 4.3.1
            Status: UNCONFIRMED
          Severity: major
          Priority: P3
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: lothar at tradescape dot biz
GCC target triplet: x86_64-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36861

[Bug c++/36861] code generation regression with -O3

[lothar at tradescape dot biz]

------- Comment #1 from lothar at tradescape dot biz  2008-07-17 07:28 -------
Created an attachment (id=15921)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=15921&action=view)
test case to reproduce the problem


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36861

[Bug c++/36861] code generation regression with -O3

[lothar at tradescape dot biz]

------- Comment #2 from lothar at tradescape dot biz  2008-07-17 16:54 -------
Created an attachment (id=15923)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=15923&action=view)
updated testcase

changed the test case to resemble my application a bit closer. Here it is
almost 40 times slower with -O3 than with -O0

The other strange thing with our app (that I can NOT recreate with a test case
so far) is that if I use -DUSE_OPTIMIZATION=1 in my app the -O3 compiled
version seems to go into an infinite loop.


real    0m1.586s
user    0m0.988s
sys     0m0.020s

real    0m40.963s
user    0m40.655s
sys     0m0.048s

real    0m2.457s
user    0m1.976s
sys     0m0.020s

real    0m0.291s
user    0m0.164s
sys     0m0.020s


-- 

lothar at tradescape dot biz changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
  Attachment #15921|0                           |1
        is obsolete|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36861

[Bug c++/36861] code generation regression with -O3

[rguenth at gcc dot gnu dot org]

------- Comment #3 from rguenth at gcc dot gnu dot org  2008-07-17 17:42 -------
Please provide preprocessed source for the testcase which you get when
appending -save-temps to the compilation command.  It will be called
avltest.ii.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rguenth at gcc dot gnu dot
                   |                            |org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36861

[Bug tree-optimization/36861] code generation regression with -O3

[pinskia at gcc dot gnu dot org]

------- Comment #4 from pinskia at gcc dot gnu dot org  2008-07-17 17:47 -------
Does -O3 -fno-tree-vectorize cause the speed back to normal?

Not everyone has boost installed :).


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|major                       |normal
          Component|c++                         |tree-optimization


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36861

[Bug tree-optimization/36861] code generation regression with -O3

[lothar at tradescape dot biz]

------- Comment #5 from lothar at tradescape dot biz  2008-07-17 17:50 -------
Created an attachment (id=15924)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=15924&action=view)
the preprocessed file for case that is slower


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36861

[Bug tree-optimization/36861] code generation regression with -O3

[lothar at tradescape dot biz]

------- Comment #6 from lothar at tradescape dot biz  2008-07-17 17:52 -------
in reply to #4:
-fno-tree-vectorize does not help

$ /opt2/linux/ix86/bin/g++-4.3.1 -m64 -O3 -fno-tree-vectorize -g -save-temps
-I/opt2/linux/x86_64/include/boost-1_35 -DUSE_OPTIMIZATION=0 -o ./avltest-no
./avltest.cpp
./avltest.cpp:115:2: warning: #warning without optimization
$ time ./avltest-no >/dev/null

real    0m40.528s
user    0m39.902s
sys     0m0.036s


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36861

[Bug tree-optimization/36861] code generation regression with -O3

[rguenth at gcc dot gnu dot org]

------- Comment #7 from rguenth at gcc dot gnu dot org  2008-07-17 18:01 -------
pointer_plus_2_bits will confuse points-to alias analysis galore.  Of course
this doesn't explain the difference to -O0.

Still, -O1 is fast, -O1 -fstrict-aliasing is _very_ slow.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |alias, missed-optimization


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36861

[Bug tree-optimization/36861] code generation regression with -O3

[lothar at tradescape dot biz]

[-- Warning: decoded text below may be mangled, UTF-8 assumed --]
[-- Attachment #1: Type: text/plain, Size: 315 bytes --]



------- Comment #8 from lothar at tradescape dot biz  2008-07-17 18:09 -------
If you have questions about the internal implementation of the boost avl tree
you can contact Ion Gaztañaga igaztanaga at gmail dot com. He is aware of this
problem already.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36861

[Bug tree-optimization/36861] [4.3 Regression] code generation regression with -O3

[rguenth at gcc dot gnu dot org]

-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|code generation regression  |[4.3 Regression] code
                   |with -O3                    |generation regression with -
                   |                            |O3
   Target Milestone|---                         |4.3.2


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36861

[Bug tree-optimization/36861] [4.3 Regression] code generation regression with -O3

[lothar at tradescape dot biz]

------- Comment #9 from lothar at tradescape dot biz  2008-07-17 20:15 -------
in reply to #7:

Does this mean the status is no longer UNCONFIRMED?


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36861

[Bug tree-optimization/36861] [4.3 Regression] code generation regression with -O3

[rguenth at gcc dot gnu dot org]

------- Comment #10 from rguenth at gcc dot gnu dot org  2008-07-17 20:19 -------
No, it's still not analyzed.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36861

[Bug tree-optimization/36861] boost's compressed avl confuses GCC

[pinskia at gcc dot gnu dot org]

------- Comment #11 from pinskia at gcc dot gnu dot org  2008-07-22 21:43 -------
I think someone needs to kick the hell out of boost for making crappy code.  I
am going to remove the regression marker for now because it is definitely a bit
weird what boost is doing and not normal for real code.


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|[4.3 Regression] code       |boost's compressed avl
                   |generation regression with -|confuses GCC
                   |O3                          |
   Target Milestone|4.3.2                       |---


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36861

[Bug tree-optimization/36861] boost's compressed avl confuses GCC

[lothar at tradescape dot biz]

------- Comment #13 from lothar at tradescape dot biz  2008-07-22 21:48 -------
(In reply to comment #11)
>From an application perspective it is still a regression, as it works happily
with gcc 4.2.3


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36861

[Bug tree-optimization/36861] boost's compressed avl confuses GCC

[pinskia at gcc dot gnu dot org]

------- Comment #14 from pinskia at gcc dot gnu dot org  2008-07-22 21:52 -------
(In reply to comment #13)
> (In reply to comment #11)
> From an application perspective it is still a regression, as it works happily
> with gcc 4.2.3

Yes but it is boost's fault that boost tries to be smart about space
optimizations when they should not have.  It really does confuse the hell out
of compilers, doing what they are doing.  C/C++ was not designed the way they
think it was.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36861

[Bug tree-optimization/36861] boost's compressed avl confuses GCC

[lothar at tradescape dot biz]

------- Comment #15 from lothar at tradescape dot biz  2008-07-22 22:01 -------
What I am worried about is that if it is not marked as a regression nobody
cares to fix it although it breaks a real application that works with gcc
4.3.2.

If you have any insights how to improve the boost intrusive library to avoid
confusing gcc I am sure the boost people are happy to hear them and to
incorporate them in the upcoming boost release.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36861

[Bug tree-optimization/36861] boost's compressed avl confuses GCC

[igaztanaga at gmail dot com]

------- Comment #17 from igaztanaga at gmail dot com  2008-07-23 08:03 -------
Andrew, I think I do write some crappy code, but the slowdown is also present
in the non-optimized version.

FWIW, there are STL implementations that embed the color bit in rbtrees in the
parent pointer so the optimization is not that strange. Maybe my implementation
is not correct and that's why the size-optimized version ends in an infinite
loop.

And we have tons of two-way pointers in production code. Too much crappy code
to ignore, I'm afraid.


-- 

igaztanaga at gmail dot com changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |igaztanaga at gmail dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36861

[Bug tree-optimization/36861] [4.3 Regression] boost's compressed avl confuses GCC

[drow at gcc dot gnu dot org]

------- Comment #18 from drow at gcc dot gnu dot org  2008-07-24 22:18 -------
Putting the regression marker back.  The code doesn't matter; if it's a
regression, then it's regression.


-- 

drow at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|boost's compressed avl      |[4.3 Regression] boost's
                   |confuses GCC                |compressed avl confuses GCC


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36861

[Bug tree-optimization/36861] [4.3 Regression] boost's compressed avl confuses GCC

[lothar at tradescape dot biz]

------- Comment #19 from lothar at tradescape dot biz  2008-07-28 18:55 -------
Created an attachment (id=15973)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=15973&action=view)
oprofile logs

I compiled optimized executables with and without -fno-strict-aliasing and
generated an oprofile log for them:

/opt2/linux/ix86/bin/g++-4.3.1 -m64 -O3 -no-strict-aliasing -g
-I/opt2/linux/x86_64/include/boost-1_35 -DUSE_OPTIMIZATION=0 -o ./avltest-non
./avltest.cpp
/opt2/linux/ix86/bin/g++-4.3.1 -m64 -O3 -fno-strict-aliasing -g
-I/opt2/linux/x86_64/include/boost-1_35 -DUSE_OPTIMIZATION=0 -o ./avltest-non
./avltest.cpp
sudo opcontrol --reset; sudo opcontrol --start; ./avltest-no; ./avltest-non ;
sudo opcontrol --stop
opannotate --source --assembly --demangle=smart --threshold=1 ./avltest-no
>./avltest-no.oprofile.txt
opannotate --source --assembly --demangle=smart --threshold=1 ./avltest-non
>./avltest-non.oprofile.txt
diff -u ./avltest-no.oprofile.txt ./avltest-non.oprofile.txt
./avltest-diff.oprofile.txt

$ time ./avltest-non
sizeof(Node)=72
sizeof(NodeSet)=40
creating file ./avltest.img done
region.get_address()=0x7f4d43ed1000
region.get_size()=3600056
p_set=0x7f4d43ed1000
p_values=0x7f4d43ed1030
created/inserted 50000 entries
region.get_address()=0x7f4d43b62000
region.get_size()=3600056
p_set=0x7f4d43b62000
p_values=0x7f4d43b62030
checked 50000 entries, result=1249975000

real    0m0.614s
user    0m0.140s
sys     0m0.016s

$ time ./avltest-no
sizeof(Node)=72
sizeof(NodeSet)=40
creating file ./avltest.img done
region.get_address()=0x7f7fdce43000
region.get_size()=3600056
p_set=0x7f7fdce43000
p_values=0x7f7fdce43030
created/inserted 50000 entries
region.get_address()=0x7f7fdcad4000
region.get_size()=3600056
p_set=0x7f7fdcad4000
p_values=0x7f7fdcad4030
checked 50000 entries, result=1249975000

real    0m22.634s
user    0m22.277s
sys     0m0.120s

I hope these tests aid in finding the problem


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36861

[Bug tree-optimization/36861] [4.3 Regression] boost's compressed avl confuses GCC

[lothar at tradescape dot biz]

------- Comment #20 from lothar at tradescape dot biz  2008-07-31 02:01 -------
With all the supplied test cases and data, why is it so hard to move the bug
out of the UNCONFIRMED state. It should be fairly easy to confirm that it is a
problem by running the attached test.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36861

[Bug tree-optimization/36861] [4.3 Regression] boost's compressed avl confuses GCC

[aldot at gcc dot gnu dot org]

------- Comment #21 from aldot at gcc dot gnu dot org  2008-07-31 07:56 -------
Lothar, see #10


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36861

[Bug tree-optimization/36861] [4.3 Regression] boost's compressed avl confuses GCC

[pinskia at gcc dot gnu dot org]

-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |pinskia at gcc dot gnu dot
                   |                            |org
   Target Milestone|---                         |4.3.3


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36861

[Bug tree-optimization/36861] [4.3/4.4 Regression] boost's compressed avl confuses GCC

[rguenth at gcc dot gnu dot org]

------- Comment #22 from rguenth at gcc dot gnu dot org  2008-09-20 14:25 -------
This PR is still very weird.  For the curious here are the numbers, now
with numbers from trunk included:

4.3 -O: 0.4s
4.3 -O -fstrict-aliasing: 59s
4.4 -O: 1m3s
4.4 -O -fstrict-aliasing: 0.4s
4.4 -O2: 0.3s
4.4 -O2 -fno-strict-aliasing: 0.2s

?!


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
      Known to fail|4.3.1                       |4.3.1 4.4.0
            Summary|[4.3 Regression] boost's    |[4.3/4.4 Regression] boost's
                   |compressed avl confuses GCC |compressed avl confuses GCC


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36861

[Bug tree-optimization/36861] [4.3/4.4 Regression] boost's compressed avl confuses GCC

[rguenth at gcc dot gnu dot org]

------- Comment #23 from rguenth at gcc dot gnu dot org  2008-09-20 14:27 -------
Created an attachment (id=16368)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=16368&action=view)
unincluded testcase

Unincluded testcase attached.

I'll leave this at P3 so some brave soul might eventually pick up the
analysis.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36861

[Bug target/36861] [4.3/4.4 Regression] boost's compressed avl confuses GCC

[lothar at tradescape dot biz]

------- Comment #25 from lothar at tradescape dot biz  2008-10-10 15:58 -------
I was running the code on my Lenovo T61 Laptop:

$ cat /proc/cpuinfo
processor       : 0
vendor_id       : GenuineIntel
cpu family      : 6
model           : 15
model name      : Intel(R) Core(TM)2 Duo CPU     T7700  @ 2.40GHz
stepping        : 10
cpu MHz         : 800.000
cache size      : 4096 KB
physical id     : 0
siblings        : 2
core id         : 0
cpu cores       : 2
fpu             : yes
fpu_exception   : yes
cpuid level     : 10
wp              : yes
flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov
pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx lm
constant_tsc arch_perfmon pebs bts rep_good pni monitor ds_cpl vmx est tm2
ssse3 cx16 xtpr lahf_lm ida
bogomips        : 4793.98
clflush size    : 64
cache_alignment : 64
address sizes   : 36 bits physical, 48 bits virtual
power management:

processor       : 1
vendor_id       : GenuineIntel
cpu family      : 6
model           : 15
model name      : Intel(R) Core(TM)2 Duo CPU     T7700  @ 2.40GHz
stepping        : 10
cpu MHz         : 800.000
cache size      : 4096 KB
physical id     : 0
siblings        : 2
core id         : 1
cpu cores       : 2
fpu             : yes
fpu_exception   : yes
cpuid level     : 10
wp              : yes
flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov
pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx lm
constant_tsc arch_perfmon pebs bts rep_good pni monitor ds_cpl vmx est tm2
ssse3 cx16 xtpr lahf_lm ida
bogomips        : 4787.49
clflush size    : 64
cache_alignment : 64
address sizes   : 36 bits physical, 48 bits virtual
power management:


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36861

[Bug target/36861] [4.3/4.4 Regression] boost's compressed avl confuses GCC

[mmitchel at gcc dot gnu dot org]

------- Comment #26 from mmitchel at gcc dot gnu dot org  2008-10-22 03:02 -------
Richard has reproduced the bad behavior.  So, this is not UNCONFIRMED.  It's an
optimization regression; hence P2.


-- 

mmitchel at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
           Priority|P3                          |P2
   Last reconfirmed|0000-00-00 00:00:00         |2008-10-22 03:02:05
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36861

[Bug target/36861] [4.3/4.4 Regression] boost's compressed avl confuses GCC

[rguenth at gcc dot gnu dot org]

------- Comment #27 from rguenth at gcc dot gnu dot org  2009-01-24 10:20 -------
GCC 4.3.3 is being released, adjusting target milestone.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|4.3.3                       |4.3.4


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36861

[Bug target/36861] [4.3/4.4 Regression] boost's compressed avl confuses GCC

[bonzini at gnu dot org]

------- Comment #28 from bonzini at gnu dot org  2009-02-03 09:54 -------
Am I right that the bug is just at -O, not at -O2?  4.4 -O2 is worse than 4.4
-O2 -fno-strict-aliasing, but are they better than 4.3 -O2 or worse?


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36861

[Bug target/36861] [4.3/4.4/4.5 Regression] boost's compressed avl confuses GCC

[rguenth at gcc dot gnu dot org]

------- Comment #29 from rguenth at gcc dot gnu dot org  2009-08-04 12:29 -------
GCC 4.3.4 is being released, adjusting target milestone.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|4.3.4                       |4.3.5


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36861

[Bug target/36861] [4.5 Regression] boost's compressed avl confuses GCC

[rguenth at gcc dot gnu dot org]

------- Comment #30 from rguenth at gcc dot gnu dot org  2010-03-16 13:15 -------
I do have updated numbers.

4.3.[012] -O3: 57s
4.3.[34] -O[123]: 0.4s
4.4.[0123] -O[123]: 0.4s
4.5 -O0: 1.6s
4.5 -O[123]: 50s (*sigh*)

that makes it fixed on the release branches and a 4.5 regression only.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
      Known to fail|4.3.1 4.4.0                 |4.3.2 4.5.0
      Known to work|4.2.3                       |4.2.3 4.3.3 4.3.4 4.4.0
                   |                            |4.4.3
            Summary|[4.3/4.4/4.5 Regression]    |[4.5 Regression] boost's
                   |boost's compressed avl      |compressed avl confuses GCC
                   |confuses GCC                |
   Target Milestone|4.3.5                       |4.5.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36861

[Bug target/36861] [4.5 Regression] boost's compressed avl confuses GCC

[rguenth at gcc dot gnu dot org]

------- Comment #31 from rguenth at gcc dot gnu dot org  2010-04-06 11:19 -------
GCC 4.5.0 is being released.  Deferring to 4.5.1.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|4.5.0                       |4.5.1


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36861

[Bug target/36861] [4.5/4.6 Regression] boost's compressed avl confuses GCC

[rguenth at gcc dot gnu dot org]

------- Comment #32 from rguenth at gcc dot gnu dot org  2010-07-05 15:40 -------
Re-confirmed for current 4.5 branch and trunk.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|2008-10-22 03:02:05         |2010-07-05 15:40:28
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36861

[Bug target/36861] [4.5/4.6 Regression] boost's compressed avl confuses GCC

[rguenth at gcc dot gnu dot org]

------- Comment #33 from rguenth at gcc dot gnu dot org  2010-07-05 16:03 -------
Using valgrind there are randomly reported errors, so this is likely either
an invalid testcase or a miscompile that manifests itself as runtime
regression.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|missed-optimization         |wrong-code


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36861

[Bug target/36861] [4.5/4.6 Regression] boost's compressed avl confuses GCC

[rguenth at gcc dot gnu dot org]

------- Comment #34 from rguenth at gcc dot gnu dot org  2010-07-31 09:29 -------
GCC 4.5.1 is being released, adjusting target milestone.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|4.5.1                       |4.5.2


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36861

[Bug target/36861] [4.5/4.6 Regression] boost's compressed avl confuses GCC

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36861

Richard Guenther <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Target Milestone|4.5.2                       |4.5.3

--- Comment #35 from Richard Guenther <rguenth at gcc dot gnu.org> 2010-12-16 13:02:44 UTC ---
GCC 4.5.2 is being released, adjusting target milestone.

[Bug target/36861] [4.5/4.6 Regression] boost's compressed avl confuses GCC

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36861

Richard Guenther <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
      Known to work|                            |
      Known to fail|                            |

--- Comment #36 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-01-18 14:43:50 UTC ---
Trunk is now very fast and segfaults the testcase ;)

[Bug target/36861] [4.5/4.6 Regression] boost's compressed avl confuses GCC

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36861

Richard Guenther <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |INVALID

--- Comment #37 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-01-18 15:19:05 UTC ---
Ok, with -O1 -fno-tree-loop-im as basic flags -fno-tree-dse makes the
difference for the segfault when we remove

 static void
boost::intrusive::avltree_algorithms<NodeTraits>::rebalance_after_i
nsertion(boost::intrusive::avltree_algorithms<NodeTraits>::node_ptr,
boost::intr
usive::avltree_algorithms<NodeTraits>::node_ptr) [with NodeTraits =
boost::intru
sive::avltree_node_traits<boost::interprocess::offset_ptr<void>, false>,
boost::
intrusive::avltree_algorithms<NodeTraits>::node_ptr =
boost::interprocess::offse
t_ptr<boost::intrusive::avltree_node<boost::interprocess::offset_ptr<void> > >] 
(struct node_ptr & restrict header, struct node_ptr & restrict x)
 {
   long int D.64530;
@@ -8290,7 +7992,6 @@
   p.19_2156 = (long int) D.64522_2154;
   D.64529_2157 = (long int) D.64513_2150;
   D.64530_2158 = p.19_2156 - D.64529_2157;
-  MEM[(struct offset_ptr *)D.64515_2149].m_offset = D.64530_2158;
   if (D.64530_2158 == 1)
     goto <bb 746>;
   else

this pointer is believed to point to

D.64515_2149, points-to NULL, points-to vars: { D.57715 CAST_RESTRICT.445 }
(includes restrict tags)

computed from

<bb 742>:
  n_2143 = (struct node_ptr & restrict) &D.57715;
  p_2144 = (struct node_ptr & restrict) &D.57716;
  D.64517_2145 = D.57715.m_offset;
  D.64516_2147 = (long unsigned int) D.64517_2145;
  D.64515_2148 = n_2143 + D.64516_2147;

<bb 743>:
  # D.64515_2149 = PHI <0B(739), D.64515_2148(742)>

which confirms the points-to calculation.

But the compressed AVL tree thinks it can represent a pointer to
a global object by using the address of a local variable and
the difference between the address of the global and the local.
Which it of course cannot according to the C++ standard.
D.57715.m_offset is set by

<bb 740>:
  p.19_2139 = (long int) D.64504_2137;
  D.64508_2140 = (long int) &D.57715;
  D.64509_2141 = p.19_2139 - D.64508_2140;
  D.57715.m_offset = D.64509_2141;
  if (D.64509_2141 == 1)
    goto <bb 741>;
  else
    goto <bb 742>;

and we have

<bb 737>:
boost::interprocess::offset_ptr<boost::intrusive::avltree_node<boost::interprocess::offset_ptr<void>
> >::offset_ptr (&D.57716, &root);
  D.64502_2134 = MEM[(const struct offset_ptr *)header_2(D)].m_offset;
  if (D.64502_2134 != 1)
    goto <bb 738>;
  else
    goto <bb 739>;

<bb 738>:
  D.64503_2136 = (long unsigned int) D.64502_2134;
  D.64504_2137 = header_2(D) + D.64503_2136;

where header_2(D) is a parameter, struct node_ptr & restrict header.  So
Boost AVL expects us to compute whatever header points-to as points-to
set of D.64515_2149 with no backing from the standard that we are required
to do that.

If we were able to "optimize" the code back to base the address on
header it would of course work (by luck).

For

extern void abort (void);
struct X { unsigned long offset; };
void foo (char *p)
{
  struct X a;
  char tmp, *q;
  a.offset = p - &tmp;
  if (a.offset == 1)
    abort ();
  q = &tmp + a.offset;
  *q = 0;
}

already early DCE kills the store (the above is what compressed AVL does).

Thus, this testcase is invalid.  Resulting performance is irrelevant.

[Bug target/36861] [4.5/4.6 Regression] boost's compressed avl confuses GCC

[igaztanaga at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36861

--- Comment #38 from Ion Gaztañaga <igaztanaga at gmail dot com> 2011-01-18 19:15:22 UTC ---
Thanks Richard for the detailed report. The fact is that the next standard is
trying to support relative pointers for container implementations (much like
Boost.Interprocess does), to be able to place containers in shared memory. In
those cases the relative pointers stores the difference between "this" and the
pointee, which is what boost::interprocess::offset_ptr does.

It is certainly outside the standard right now and the standard won't include
such "relative pointer" class, but it will require to container implementations
to support them as allocator::pointer types. It would be important to find a
way to use smart pointers, maybe with some existing annotation/attribute to
avoid optimizing away that relative addressing, so that there is always away to
compute the pointee address based on "this" + this->m_offset. It is also
important to support smart pointers created on the stack pointing to global
objects (temporaries, function arguments, etc.)

I know this is outside the bug, but it will be certainly a common question when
using GCC and next standard containers. Thanks

[Bug target/36861] [4.5/4.6 Regression] boost's compressed avl confuses GCC

[rguenther at suse dot de]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36861

--- Comment #39 from rguenther at suse dot de <rguenther at suse dot de> 2011-01-19 09:54:19 UTC ---
On Tue, 18 Jan 2011, igaztanaga at gmail dot com wrote:

> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36861
> 
> --- Comment #38 from Ion Gaztañaga <igaztanaga at gmail dot com> 2011-01-18 19:15:22 UTC ---
> Thanks Richard for the detailed report. The fact is that the next standard is
> trying to support relative pointers for container implementations (much like
> Boost.Interprocess does), to be able to place containers in shared memory. In
> those cases the relative pointers stores the difference between "this" and the
> pointee, which is what boost::interprocess::offset_ptr does.

But then the pointer arithmetic will not cause the pointer to move
from one object to another - the object will be simply the shared memory
segment (which isn't statically allocated either) - that's perfectly
valid in C and C++ right now.

> It is certainly outside the standard right now and the standard won't include
> such "relative pointer" class, but it will require to container implementations
> to support them as allocator::pointer types. It would be important to find a
> way to use smart pointers, maybe with some existing annotation/attribute to
> avoid optimizing away that relative addressing, so that there is always away to
> compute the pointee address based on "this" + this->m_offset. It is also
> important to support smart pointers created on the stack pointing to global
> objects (temporaries, function arguments, etc.)
> 
> I know this is outside the bug, but it will be certainly a common question when
> using GCC and next standard containers. Thanks

The point with the case in question is that a pointer to a global
object is computed relative to the address _of an automatic variable_.
I hope no standard will ever allow that ;)  It might be a simple
oversight (and thus bug) in Boost AVL though.

If the object were a global one then you wouldn't trigger the particular
bug (though still GCC would conclude that the pointer is pointing to
that global object, which, when in statically allocated storage, can
still lead to "miscompilations").

Richard.

[Bug target/36861] [4.5/4.6 Regression] boost's compressed avl confuses GCC

[igaztanaga at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36861

--- Comment #40 from Ion Gaztañaga <igaztanaga at gmail dot com> 2011-01-19 16:07:01 UTC ---
(In reply to comment #39)
> The point with the case in question is that a pointer to a global
> object is computed relative to the address _of an automatic variable_.
> I hope no standard will ever allow that ;)  It might be a simple
> oversight (and thus bug) in Boost AVL though.

Is not an oversight, it's a basic feature to build easy relative addressing to
be used in shared memory and memory mapped files shared between several
processes. A pointer only stores the distance between the "smart pointer" and
the pointee, so it can be mapped in different addresses in several processes.
Support for automatic storage smart pointers is needed just to support
iterators that could be stored in shared memory (thus, storing smart pointers
in shared memory) and also be used by programmers in a loop (automatic
storage).

Until now, this illegal scheme was working in all known compilers (although
outside the standard, I agree). Since the standard does not support shared
memory, if someday shared memory support is added to the standard, it will
surely require some changes to support this widely used relative addressing.
Until then, I agree, this bug report is invalid.

The only issue I would like to know is if this relative addressing scheme for
automatic variables can be used with GCC with some kind of existing annotation,
anything that could force that automatic variable to have an address that could
be used to point to shared memory elements.

Ion

[Bug target/36861] [4.5/4.6 Regression] boost's compressed avl confuses GCC

[rguenther at suse dot de]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36861

--- Comment #41 from rguenther at suse dot de <rguenther at suse dot de> 2011-01-19 16:40:15 UTC ---
On Wed, 19 Jan 2011, igaztanaga at gmail dot com wrote:

> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36861
> 
> --- Comment #40 from Ion Gaztañaga <igaztanaga at gmail dot com> 2011-01-19 16:07:01 UTC ---
> (In reply to comment #39)
> > The point with the case in question is that a pointer to a global
> > object is computed relative to the address _of an automatic variable_.
> > I hope no standard will ever allow that ;)  It might be a simple
> > oversight (and thus bug) in Boost AVL though.
> 
> Is not an oversight, it's a basic feature to build easy relative addressing to
> be used in shared memory and memory mapped files shared between several
> processes. A pointer only stores the distance between the "smart pointer" and
> the pointee, so it can be mapped in different addresses in several processes.
> Support for automatic storage smart pointers is needed just to support
> iterators that could be stored in shared memory (thus, storing smart pointers
> in shared memory) and also be used by programmers in a loop (automatic
> storage).

The case in question seems to use random automatic storage to represent
a pointer to unrelated storage.  That is never going to work.  And if
it will ever be required to work all points-to analysis will just
be useless which is not something you want.

> Until now, this illegal scheme was working in all known compilers (although
> outside the standard, I agree). Since the standard does not support shared
> memory, if someday shared memory support is added to the standard, it will
> surely require some changes to support this widely used relative addressing.
> Until then, I agree, this bug report is invalid.
> 
> The only issue I would like to know is if this relative addressing scheme for
> automatic variables can be used with GCC with some kind of existing annotation,
> anything that could force that automatic variable to have an address that could
> be used to point to shared memory elements.

No, there is no way to do that.  Sorry, but you really have to
re-design this thing to be standard conformant.

Richard.

[Bug target/36861] [4.5/4.6 Regression] boost's compressed avl confuses GCC

[igaztanaga at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36861

--- Comment #42 from Ion Gaztañaga <igaztanaga at gmail dot com> 2011-01-20 11:52:36 UTC ---
> No, there is no way to do that.  Sorry, but you really have to
> re-design this thing to be standard conformant.

Thanks for your detailed report, I was not aware of this problem so I'll try to
find a solution.

Ion