[Bug c++/26788] New: optimization of expression templates not as performant as g++ 4.0.2

[Bug c++/26788] New: optimization of expression templates not as performant as g++ 4.0.2

[roebel at ircam dot fr]

Hi,

I just installed gcc 4.1.0 to compile my template expression 
matrix arithmetric library (a la Blitz). 
I recently did benchmarks with g++ 3.4.4
and 4.0.2 an I was pretty much impressed that g++ 4.0.2 managed to
optimize the expressions such that I obtained performance nearly
twice as fast as with g++ 3.4.4, and even better
the performance was the same as my hand coded pointer only implementation.
I was rather happy with this result. It seems that the handling of 
pointer arrays that are stored in a struct that represents the expression
has been significantly improved.  

Now, the downside. I tried 4.1.0 and I noticed that the performance dropped
down too a level even worse than gcc 3.4.4. I wondered about the reason and 
scanned the optimization parameters. I found salias-max-implicit-fields
with a default value of 5. I guessed that might be the reason
and increased the value to 50. With this value I've got back the impressive
performance of g++ 4.0.2.

I wonder why the default value has been set so low that apparently it
cripples the optimizer to a level of optimization consierably
below what has been achieved with g++ 4.0.2 (where this option does not exist).
Does this option negatively affects performance elsewhere? If not
it seems to me that a default value that resembles the 
settings in gcc 4.0.2 would be more sensible.

Kind regards,

and thanks anyway for this great compiler suite.

Axel


-- 
           Summary: optimization of expression templates not as performant
                    as g++ 4.0.2
           Product: gcc
           Version: 4.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: roebel at ircam dot fr
 GCC build triplet: i686-pc-linux-gnu
  GCC host triplet: i686-pc-linux-gnu
GCC target triplet: i686-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26788

[Bug c++/26788] optimization of expression templates not as performant as g++ 4.0.2

[pinskia at gcc dot gnu dot org]

------- Comment #1 from pinskia at gcc dot gnu dot org  2006-03-21 21:14 -------
salias-max-implicit-fields did not exist in 4.0.x.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26788

[Bug c++/26788] optimization of expression templates not as performant as g++ 4.0.2

[pinskia at gcc dot gnu dot org]

------- Comment #2 from pinskia at gcc dot gnu dot org  2006-03-21 21:27 -------
And the reason why salias-max-implicit-fields is set so low is to keep the
compile time in check since we get bug reports about that also.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26788

[Bug c++/26788] optimization of expression templates not as performant as g++ 4.0.2

[pinskia at gcc dot gnu dot org]

------- Comment #3 from pinskia at gcc dot gnu dot org  2006-03-21 21:28 -------
Also do you have a testcase that can be attached to the bug since the
information here is not enough to figure out what is going wrong.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26788

[Bug c++/26788] optimization of expression templates not as performant as g++ 4.0.2

[rguenth at gcc dot gnu dot org]

------- Comment #4 from rguenth at gcc dot gnu dot org  2006-03-22 09:12 -------
If the salias-max-implicit-fields setting helps you then this is a PTA issue. 
I never hit PTA issues with the expression templates in POOMA, so it might be
interesting to get a testcase for this.  A testcase is also necessary to do
anything about it.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |rguenth at gcc dot gnu dot
                   |                            |org
           Keywords|                            |alias, missed-optimization


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26788

[Bug c++/26788] optimization of expression templates not as performant as g++ 4.0.2

[roebel at ircam dot fr]

------- Comment #5 from roebel at ircam dot fr  2006-03-22 11:13 -------
Created an attachment (id=11090)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=11090&action=view)
Results file for testcase

As you requested I provide a testcase. It consists of 2 shell scripts
that run the different compilers and then run the testcase.
The testcase has two cases and two compilation modes:

switch 1
compiled with -DHAND it gives hand optimized pointer only version
compiled with -DMATMTL it gives the equivalent expression templates version
switch 2
compiled with -DBENCH=1 it calculates an addition of three vectors
compiled with -DBENCH=2 it calculates an addition of three vectors with some
scalar multiplications

The name of the excutable will indicate the experiment by two final characters
H1 stands for hand optimized first benchmark, M2 stands for matmtl second
benchmark ...

The two scripts comp.sh and master.sh run the whole experiment:
comp.sh runs the experiment for a single compiler and a user supplied set of 
vector sizes. Note, that each experiment always uses 100000000
vector element operations. By means of the vector size the amount of overhead
can be controlled.
master.sh runs comp.sh with a single compiler and the vector size arguments 5
and 1000

Results are produced with

./master.sh 2>&1 | tee mout
egrep "#|user"  mout

First result is that gcc 4.1.0 with --param  salias-max-implicit-fields=50
is a real success. As you see the compile time does not change "at least for
this testcase" but the performance is identical to the pointer only
case!!!!!!!!!!!!!!!!!!!!

second result is that for gcc 4.1.0 with default parameter set
we get performance worse then gcc 4.0.2 especially for small vectors
(large overhead). The larger the vectors become the more
gcc 4.1.0 approaches 4.0.2

###############################################################
# g++ 4.0.2 the reference
###################################################
#compile times
user    0m0.702s
user    0m0.697s
user    0m1.066s
user    0m1.077s
#run times : vector size 5
# benchmarkredH1
user    0m0.295s
# benchmarkredM1
user    0m0.307s
# benchmarkredH2
user    0m0.381s
# benchmarkredM2
user    0m0.412s
#run times : vector size 1000
# benchmarkredH1
user    0m0.230s
# benchmarkredM1
user    0m0.243s
# benchmarkredH2
user    0m0.287s
# benchmarkredM2
user    0m0.370s
# g++ 4.1.0 default
###################################################
#compile times
user    0m0.747s
user    0m0.752s
user    0m1.211s
user    0m1.227s
#run times : vector size 5
# benchmarkredH1
user    0m0.264s
# benchmarkredM1
user    0m0.519s
# benchmarkredH2
user    0m0.347s
# benchmarkredM2
user    0m1.211s
#run times : vector size 1000
# benchmarkredH1
user    0m0.222s
# benchmarkredM1
user    0m0.286s
# benchmarkredH2
user    0m0.298s
# benchmarkredM2
user    0m0.375s
# g++ 4.1.0 salias=50
###################################################
#compile times
user    0m0.753s
user    0m0.741s
user    0m1.225s
user    0m1.239s
#run times : vector size 5
# benchmarkredH1
user    0m0.262s
# benchmarkredM1
user    0m0.307s
# benchmarkredH2
user    0m0.344s
# benchmarkredM2
user    0m0.313s
#run times : vector size 1000
# benchmarkredH1
user    0m0.223s
# benchmarkredM1
user    0m0.234s
# benchmarkredH2
user    0m0.299s
# benchmarkredM2
user    0m0.260s


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26788

[Bug c++/26788] optimization of expression templates not as performant as g++ 4.0.2

[roebel at ircam dot fr]

------- Comment #6 from roebel at ircam dot fr  2006-03-22 11:14 -------
Created an attachment (id=11091)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=11091&action=view)
master shell script

for comments 
see 11090: Results file for testcase


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26788

[Bug c++/26788] optimization of expression templates not as performant as g++ 4.0.2

[roebel at ircam dot fr]

------- Comment #7 from roebel at ircam dot fr  2006-03-22 11:15 -------
Created an attachment (id=11092)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=11092&action=view)
single experiment shell script

for comments 
see 11090: Results file for testcase


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26788

[Bug c++/26788] optimization of expression templates not as performant as g++ 4.0.2

[roebel at ircam dot fr]

------- Comment #8 from roebel at ircam dot fr  2006-03-22 11:16 -------
Created an attachment (id=11093)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=11093&action=view)
testcase source file

for comments 
see 11090: Results file for testcase


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26788

[Bug c++/26788] optimization of expression templates not as performant as g++ 4.0.2

[rguenth at gcc dot gnu dot org]

------- Comment #9 from rguenth at gcc dot gnu dot org  2006-03-22 11:39 -------
This is another case of find_used_portions missing explicit uses due to C++ and
lots of inlining without any cleanup after that.  And inserting cleanup being
difficult because structure-aliasing pass running before going into SSA.  A
forwprop pass before it would do wonders here.

Danny - any plans to look at making salias pass work on SSA form?  With
inlining
on SSA like on IPA branch this looks necessary anyway (Honza simply moved the
pass to before (final) inlining, which will make the situation just worse).


-- 

rguenth at gcc dot gnu dot org changed:

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


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26788

[Bug c++/26788] optimization of expression templates not as performant as g++ 4.0.2

[roebel at ircam dot fr]

------- Comment #10 from roebel at ircam dot fr  2006-03-22 11:55 -------

Not that I understand what you just said, but, I wanted to mention, that 
in contrast to my initial email the data I just sent
indicates a small performance penalty  of about 25% for g++ 4.0.2 
for large vectors on a pentium 4 (that are the results I've sent) 
while there is no such 
penalty for large vectors on a pentium m. On a pentium m g++ 4.0.2
works as well as g++ 4.1.0 on pentium 4 with the --param salias...=50.
Unfortunately, I dont have gcc 4.1.0 on my pentium m machine

thanks,

Axel


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26788

[Bug c++/26788] optimization of expression templates not as performant as g++ 4.0.2

[pinskia at gcc dot gnu dot org]

------- Comment #11 from pinskia at gcc dot gnu dot org  2006-04-30 04:11 -------
(In reply to comment #9)
But that only applies to 4.2 and not 4.1.0.


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |minor


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26788

[Bug tree-optimization/26788] optimization of expression templates not as performant as g++ 4.0.2

[rguenth at gcc dot gnu dot org]

------- Comment #12 from rguenth at gcc dot gnu dot org  2008-01-26 22:58 -------
Can you check 4.2 and 4.3?


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |WAITING


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26788

[Bug tree-optimization/26788] optimization of expression templates not as performant as g++ 4.0.2

[roebel at ircam dot fr]

------- Comment #13 from roebel at ircam dot fr  2008-01-27 12:35 -------
Hi,

I run the tests with g++ 422 and it seems to  me the issue is closed.
Compilation without the salias-max-implicit-fields flag is nor producing
any substantial increase in run time any more and with and without
this parameter the hand optimized and compiler template version 
of the code have very similar run time.

I would be really happy with this, if gcc422 would produce
correct code in all my projects. I tried it already a while ago
and found a problem with std::set where the optimized version of the program
simply did and up with duplicate entries in the set 
(while gcc 4.1.2 has no problems with the very same code)!!!

Besides that show stopper we had other problems with code using
sse/sse2 intrinsics  producing wrong results when optimization was enabled.

All this may have changed in gcc4.3. I will give it another trial.

Thanks


-- 

roebel at ircam dot fr changed:

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


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26788