New GCC releases comparison and comparison of GCC4.4 and LLVM2.5 on SPEC2000

New GCC releases comparison and comparison of GCC4.4 and LLVM2.5 on SPEC2000

[Vladimir Makarov]

A few people asked me to do a new comparison of GCC releases and LLVM as 
the new GCC release and LLVM were out recently.

You can find the comparison on http://vmakarov.fedorapeople.org/spec/

The comparison for x86 (32-bit mode) was done on Pentium4 and  for 
x86_64 (64-bit mode) on Core I7.

Some changes in the performance were big since GCC 3.2 and it is 
sometimes hard to see small changes on the posted graphs.   Therefore I 
put original tables used to generate the graphs.
 

Re: New GCC releases comparison and comparison of GCC4.4 and LLVM2.5 on SPEC2000

[Chris Lattner]

On May 12, 2009, at 6:56 AM, Vladimir Makarov wrote:

> A few people asked me to do a new comparison of GCC releases and  
> LLVM as the new GCC release and LLVM were out recently.
>
> You can find the comparison on http://vmakarov.fedorapeople.org/spec/
>
> The comparison for x86 (32-bit mode) was done on Pentium4 and  for  
> x86_64 (64-bit mode) on Core I7.
>
> Some changes in the performance were big since GCC 3.2 and it is  
> sometimes hard to see small changes on the posted graphs.    
> Therefore I put original tables used to generate the graphs.

Looking at the llvm 2.5 vs gcc 4.4 comparison is very interesting,  
thank you for putting this together Vladimir!  I find these numbers  
particularly interesting because you're comparing simple options like - 
O2 and -O3 instead of the crazy spec tuning mix :).  This is much more  
likely to be representative of what real users will get on their apps.

Some random thoughts:

1. I have a hard time understanding the code size numbers.  Does 10%  
mean that GCC is generating 10% bigger or 10% smaller code than llvm?

2. You change two variables in your configurations: micro architecture  
and pointer size.  Would you be willing to run x86-32 Core i7 numbers  
as well?  LLVM in particular is completely untuned for the (really old  
and quirky) "netburst" architecture, but I'm interested to see how it  
runs for you on more modern Core i7 or Core2 processors in 32-bit mode.

3. Your SPEC FP benchmarks tell me two things: GCC 4.4's fortran  
support is dramatically better than 4.2's (which llvm 2.5 uses), and  
your art/mgrid hacks apparently do great stuff :).

4. Your SPEC INT numbers are more interesting to me.  It looks like  
you guys have some significant wins in 175.vpr, 197.crafty, and other  
benchmarks.  At some point, I'll have to see what you guys are doing :)

Thanks for the info, great stuff!

-Chris

Re: New GCC releases comparison and comparison of GCC4.4 and LLVM2.5 on SPEC2000

[Vladimir Makarov]

Chris Lattner wrote:
>
> On May 12, 2009, at 6:56 AM, Vladimir Makarov wrote:
>
>> A few people asked me to do a new comparison of GCC releases and LLVM 
>> as the new GCC release and LLVM were out recently.
>>
>> You can find the comparison on http://vmakarov.fedorapeople.org/spec/
>>
>> The comparison for x86 (32-bit mode) was done on Pentium4 and  for 
>> x86_64 (64-bit mode) on Core I7.
>>
>> Some changes in the performance were big since GCC 3.2 and it is 
>> sometimes hard to see small changes on the posted graphs.   Therefore 
>> I put original tables used to generate the graphs.
>
> Looking at the llvm 2.5 vs gcc 4.4 comparison is very interesting, 
> thank you for putting this together Vladimir!  I find these numbers 
> particularly interesting because you're comparing simple options like 
> -O2 and -O3 instead of the crazy spec tuning mix :).  This is much 
> more likely to be representative of what real users will get on their 
> apps.
>
> Some random thoughts:
>
> 1. I have a hard time understanding the code size numbers.  Does 10% 
> mean that GCC is generating 10% bigger or 10% smaller code than llvm?
>
The change is reported relative to LLVM.  So 10% means that GCC 
generates 10% bigger code than LLVM and -10% means that GCC generates 
10% less code.
> 2. You change two variables in your configurations: micro architecture 
> and pointer size.  Would you be willing to run x86-32 Core i7 numbers 
> as well?  LLVM in particular is completely untuned for the (really old 
> and quirky) "netburst" architecture, but I'm interested to see how it 
> runs for you on more modern Core i7 or Core2 processors in 32-bit mode.
>
I used the same processor (P4) and options for x86 as for the GCC 
release comparison.  I did not know that LLVM is badly tuned for P4, 
sorry.  I could do the same comparison for x86 on Core i7 without 
specific tuning (there is no tuning for i7 yet) but it takes a lot of 
time.  May be it will be ready on next week.
> 3. Your SPEC FP benchmarks tell me two things: GCC 4.4's fortran 
> support is dramatically better than 4.2's (which llvm 2.5 uses), and 
> your art/mgrid hacks apparently do great stuff :).
>
> 4. Your SPEC INT numbers are more interesting to me.  It looks like 
> you guys have some significant wins in 175.vpr, 197.crafty, and other 
> benchmarks.  At some point, I'll have to see what you guys are doing :)
>
> Thanks for the info, great stuff!
>
> -Chris
>

Re: New GCC releases comparison and comparison of GCC4.4 and LLVM2.5 on SPEC2000

[Joseph S. Myers]

On Tue, 12 May 2009, Chris Lattner wrote:

> 1. I have a hard time understanding the code size numbers.  Does 10% mean that
> GCC is generating 10% bigger or 10% smaller code than llvm?

I have a different comment on the code size numbers: could we have 
comparisons of code size for -Os rather than (or in addition to) -O2 and 
-O3?  If someone is particularly concerned with code size, -Os is what 
they are expected to use.

-- 
Joseph S. Myers
joseph@codesourcery.com

Re: New GCC releases comparison and comparison of GCC4.4 and LLVM2.5 on SPEC2000

[Vladimir Makarov]

Joseph S. Myers wrote:
> On Tue, 12 May 2009, Chris Lattner wrote:
>
>   
>> 1. I have a hard time understanding the code size numbers.  Does 10% mean that
>> GCC is generating 10% bigger or 10% smaller code than llvm?
>>     
>
> I have a different comment on the code size numbers: could we have 
> comparisons of code size for -Os rather than (or in addition to) -O2 and 
> -O3?  If someone is particularly concerned with code size, -Os is what 
> they are expected to use.
>
>   
Thanks for pointing this, Joseph.

Yes, it would be interesting to see how GCC code size is changed with 
-Os (as the performance too).  But probably it is even more interesting 
for embedded processors.  When I am less busy, I'll try to do it.

Re: New GCC releases comparison and comparison of GCC4.4 and LLVM2.5 on SPEC2000

[Chris Lattner]

On May 12, 2009, at 11:05 AM, Vladimir Makarov wrote:
> Chris Lattner wrote:
>> On May 12, 2009, at 6:56 AM, Vladimir Makarov wrote:
>>
>>> A few people asked me to do a new comparison of GCC releases and  
>>> LLVM as the new GCC release and LLVM were out recently.
>>>
>>> You can find the comparison on http://vmakarov.fedorapeople.org/ 
>>> spec/
>>>
>>> The comparison for x86 (32-bit mode) was done on Pentium4 and  for  
>>> x86_64 (64-bit mode) on Core I7.
>>>
>>> Some changes in the performance were big since GCC 3.2 and it is  
>>> sometimes hard to see small changes on the posted graphs.    
>>> Therefore I put original tables used to generate the graphs.
>>
>> Looking at the llvm 2.5 vs gcc 4.4 comparison is very interesting,  
>> thank you for putting this together Vladimir!  I find these numbers  
>> particularly interesting because you're comparing simple options  
>> like -O2 and -O3 instead of the crazy spec tuning mix :).  This is  
>> much more likely to be representative of what real users will get  
>> on their apps.
>>
>> Some random thoughts:
>>
>> 1. I have a hard time understanding the code size numbers.  Does  
>> 10% mean that GCC is generating 10% bigger or 10% smaller code than  
>> llvm?
>>
> The change is reported relative to LLVM.  So 10% means that GCC  
> generates 10% bigger code than LLVM and -10% means that GCC  
> generates 10% less code.

Ok!  It is interesting that GCC seems to generate consistently larger  
code at both -O2 and -O3 in x86-64 mode (over 20% larger in -O3).   
Perhaps that also is impacting the compile time numbers as well.

>> 2. You change two variables in your configurations: micro  
>> architecture and pointer size.  Would you be willing to run x86-32  
>> Core i7 numbers as well?  LLVM in particular is completely untuned  
>> for the (really old and quirky) "netburst" architecture, but I'm  
>> interested to see how it runs for you on more modern Core i7 or  
>> Core2 processors in 32-bit mode.
>>
> I used the same processor (P4) and options for x86 as for the GCC  
> release comparison.  I did not know that LLVM is badly tuned for P4,  
> sorry.  I could do the same comparison for x86 on Core i7 without  
> specific tuning (there is no tuning for i7 yet) but it takes a lot  
> of time.  May be it will be ready on next week.

No problem at all, I appreciate you running the numbers!

It would also be very interesting to include LLVM's LTO support, which  
gives a pretty dramatic win on SPEC.  However, I don't know how  
difficult it is to use on linux (on the mac, you just pass -O4 at  
compile time, and everything works).  I've heard that Gold has a new  
plugin to make LTO transparent on linux as well, but I have no  
experience with it, and it is probably more trouble than you want to  
take.  Does gcc 4.4 include the LTO branch yet?

-Chris

>
>> 3. Your SPEC FP benchmarks tell me two things: GCC 4.4's fortran  
>> support is dramatically better than 4.2's (which llvm 2.5 uses),  
>> and your art/mgrid hacks apparently do great stuff :).
>>
>> 4. Your SPEC INT numbers are more interesting to me.  It looks like  
>> you guys have some significant wins in 175.vpr, 197.crafty, and  
>> other benchmarks.  At some point, I'll have to see what you guys  
>> are doing :)
>>
>> Thanks for the info, great stuff!
>>
>> -Chris
>>
>

Re: New GCC releases comparison and comparison of GCC4.4 and LLVM2.5 on SPEC2000

[Vladimir Makarov]

Vladimir Makarov wrote:
> Chris Lattner wrote:
>> 2. You change two variables in your configurations: micro 
>> architecture and pointer size.  Would you be willing to run x86-32 
>> Core i7 numbers as well?  LLVM in particular is completely untuned 
>> for the (really old and quirky) "netburst" architecture, but I'm 
>> interested to see how it runs for you on more modern Core i7 or Core2 
>> processors in 32-bit mode.
>>
> I used the same processor (P4) and options for x86 as for the GCC 
> release comparison.
I was wrong here  GCC-LLVM comparison does not use -mtune=pentium4 as it 
was used for GCC releases.  So default x86 tunings for  the compilers 
were used for the 32-bit comparison.  Still the results might look 
different on Core i7.

Sorry, I missed to mention that I used an additional option -mpc64 for 
32-bit GCC4.4.  It is not possible to generate SPECFP2000 expected 
results  by GCC4.4 without this option. LLVM does not support this 
option.  And this option can significantly improve the performance.  So 
32-bit comparison of SPECFP2000 should be taken with a grain of salt.

I've just corrected page 
http://vmakarov.fedorapeople.org/spec/llvmgcc32.html by adding these 
comments.

Re: New GCC releases comparison and comparison of GCC4.4 and LLVM2.5 on SPEC2000

[Rafael Espindola]

> It would also be very interesting to include LLVM's LTO support, which gives
> a pretty dramatic win on SPEC.  However, I don't know how difficult it is to
> use on linux (on the mac, you just pass -O4 at compile time, and everything
> works).  I've heard that Gold has a new plugin to make LTO transparent on
> linux as well, but I have no experience with it, and it is probably more
> trouble than you want to take.  Does gcc 4.4 include the LTO branch yet?

For spec all that you (should) need is to link with a gold with
plugins enabled and pass -use-gold-plugin to llvm-gcc. For software
that uses static libraries you will also need the bfd plugin support
(currently in code review). I am going on vacation tomorrow, but might
read my mail from time to time. Ping me if you need help.

The current trunk includes some patches from LTO, but not the streamer.

> -Chris

Cheers,
-- 
Rafael Avila de Espindola

Google | Gordon House | Barrow Street | Dublin 4 | Ireland
Registered in Dublin, Ireland | Registration Number: 368047

Re: New GCC releases comparison and comparison of GCC4.4 and LLVM2.5 on SPEC2000

[Steven Bosscher]

On Tue, May 12, 2009 at 7:45 PM, Chris Lattner <clattner@apple.com> wrote:
> 2. You change two variables in your configurations: micro architecture and
> pointer size.  Would you be willing to run x86-32 Core i7 numbers as well?
>  LLVM in particular is completely untuned for the (really old and quirky)
> "netburst" architecture, but I'm interested to see how it runs for you on
> more modern Core i7 or Core2 processors in 32-bit mode.

FWIW, GCC is also completely untuned for NetBurst. There isn't even a
scheduler description for the P4, and there also isn't anything for
the funny branch predictor.


> 3. Your SPEC FP benchmarks tell me two things: GCC 4.4's fortran support is
> dramatically better than 4.2's (which llvm 2.5 uses), and your art/mgrid
> hacks apparently do great stuff :).

Something like the "art hack" is in ipa-struct-reorg, but it is not
enabled at any level. If gcc outperforms llvm on art by much, it's
more likely that some important opportunities for art are being
overlooked by llvm.

There also isn't anything special done for mgrid, except predictive
commoning (CSE around loops) which is not a hack, in the sense it is
helpful for a lot of numerical code and triggers several times in
things like generic Fortran blas/lapack routines.

Hope this helps,

Ciao!
Steven

Re: New GCC releases comparison and comparison of GCC4.4 and LLVM2.5 on SPEC2000

[Paolo Bonzini]

>> 3. Your SPEC FP benchmarks tell me two things: GCC 4.4's fortran support is
>> dramatically better than 4.2's (which llvm 2.5 uses), and your art/mgrid
>> hacks apparently do great stuff :).
> 
> Something like the "art hack" is in ipa-struct-reorg, but it is not
> enabled at any level. If gcc outperforms llvm on art by much, it's
> more likely that some important opportunities for art are being
> overlooked by llvm.
> 
> There also isn't anything special done for mgrid, except predictive
> commoning (CSE around loops) which is not a hack, in the sense it is
> helpful for a lot of numerical code and triggers several times in
> things like generic Fortran blas/lapack routines.

Indeed, we have a couple of benchmark-inspired optimizations for
SPEC2006 (division/modulo power-of-two, see PR26026; and ifcombine), and
we optimize MATMUL (TRANSPOSE (A), B) which helps galgel a lot.  But
both of this may trigger quite a lot on other code, and LLVM also
benefits from the galgel one :-) because it's done in the front-end.

Paolo

Re: New GCC releases comparison and comparison of GCC4.4 and LLVM2.5 on SPEC2000

[Andi Kleen]

"Joseph S. Myers" <joseph@codesourcery.com> writes:

> On Tue, 12 May 2009, Chris Lattner wrote:
>
>> 1. I have a hard time understanding the code size numbers.  Does 10% mean that
>> GCC is generating 10% bigger or 10% smaller code than llvm?
>
> I have a different comment on the code size numbers: could we have 
> comparisons of code size for -Os rather than (or in addition to) -O2 and 
> -O3?  If someone is particularly concerned with code size, -Os is what 
> they are expected to use.

It's a slippery slope that -O2 is getting so bad regarding
code size.  What should people do who need performance, but cannot
completely disregard code size (and can't use profile feedback for
some reason).

Also with limited caches code size for large programs typically tends
to affect performance too.

In my experience -Os has some significant drawbacks because it
prioritizes everything code size over everything else. For example one
-Os problem case we ran into was on x86 it always uses a fully generic
hardware division when dividing by constant even for cases where you
can generate a much faster sequence by spending a few more bytes.
Also it disables a lot of other useful optimizations.

-Andi

-- 
ak@linux.intel.com -- Speaking for myself only.

Re: New GCC releases comparison and comparison of GCC4.4 and LLVM2.5 on SPEC2000

[Jakub Jelinek]

On Wed, May 13, 2009 at 09:33:20AM +0200, Andi Kleen wrote:
> "Joseph S. Myers" <joseph@codesourcery.com> writes:
> 
> > On Tue, 12 May 2009, Chris Lattner wrote:
> >
> >> 1. I have a hard time understanding the code size numbers.  Does 10% mean that
> >> GCC is generating 10% bigger or 10% smaller code than llvm?
> >
> > I have a different comment on the code size numbers: could we have 
> > comparisons of code size for -Os rather than (or in addition to) -O2 and 
> > -O3?  If someone is particularly concerned with code size, -Os is what 
> > they are expected to use.
> 
> It's a slippery slope that -O2 is getting so bad regarding
> code size.  What should people do who need performance, but cannot
> completely disregard code size (and can't use profile feedback for
> some reason).

For x86_64/i?86 http://gcc.gnu.org/ml/gcc-patches/2009-05/msg00702.html
patch might buy back around 2% for -O2 code (at least it does so for cc1plus
binary), and perhaps improving min_insn_size might give further code size
improvements at -O2.

	Jakub

Re: New GCC releases comparison and comparison of GCC4.4 and LLVM2.5 on SPEC2000

[Paolo Bonzini]

Andi Kleen wrote:
> "Joseph S. Myers" <joseph@codesourcery.com> writes:
> 
>> On Tue, 12 May 2009, Chris Lattner wrote:
>>
>>> 1. I have a hard time understanding the code size numbers.  Does 10% mean that
>>> GCC is generating 10% bigger or 10% smaller code than llvm?
>> I have a different comment on the code size numbers: could we have 
>> comparisons of code size for -Os rather than (or in addition to) -O2 and 
>> -O3?  If someone is particularly concerned with code size, -Os is what 
>> they are expected to use.
> 
> It's a slippery slope that -O2 is getting so bad regarding
> code size.  What should people do who need performance, but cannot
> completely disregard code size (and can't use profile feedback for
> some reason).

From looking http://vmakarov.fedorapeople.org/spec/I2Size32.png it does
not look that bad at all.  For SpecFP it is different, but code size is
rarely a problem in FP benchmarks (hot loops are really small or really
huge anyway).  So it looks like we're doing the right thing in that respect.

Rather, we should seriously understand what caused the compilation time
jump in 4.2, and whether those are still a problem.  We made a good job
in 4.0 and 4.3 offsetting the slowdowns from infrastructure changes with
speedups from other changes; and 4.4 while slower than 4.3 at least
stays below 4.2.  But, 4.2 was a disaster for compilation time.

Paolo

Re: New GCC releases comparison and comparison of GCC4.4 and LLVM2.5 on SPEC2000

[Paolo Bonzini]

Andi Kleen wrote:
> "Joseph S. Myers" <joseph@codesourcery.com> writes:
> 
>> On Tue, 12 May 2009, Chris Lattner wrote:
>>
>>> 1. I have a hard time understanding the code size numbers.  Does 10% mean that
>>> GCC is generating 10% bigger or 10% smaller code than llvm?
>> I have a different comment on the code size numbers: could we have 
>> comparisons of code size for -Os rather than (or in addition to) -O2 and 
>> -O3?  If someone is particularly concerned with code size, -Os is what 
>> they are expected to use.
> 
> It's a slippery slope that -O2 is getting so bad regarding
> code size.  What should people do who need performance, but cannot
> completely disregard code size (and can't use profile feedback for
> some reason).

From looking http://vmakarov.fedorapeople.org/spec/I2Size32.png it does
not look that bad at all.  For SpecFP it is different, but code size is
rarely a problem in FP benchmarks (hot loops are really small or really
huge anyway).  So it looks like we're doing the right thing in that respect.

Rather, we should seriously understand what caused the compilation time
jump in 4.2, and whether those are still a problem.  We made a good job
in 4.0 and 4.3 offsetting the slowdowns from infrastructure changes with
speedups from other changes; and 4.4 while slower than 4.3 at least
stays below 4.2.  But, 4.2 was a disaster for compilation time.

Paolo

Re: New GCC releases comparison and comparison of GCC4.4 and LLVM2.5 on SPEC2000

[Andi Kleen]

> From looking http://vmakarov.fedorapeople.org/spec/I2Size32.png it does
> not look that bad at all.  For SpecFP it is different, but code size is

The code size seems to be much worse than LLVM at least, unless
I misread the graphs. 

Also my comment was in regard of the suggestion to try -Os -- -Os 
is not the answer for -O2 code size regressions.

> Rather, we should seriously understand what caused the compilation time
> jump in 4.2, and whether those are still a problem.  We made a good job
> in 4.0 and 4.3 offsetting the slowdowns from infrastructure changes with
> speedups from other changes; and 4.4 while slower than 4.3 at least
> stays below 4.2.  But, 4.2 was a disaster for compilation time.

Yes that would be useful, although I admit for me personally
make -j and icecream do a pretty good job at hiding that pain.

-Andi

-- 
ak@linux.intel.com -- Speaking for myself only.

Re: New GCC releases comparison and comparison of GCC4.4 and LLVM2.5 on SPEC2000

[Duncan Sands]

Hi,

> Sorry, I missed to mention that I used an additional option -mpc64 for 
> 32-bit GCC4.4.  It is not possible to generate SPECFP2000 expected 
> results  by GCC4.4 without this option. LLVM does not support this 
> option.  And this option can significantly improve the performance.  So 
> 32-bit comparison of SPECFP2000 should be taken with a grain of salt.

what does -mpc64 do exactly?  The gcc docs say:
  `-mpc64' rounds the the significands of results of floating-point operations to 53 bits (double precision)
Does this mean that a rounding operation is performed after each fp
operation, or that optimizations are permitted that don't result in
accurate extended double precision values as long as they are correct
to 53 bits, or something else?

The LLVM code generators have an option called -limit-float-precision:
  -limit-float-precision=<uint>       - Generate low-precision inline sequences for some float libcalls
I'm not sure what it does exactly, but perhaps it is similar to -mpc64?

Ciao,

Duncan.

Re: New GCC releases comparison and comparison of GCC4.4 and LLVM2.5 on SPEC2000

[Steven Bosscher]

On Wed, May 13, 2009 at 1:41 PM, Andi Kleen <andi@firstfloor.org> wrote:
>> Rather, we should seriously understand what caused the compilation time
>> jump in 4.2, and whether those are still a problem.  We made a good job
>> in 4.0 and 4.3 offsetting the slowdowns from infrastructure changes with
>> speedups from other changes; and 4.4 while slower than 4.3 at least
>> stays below 4.2.  But, 4.2 was a disaster for compilation time.
>
> Yes that would be useful, although I admit for me personally
> make -j and icecream do a pretty good job at hiding that pain.

Yes, well...

Given the continuous complaints/bashes from some of your fellow kernel
hackers and many others, it still is still important to address the
compilation time issues. There still is enough low-hanging fruit.

Ciao!
Steven

Re: New GCC releases comparison and comparison of GCC4.4 and LLVM2.5 on SPEC2000

[Dave Korn]

Steven Bosscher wrote:
> On Wed, May 13, 2009 at 1:41 PM, Andi Kleen <andi@firstfloor.org> wrote:
>>> Rather, we should seriously understand what caused the compilation time
>>> jump in 4.2, and whether those are still a problem.  We made a good job
>>> in 4.0 and 4.3 offsetting the slowdowns from infrastructure changes with
>>> speedups from other changes; and 4.4 while slower than 4.3 at least
>>> stays below 4.2.  But, 4.2 was a disaster for compilation time.
>> Yes that would be useful, although I admit for me personally
>> make -j and icecream do a pretty good job at hiding that pain.
> 
> Yes, well...
> 
> Given the continuous complaints/bashes from some of your fellow kernel
> hackers and many others, it still is still important to address the
> compilation time issues. There still is enough low-hanging fruit.


  Us Cygwin folks will not be complaining about any improvements in
compile-time issues either :-)  We have a fair amount of emulation overhead to
live with as it is.

    cheers,
      DaveK

Re: New GCC releases comparison and comparison of GCC4.4 and LLVM2.5 on SPEC2000

[Richard Guenther]

On Wed, May 13, 2009 at 1:51 PM, Duncan Sands
<duncan.sands@math.u-psud.fr> wrote:
> Hi,
>
>> Sorry, I missed to mention that I used an additional option -mpc64 for
>> 32-bit GCC4.4.  It is not possible to generate SPECFP2000 expected
>> results  by GCC4.4 without this option. LLVM does not support this
>> option.  And this option can significantly improve the performance.  So
>> 32-bit comparison of SPECFP2000 should be taken with a grain of salt.
>
> what does -mpc64 do exactly?  The gcc docs say:
>  `-mpc64' rounds the the significands of results of floating-point operations to 53 bits (double precision)
> Does this mean that a rounding operation is performed after each fp
> operation, or that optimizations are permitted that don't result in
> accurate extended double precision values as long as they are correct
> to 53 bits, or something else?
>
> The LLVM code generators have an option called -limit-float-precision:
>  -limit-float-precision=<uint>       - Generate low-precision inline sequences for some float libcalls
> I'm not sure what it does exactly, but perhaps it is similar to -mpc64?

-mpc64 sets the x87 floating point control register to not use the 80bit
extended precision.  This causes some x87 floating point operations
to operate faster and there are no issues with the extra roundings you
get when storing an 80bit precision register to a 64bit memory location.

Does LLVM support x87 arithmetic at all or does it default to SSE
arithmetic in 32bits?  I guess using SSE math for both would be a more
fair comparison?

Richard.

Re: New GCC releases comparison and comparison of GCC4.4 and LLVM2.5 on SPEC2000

[Joseph S. Myers]

On Wed, 13 May 2009, Richard Guenther wrote:

> -mpc64 sets the x87 floating point control register to not use the 80bit
> extended precision.  This causes some x87 floating point operations
> to operate faster and there are no issues with the extra roundings you
> get when storing an 80bit precision register to a 64bit memory location.

Note that this is purely a link-time option; it does not cause 
TARGET_96_ROUND_53_LONG_DOUBLE to be set so the compiler will not know 
internally how long double behaves in the processor.  Note also that you 
still have the extended exponent range and associated issues with storing 
to memory with that (including extra roundings for subnormals).

-- 
Joseph S. Myers
joseph@codesourcery.com

Re: New GCC releases comparison and comparison of GCC4.4 and LLVM2.5 on SPEC2000

[Duncan Sands]

Hi Richard,

> -mpc64 sets the x87 floating point control register to not use the 80bit
> extended precision.  This causes some x87 floating point operations
> to operate faster and there are no issues with the extra roundings you
> get when storing an 80bit precision register to a 64bit memory location.

I see, thanks for the explanation.

> Does LLVM support x87 arithmetic at all or does it default to SSE
> arithmetic in 32bits?  I guess using SSE math for both would be a more
> fair comparison?

LLVM does support the x86 floating point stack, though it doesn't support
all asm expressions for it (which is why llvm-gcc disables math inlines).
My understanding is that no effort has been made to produce optimal code
when using the x86 fp stack, and all the effort when into SSE instead.

Ciao,

Duncan.

Re: New GCC releases comparison and comparison of GCC4.4 and LLVM2.5 on SPEC2000

[Paolo Bonzini]

Andi Kleen wrote:
>> From looking http://vmakarov.fedorapeople.org/spec/I2Size32.png it does
>> not look that bad at all.  For SpecFP it is different, but code size is
> 
> The code size seems to be much worse than LLVM at least, unless
> I misread the graphs. 

Not really, see http://vmakarov.fedorapeople.org/spec/GLI2Size32.png and
http://vmakarov.fedorapeople.org/spec/GLF2Size32.png

That said, the LLVM results are totally interesting and kudos must go to
the LLVM people.  (I would love to have a DAG-based instruction
selection pass...)

Paolo

Re: New GCC releases comparison and comparison of GCC4.4 and LLVM2.5 on SPEC2000

[Vladimir Makarov]

Paolo Bonzini wrote:
>
> Rather, we should seriously understand what caused the compilation time
> jump in 4.2, and whether those are still a problem.  We made a good job
> in 4.0 and 4.3 offsetting the slowdowns from infrastructure changes with
> speedups from other changes; and 4.4 while slower than 4.3 at least
> stays below 4.2.  But, 4.2 was a disaster for compilation time.
>
>   
It was interesting for me that GCC4.4 is faster than GCC4.3 on Intel 
Core I7.  This is  not true for most other processors (at least for P4, 
Core2, Power4/5/6 etc).  Intel Core I7 has 3rd level big and fast cache 
and memory controller on the chip.  I guess that slowdown in GCC is 
mostly because of data and/or code locality.

Fortunately, GCC4.5 will be 5% faster because of Richard Guenter's work 
on improving aliasing (of course if it will be not eaten by a new 
optimization).  People are complaining about GCC compilation speed and 
of course we should work on its speedup.  But GCC is not so bad, for 
example SUN Studio compiler is almost 2 times slower than GCC.  IMHO, 
GCC performace is still #1 priority for us (although people working on 
embedded processors could disagree with me).  I think that because in my 
experience the performance improvement is much harder to achieve than 
compilation speed and code size improvements.

Re: New GCC releases comparison and comparison of GCC4.4 and LLVM2.5 on SPEC2000

[Jan Hubicka]

> Paolo Bonzini wrote:
> >
> >Rather, we should seriously understand what caused the compilation time
> >jump in 4.2, and whether those are still a problem.  We made a good job
> >in 4.0 and 4.3 offsetting the slowdowns from infrastructure changes with
> >speedups from other changes; and 4.4 while slower than 4.3 at least
> >stays below 4.2.  But, 4.2 was a disaster for compilation time.
> >
> >  
> It was interesting for me that GCC4.4 is faster than GCC4.3 on Intel 
> Core I7.  This is  not true for most other processors (at least for P4, 
> Core2, Power4/5/6 etc).  Intel Core I7 has 3rd level big and fast cache 
> and memory controller on the chip.  I guess that slowdown in GCC is 
> mostly because of data and/or code locality.

Note that one of important reasons for slowdowns is the startup cost.
Binarry is bigger, we need more libraries and we do more initialization.
With per-function flags via attributes, we are now initializing all the
builtins and amount of builtins in i386 backend has grown to quite
extreme sizes.  In 4.3 timeframe I did some work on optimizing startup
costs that translated quite noticeably to kernel compilation time.  It
might be interesting to consider some scheme for initializing them
lazilly.

Honza
> 
> Fortunately, GCC4.5 will be 5% faster because of Richard Guenter's work 
> on improving aliasing (of course if it will be not eaten by a new 
> optimization).  People are complaining about GCC compilation speed and 
> of course we should work on its speedup.  But GCC is not so bad, for 
> example SUN Studio compiler is almost 2 times slower than GCC.  IMHO, 
> GCC performace is still #1 priority for us (although people working on 
> embedded processors could disagree with me).  I think that because in my 
> experience the performance improvement is much harder to achieve than 
> compilation speed and code size improvements.

Re: New GCC releases comparison and comparison of GCC4.4 and LLVM2.5 on SPEC2000

[Michael Meissner]

On Wed, May 13, 2009 at 05:42:03PM +0200, Jan Hubicka wrote:
> > Paolo Bonzini wrote:
> > >
> > >Rather, we should seriously understand what caused the compilation time
> > >jump in 4.2, and whether those are still a problem.  We made a good job
> > >in 4.0 and 4.3 offsetting the slowdowns from infrastructure changes with
> > >speedups from other changes; and 4.4 while slower than 4.3 at least
> > >stays below 4.2.  But, 4.2 was a disaster for compilation time.
> > >
> > >  
> > It was interesting for me that GCC4.4 is faster than GCC4.3 on Intel 
> > Core I7.  This is  not true for most other processors (at least for P4, 
> > Core2, Power4/5/6 etc).  Intel Core I7 has 3rd level big and fast cache 
> > and memory controller on the chip.  I guess that slowdown in GCC is 
> > mostly because of data and/or code locality.
> 
> Note that one of important reasons for slowdowns is the startup cost.
> Binarry is bigger, we need more libraries and we do more initialization.
> With per-function flags via attributes, we are now initializing all the
> builtins and amount of builtins in i386 backend has grown to quite
> extreme sizes.  In 4.3 timeframe I did some work on optimizing startup
> costs that translated quite noticeably to kernel compilation time.  It
> might be interesting to consider some scheme for initializing them
> lazilly.

For C and C++ on x86/x86_64, the compiler does not create the decls for the
builtin functions until it is compiling an ISA that supports the builtin.  So
on x86_64, it will create the sse and sse2 builtins by default, but not the
sse4, sse5, avx builtins until you use the appropriate -m<xxx> flag or specify
a target specific option in the attributes or pragmas.  Yes, an initial version
of my patches did create all of the nodes, but the August 29th checkin did
restrict building the builtins to the ISA supported.

Now, it does create the builtins all of the time for other languages, since
they don't register the appropriate create a builtin function in external scope
hook.

However, that still leaves the compiler creating a lot of builtins that mostly
aren't used.  It may be useful to register names, and have a call back to
create the builtin if the name is actually used.  It certainly would eliminate
a lot of memory space, and time in the compiler.

-- 
Michael Meissner, IBM
4 Technology Place Drive, MS 2203A, Westford, MA, 01886, USA
meissner@linux.vnet.ibm.com

Re: New GCC releases comparison and comparison of GCC4.4 and LLVM2.5 on SPEC2000

[Evan Cheng]

On May 13, 2009, at 4:51 AM, Duncan Sands wrote:

> Hi,
>
>> Sorry, I missed to mention that I used an additional option -mpc64  
>> for
>> 32-bit GCC4.4.  It is not possible to generate SPECFP2000 expected
>> results  by GCC4.4 without this option. LLVM does not support this
>> option.  And this option can significantly improve the  
>> performance.  So
>> 32-bit comparison of SPECFP2000 should be taken with a grain of salt.
>
> what does -mpc64 do exactly?  The gcc docs say:
>  `-mpc64' rounds the the significands of results of floating-point  
> operations to 53 bits (double precision)
> Does this mean that a rounding operation is performed after each fp
> operation, or that optimizations are permitted that don't result in
> accurate extended double precision values as long as they are correct
> to 53 bits, or something else?
>
> The LLVM code generators have an option called -limit-float-precision:
>  -limit-float-precision=<uint>       - Generate low-precision inline  
> sequences for some float libcalls
> I'm not sure what it does exactly, but perhaps it is similar to - 
> mpc64?

No, that inline a small set of libcalls into sequences code that  
implement low precision math (6, 8, 12 bits).

Evan

>
> Ciao,
>
> Duncan.

Re: New GCC releases comparison and comparison of GCC4.4 and LLVM2.5 on SPEC2000

[Toon Moene]

Vladimir Makarov wrote:

> People are complaining about GCC compilation speed and 
> of course we should work on its speedup.  But GCC is not so bad, for 
> example SUN Studio compiler is almost 2 times slower than GCC.

Well, outside of these comparisons, I often completely flatten my 
colleagues (at home and abroad) because I compile the full 10^6 lines of 
Fortran code HIRLAM weather forecasting system in less than 5 minutes on 
my quad core PC (using make -j8).  That's using -O3 (i.e., including 
vectorization).

-- 
Toon Moene - e-mail: toon@moene.org - phone: +31 346 214290
Saturnushof 14, 3738 XG  Maartensdijk, The Netherlands
At home: http://moene.org/~toon/
Progress of GNU Fortran: http://gcc.gnu.org/gcc-4.4/changes.html

Re: New GCC releases comparison and comparison of GCC4.4 and LLVM2.5 on SPEC2000

[Chris Lattner]

On May 13, 2009, at 5:26 AM, Duncan Sands wrote:

> Hi Richard,
>
>> -mpc64 sets the x87 floating point control register to not use the  
>> 80bit
>> extended precision.  This causes some x87 floating point operations
>> to operate faster and there are no issues with the extra roundings  
>> you
>> get when storing an 80bit precision register to a 64bit memory  
>> location.

However, this does break long double, right?

>> Does LLVM support x87 arithmetic at all or does it default to SSE
>> arithmetic in 32bits?  I guess using SSE math for both would be a  
>> more
>> fair comparison?
>
> LLVM does support the x86 floating point stack, though it doesn't  
> support
> all asm expressions for it (which is why llvm-gcc disables math  
> inlines).
> My understanding is that no effort has been made to produce optimal  
> code
> when using the x86 fp stack, and all the effort when into SSE instead.

As long as you configure llvm-gcc with --with-arch=pentium-4 or later,  
you should be fine.  I personally use --with-arch=nocona.

-Chris

Re: New GCC releases comparison and comparison of GCC4.4 and LLVM2.5 on SPEC2000

[Vincent Lefevre]

On 2009-05-25 12:53:49 -0700, Chris Lattner wrote:
> On May 13, 2009, at 5:26 AM, Duncan Sands wrote:
>>> -mpc64 sets the x87 floating point control register to not use the
>>> 80bit extended precision. This causes some x87 floating point
>>> operations to operate faster and there are no issues with the
>>> extra roundings you get when storing an 80bit precision register
>>> to a 64bit memory location.
>
> However, this does break long double, right?

Unless the parameters associated with long double (among LDBL_MANT_DIG,
LDBL_DIG, LDBL_MIN_EXP, LDBL_MIN_10_EXP, LDBL_MAX_EXP, LDBL_MAX_10_EXP,
LDBL_MAX, LDBL_EPSILON, LDBL_MIN) are changed accordingly.

-- 
Vincent Lefèvre <vincent@vinc17.org> - Web: <http://www.vinc17.org/>
100% accessible validated (X)HTML - Blog: <http://www.vinc17.org/blog/>
Work: CR INRIA - computer arithmetic / Arenaire project (LIP, ENS-Lyon)

Re: New GCC releases comparison and comparison of GCC4.4 and LLVM2.5 on SPEC2000

[Joseph S. Myers]

On Mon, 25 May 2009, Chris Lattner wrote:

> On May 13, 2009, at 5:26 AM, Duncan Sands wrote:
> 
> > Hi Richard,
> > 
> > > -mpc64 sets the x87 floating point control register to not use the 80bit
> > > extended precision.  This causes some x87 floating point operations
> > > to operate faster and there are no issues with the extra roundings you
> > > get when storing an 80bit precision register to a 64bit memory location.
> 
> However, this does break long double, right?

I already said in <http://gcc.gnu.org/ml/gcc/2009-05/msg00341.html> that 
it is a purely link-time option that does not cause 
TARGET_96_ROUND_53_LONG_DOUBLE to be set; fixing that would make long 
double behave correctly, although not necessarily calls to libgcc, 
libstdc++, libm or other library functions that did not expect to be 
called in the different mode, unless extra multilibs built to expect that 
mode are added (and the source code to libraries such as libm adjusted to 
handle this).

-- 
Joseph S. Myers
joseph@codesourcery.com