Re: powercp-linux cross GCC 4.2 vs GCC 4.0.0: -Os code size regression?

[Sergei Poselenov]

Hello,

I've just noted an error in my calculations: not 40%, but 10%
regression (used gdb to do the calculations and forgot to convert
inputs to float). Sorry.
But the problem still persists for me - I'm building an embedded
firmware (U-Boot) and it doesn't fit into the reserved space
anymore.

Andrew Haley wrote:
Sergei Poselenov writes:
 > Hello all,
 > 
 > I'm using the ppc-linux gcc-4.2.2 compiler and noted the code
 > size have increased significantly (about 40%!), comparing with
 > old 4.0.0 when using the -Os option. Same code, same compile-
 > and configuration-time options. Binutils are differ
 > (2.16.1 vs 2.17.50), though.
 > 
 > I've looked at the CSiBE testing results for ppc-elf with -Os,
 > comparing gcc_4_0_0 with mainline and found that the mainline
 > actually optimizes better, at least for the CSiBE test environment.
 > After some analysis I've came to the following results:
 >   Number of packages in the CSiBE test environment: 863
 >   N of packages where mainline GCC optimizes better:   290
 >   N of packages where mainline GCC optimizes worse: 436
 > 
 > And the regression in code size is up to 40%, like in my case.

40% seems severe, but it may be an outlier.  What is the average
increase in code size, including the packages where it got better?



Specifically, in my case the digits are as follows (as reported by
'size'):
gcc 4.2.2:
   text    data     bss     dec     hex filename
   2696      60    1536    4292    10c4 interrupts.o

gcc 4.0.0:
 text    data     bss     dec     hex filename
   2424      88    1536    4048     fd0 interrupts.o

(about 10% regression)

As for the CSiBE results - the average regression is
3%, including top 3 winners:
100% (32768 vs 16384 for "linux-2.4.23-pre3-testplatform - 
arch/testplatform/kernel/init_task")
35% (1440 vs 1064 for "teem-1.6.0-src - src/air/enum")
34% (1712 vs 1280 for "teem-1.6.0-src - src/nrrd/encodingHex")

(See 
http://www.inf.u-szeged.hu/csibe/osingle.php?branchid=mainline&targetid=ppc-elf&timestamp=2008-01-15%2012:00:00&flags=-Os&csibever=2.x.x&projectid=---%20all%20files%20---&finish_button=Finish
for mainline results, and
http://www.inf.u-szeged.hu/csibe/osingle.php?branchid=gcc_4_0_0_release&targetid=ppc-elf&timestamp=2003-01-01%2012:00:00&flags=-Os&csibever=2.x.x&projectid=---%20all%20files%20---&finish_button=Finish
for gcc_4_0_0_release results)

For comparison bar chart see
http://www.inf.u-szeged.hu/csibe/ocomp.php?branchid_a=gcc_4_0_0_release&branchid_b=mainline&targetid_a=ppc-elf&targetid_b=ppc-elf&timestamp_a=2003-01-01%2012:00:00&timestamp_b=2008-01-15%2012:00:00&flags_a=-Os&flags_b=-Os&csibever_a=2.x.x&csibever_b=2.x.x&dataview=Code%20size&viewmode=Bar%20chart&finish_button=Finish


 > What I'm missing here? Apparently, just "-Os" is not enough to
 > get the best size-optimized code?

Not necessarily.  -Os is mostly [*] the same as -O2 but with
optimizations that might reasonably be expected to cause code size
increase disabled.  You might find some better combination of options
for your code base.


Understood. I raised the question in a hope that somebody already
have a set of useful options for powerpc cross to try with -Os.

Thanks,
Sergei
Andrew.

[*] Full disclosure:  There are a few optimizations, such as code
hoisting, that are specially used with -Os.  There are also some
passes with special tuning for -Os.