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

[Andrew Haley <aph at redhat dot com>]

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?

 > 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.

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.

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