maria-discuss team mailing list archive

[Reindl Harald <h.reindl@xxxxxxxxxxxxx>]

Am 21.01.2017 um 06:59 schrieb Reindl Harald:
> why does MariaDB use "-g -static-libgcc" in "mysql_release.cmake" and so
> besides override -g0 break -flto builds?

-O3 -g -static-libgcc

it also should not override with -O3 because the previous -O3 followed 
by some -fno-options has a reason - not enable optimizations which are 
pointless and in doubt doing more harm than good

-O3 -fno-strict-aliasing -fno-gcse-after-reload -fno-inline-functions 
-fno-ipa-cp -fno-ipa-cp-clone -fno-peel-loops -fno-predictive-commoning 
-fno-tree-loop-distribute-patterns -fno-unswitch-loops

see deeper informations below, so the blank -O3 is pointless and not 
breaking -flto would gain much more in context of performance
________________________________

https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html
-O3 enables options which are clearly statet as "generally profitable 
only with profile feedback" and so just blows up the binary size for no 
benefit

doing a profile-guided-optimization is not done with just a simple build

you need two builds and running the application with typical 
use-cases/benchmarks while the first build instructs GCC to generate 
profiling code, the usage of the intermediate-build then stores runtime 
informations in .gcda files and after a "make prof-clean" which deletes 
anything but the .gcda files "-fprofile-use" is using that informations 
for otpimize the binary

-fprofile-use=path
Enable profile feedback-directed optimizations, and the following 
optimizations which are generally profitable only with profile feedback 
available: -fbranch-probabilities, -fvpt, -funroll-loops, -fpeel-loops, 
-ftracer, -ftree-vectorize, and ftree-loop-distribute-patterns.

-fauto-profile=path
Enable sampling-based feedback-directed optimizations, and the following 
optimizations which are generally profitable only with profile feedback 
available: -fbranch-probabilities, -fvpt, -funroll-loops, -fpeel-loops, 
-ftracer, -ftree-vectorize, -finline-functions, -fipa-cp, 
-fipa-cp-clone, -fpredictive-commoning, -funswitch-loops, 
-fgcse-after-reload, and -ftree-loop-distribute-patterns.