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Has anyone seen any real world numbers for different programs which are using the feedback optimization that C/C++ compilers offer to support the branch prediction, cache preloading functions etc.
I searched for it and amazingly not even the popular interpreter development groups seem to have checked the effect. And increasing ruby,python,php etc. performance by 10% or so should be considered usefull.
Is there really no benefit or is the whole developer community just to lazy to use it?
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Compiler optimization is generally implemented using a sequence of optimizing transformations, algorithms which take a program and transform it to produce a semantically equivalent output program that uses fewer resources or executes faster.
The compiler optimizes to reduce the size of the binary instead of execution speed. If you do not specify an optimization option, gcc attempts to reduce the compilation time and to make debugging always yield the result expected from reading the source code.
No. GCC passes your assembly source through the preprocessor and then to the assembler. At no time are any optimisations performed.
Use the command-line option -O0 (-[capital o][zero]) to disable optimization, and -S to get assembly file. Look here to see more gcc command-line options.
10% is a good ballpark figure. That said, ...
You have to REALLY care about the performance to go this route. The product I work on (DB2) uses PGO and other invasive and agressive optimizations. Among the costs are significant build time (triple on some platforms) and development and support nightmares.
When something goes wrong it can be non-trivial to map the fault location in the optimized code back to the source. Developers don't usually expect that functions in different modules can end up merged and inlined and this can have "interesting" effects.
Problems with pointer aliasing, which are nasty to track down also usually show up with these sorts of optimizations. You have the additional fun of having non-deterministic builds (an aliasing problem can show up in monday's build, vanish again till thursday's, ...).
The line between what is correct or incorrect compiler behaviour under these sorts of aggressive optimizations also becomes fairly blurred. Even with the luxury of having our compiler guys in house (literally) the optimization issues (either in our source or the compiler) are still not easy to understand and resolve.
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