Optimization is the process of ' Guided Mode automatically diagnoses your model, finds (and even fixes) modeling problems, and helps you select the best optimization method and Solver Engine. Structure optimization This program performs basic mean-variance optimization with arbitrary bounds on portfolio weights. The user can choose to compute the portfolio that either.
- The exercise Optimization appears under the Differential calculus Math Mission. This exercise teaches users optimization with Calculus.
- The use of optimization software requires that the function f is defined in a suitable programming language and connected at compile or run time to the optimization.
GCC optimization - Gentoo Wiki. This guide provides an introduction to optimizing compiled code using safe, sane CFLAGS and CXXFLAGS. It also as describes the theory behind optimizing in general. While these variables are not standardized, their use is essentially ubiquitous and any correctly written build should understand these for passing extra or custom options when it invokes the compiler. See the GNU make info page for a list of some of the commonly used variables in this category.
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The GCC manual maintains a complete list of available options and their purposes. In Gentoo- based systems, set the CFLAGS and CXXFLAGS variables in /etc/portage/make. Variables set in this file will be exported to the environment of programs invoked by portage such that all packages will be compiled using these options as a base.
Make sure the CFLAGS declaration is on a single line, with as little whitespace as possible to avoid issues. See bug #5. 00. 03. As seen in the example above the CXXFLAGS variable is set to use all the options present in CFLAGS.
Most every system should be configured in this manner. Additional options for CXXFLAGS are less common and don't usually apply generally enough to deserve setting them globally. The point of diminishing performance returns is reached rather quickly when dealing with CFLAGS. Don't set them arbitrarily. Individual packages further modify these options either in the ebuild or the build system itself to generate the final set of flags used when invoking the compiler. These will hold in good stead and will be endearing to developers the next time a problem is reported on Bugzilla. Remember: aggressive flags can ruin code!).
Optimizing. The basics. The goal behind CFLAGS and CXXFLAGS is to create code tailor- made to the system; it should function perfectly while being lean and fast, if possible.
Sometimes these conditions are mutually exclusive, so this guide will stick to combinations known to work well. Ideally, they are the best available for any CPU architecture. For informational purposes, aggressive flag use will be covered later.
Not every option listed on the GCC manual (there are hundreds) will be discussed, but basic, most common flags will be reviewed. If still stumped after viewing the manual, try a search engine or check out the GCC mailing lists.- march. The first and most important option is - march. This tells the compiler what code it should produce for the system's processor architecture (or arch); it tells GCC that it should produce code for a certain kind of CPU. Different CPUs have different capabilities, support different instruction sets, and have different ways of executing code. The - march flag will instruct the compiler to produce specific code for the system's CPU, with all its capabilities, features, instruction sets, quirks, and so on. To find out, run the following command.
To get more details, including march and mtune values, two commands can be used. In this case, the options shown are those enabled for the selected target: user $gcc - c - Q - march=native - -help=target The second command will show the compiler directives for building the header file, but without actually performing the steps and instead showing them on the screen (- ###). The final output line is the command that holds all the optimization options and architecture selection: user $gcc - ### - march=native /usr/include/stdlib. Now lets see - march in action. This example is for an older Pentium III chip.
FILE/etc/portage/make. Pentium III example. CFLAGS=. When this flag is used, GCC will attempt to detect the processor and automatically set appropriate flags for it. However, this should not be used when intending to compile packages for different CPUs! Warning. Do not use - march=native or - mtune=native in the CFLAGS or CXXFLAGS variables of make.
If compiling packages on one computer in order to run them on a different computer (such as when using a fast computer to build for an older, slower machine), then do not use - march=native. The applications built with - march=native on an AMD Athlon 6. CPU will not be able to run on an old VIA C3 CPU. These flags are normally only used when there is no available - march option; certain processor architectures may require - mtune or even - mcpu. Unfortunately, GCC's behavior isn't very consistent with how each flag behaves from one architecture to the next.
Consider using - mtune when generating code for older CPUs such as i. Do not use - mcpu on x. On these architectures, - mtune / - mcpu will sometimes behave just like - march (on x.
Again, GCC's behavior and flag naming is not consistent across architectures, so be sure to check the GCC manual to determine which one should be used. Also, read the GCC manual's list of architecture- specific options, as well as more detailed explanations about the differences between - march, - mcpu, and - mtune.- ONext up is the - O variable. This variable controls the overall level of optimization. Changing this value will make the code compilation take more time and will use much more memory, especially as the level of optimization is increased. Only use one of them in /etc/portage/make. With the exception of - O0, the - O settings each activate several additional flags, so be sure to read the GCC manual's chapter on optimization options to learn which flags are activated at each - O level, as well as some explanations as to what they do. This reduces compilation time and can improve debugging info, but some applications will not work properly without optimization enabled.
This option is not recommended except for debugging purposes.- O1: the most basic optimization level. The compiler will try to produce faster, smaller code without taking much compilation time. It is basic, but it should get the job done all the time.- O2: A step up from - O1. The recommended level of optimization unless the system has special needs. With - O2, the compiler will attempt to increase code performance without compromising on size, and without taking too much compilation time.- O3: the highest level of optimization possible.
It enables optimizations that are expensive in terms of compile time and memory usage. Using - O3 is not recommended.- Os: optimizes code for size.
It activates all - O2 options that do not increase the size of the generated code. It can be useful for machines that have extremely limited disk storage space and/or CPUs with small cache sizes.- Og: In GCC 4. Og, has been introduced. It addresses the need for fast compilation and a superior debugging experience while providing a reasonable level of runtime performance.
Overall experience for development should be better than the default optimization level - O0. This option breaks strict standards compliance, and is not recommended for use. As previously mentioned, - O2 is the recommended optimization level.
If package compilation fails and while not using - O2, try rebuilding with that option. As a fallback option, try setting the CFLAGS and CXXFLAGS to a lower optimization level, such as - O1 or even - O0 - g. This flag has no effect on the generated code, but it makes the compilation process faster. It tells the compiler to use pipes instead of temporary files during the different stages of compilation, which uses more memory. On systems with low memory, GCC might get killed. In those cases do not use this flag.
It is turned on at all levels of - O (except - O0) on architectures where doing so does not interfere with debugging (such as x. In that case add it to the flags. Though the GCC manual does not specify all architectures, it is turned on by using the - O option.
It's still necessary to explicitly enable the - fomit- frame- pointer option, to activate it on x. GCC up to version 4. Os on x. 86- 3. 2 with any version of GCC. However, using - fomit- frame- pointer will make debugging hard or impossible.
So while the flag can help, it also makes debugging harder; backtraces in particular will be useless. When not doing software debugging and no other debugging- related CFLAGS such as - ggdb have been used, then try using - fomit- frame- pointer. Using the latter flag is discouraged, as - fomit- frame- pointer already does the job properly.
Furthermore, - momit- leaf- frame- pointer has been shown to negatively impact code performance.- msse, - msse. These flags enable the Streaming SIMD Extentions (SSE), SSE2, SSE3, MMX, and 3. DNow! These are useful primarily in multimedia, gaming, and other floating point- intensive computing tasks, though they also contain several other mathematical enhancements. These instruction sets are found in more modern CPUs. The output will include any supported additional instruction sets. Note that pni is just a different name for SSE3.
Normally none of these flags need to be added to /etc/portage/make. Some notable exceptions are newer VIA and AMD6. CPUs that support instructions not implied by - march (such as SSE3).
For CPUs like these additional flags will need to be enabled where appropriate after checking /proc/cpuinfo. If an instruction is listed, then it does not need to be separately specified; it will be turned on by using the proper - march setting. Optimization FAQs. But I get better performance with - funroll- loops - fomg- optimize!
No, people only think they do because someone has convinced them that more flags are better. Aggressive flags will only hurt applications when used system- wide. Even the GCC manual says that using - funroll- loops and - funroll- all- loops will make code larger and run more slowly. Yet for some reason, these two flags, along with - ffast- math, - fforce- mem, - fforce- addr, and similar flags, continue to be very popular among ricers who want the biggest bragging rights. Take a good look around the Gentoo Forums and Bugzilla to see what those flags do: nothing good! They will only hurt performance.