What techniques can be used to speed up C++ compilation times?
This question came up in some comments to Stack Overflow question C++ programming style, and I'm interested to hear what ideas there are.
I've seen a related question, Why does C++ compilation take so long?, but that doesn't provide many solutions.
Take a look at the Pimpl idiom here, and here, also known as an opaque pointer or handle classes. Not only does it speed up compilation, it also increases exception safety when combined with a non-throwing swap function. The Pimpl idiom lets you reduce the dependencies between headers and reduces the amount of recompilation that needs to be done.
Wherever possible, use forward declarations. If the compiler only needs to know that SomeIdentifier is a struct or a pointer or whatever, don't include the entire definition, forcing the compiler to do more work than it needs to. This can have a cascading effect, making this way slower than they need to be.
The I/O streams are particularly known for slowing down builds. If you need them in a header file, try #including <iosfwd> instead of <iostream> and #include the <iostream> header in the implementation file only. The <iosfwd> header holds forward declarations only. Unfortunately the other standard headers don't have a respective declarations header.
Prefer pass-by-reference to pass-by-value in function signatures. This will eliminate the need to #include the respective type definitions in the header file and you will only need to forward-declare the type. Of course, prefer const references to non-const references to avoid obscure bugs, but this is an issue for another question.
Use guard conditions to keep header files from being included more than once in a single translation unit.
#pragma once
#ifndef filename_h
#define filename_h
// Header declarations / definitions
#endif
By using both the pragma and the ifndef, you get the portability of the plain macro solution, as well as the compilation speed optimization that some compilers can do in the presence of the pragma once directive.
The more modular and less interdependent your code design is in general, the less often you will have to recompile everything. You can also end up reducing the amount of work the compiler has to do on any individual block at the same time, by virtue of the fact that it has less to keep track of.
These are used to compile a common section of included headers once for many translation units. The compiler compiles it once, and saves its internal state. That state can then be loaded quickly to get a head start in compiling another file with that same set of headers.
Be careful that you only include rarely changed stuff in the precompiled headers, or you could end up doing full rebuilds more often than necessary. This is a good place for STL headers and other library include files.
ccache is another utility that takes advantage of caching techniques to speed things up.
Many compilers / IDEs support using multiple cores/CPUs to do compilation simultaneously. In GNU Make (usually used with GCC), use the -j [N] option. In Visual Studio, there's an option under preferences to allow it to build multiple projects in parallel. You can also use the /MP option for file-level paralellism, instead of just project-level paralellism.
Other parallel utilities:
The more the compiler tries to optimize, the harder it has to work.
Moving your less frequently modified code into libraries can reduce compile time. By using shared libraries (.so or .dll), you can reduce linking time as well.
More RAM, faster hard drives (including SSDs), and more CPUs/cores will all make a difference in compilation speed.
#pragma and guards, you should reverse the order. Some compilers will process an include guard as a #pragma once if it is the first thing in the preprocessed file, but will fail to do so if you add anything other than a comment outside of the #ifndef –
Phuongphycology I work on the STAPL project which is a heavily-templated C++ library. Once in a while, we have to revisit all the techniques to reduce compilation time. In here, I have summarized the techniques we use. Some of these techniques are already listed above:
Although there is no proven correlation between the symbol lengths and compilation time, we have observed that smaller average symbol sizes can improve compilation time on all compilers. So your first goals it to find the largest symbols in your code.
You can use the nm command to list the symbols based on their sizes:
nm --print-size --size-sort --radix=d YOUR_BINARY
In this command the --radix=d lets you see the sizes in decimal numbers (default is hex). Now by looking at the largest symbol, identify if you can break the corresponding class and try to redesign it by factoring the non-templated parts in a base class, or by splitting the class into multiple classes.
You can run the regular nm command and pipe it to your favorite script (AWK, Python, etc.) to sort the symbols based on their length. Based on our experience, this method identifies the largest trouble making candidates better than method 1.
"Templight is a Clang-based tool to profile the time and memory consumption of template instantiations and to perform interactive debugging sessions to gain introspection into the template instantiation process".
You can install Templight by checking out LLVM and Clang (instructions) and applying the Templight patch on it. The default setting for LLVM and Clang is on debug and assertions, and these can impact your compilation time significantly. It does seem like Templight needs both, so you have to use the default settings. The process of installing LLVM and Clang should take about an hour or so.
After applying the patch you can use templight++ located in the build folder you specified upon installation to compile your code.
Make sure that templight++ is in your PATH. Now to compile add the following switches to your CXXFLAGS in your Makefile or to your command line options:
CXXFLAGS+=-Xtemplight -profiler -Xtemplight -memory -Xtemplight -ignore-system
Or
templight++ -Xtemplight -profiler -Xtemplight -memory -Xtemplight -ignore-system
After compilation is done, you will have a .trace.memory.pbf and .trace.pbf generated in the same folder. To visualize these traces, you can use the Templight Tools that can convert these to other formats. Follow these instructions to install templight-convert. We usually use the callgrind output. You can also use the GraphViz output if your project is small:
$ templight-convert --format callgrind YOUR_BINARY --output YOUR_BINARY.trace
$ templight-convert --format graphviz YOUR_BINARY --output YOUR_BINARY.dot
The callgrind file generated can be opened using kcachegrind in which you can trace the most time/memory consuming instantiation.
Although there are no exact solution for reducing the number of template instantiations, there are a few guidelines that can help:
For example, if you have a class,
template <typename T, typename U>
struct foo { };
and both of T and U can have 10 different options, you have increased the possible template instantiations of this class to 100. One way to resolve this is to abstract the common part of the code to a different class. The other method is to use inheritance inversion (reversing the class hierarchy), but make sure that your design goals are not compromised before using this technique.
Using this technique, you can compile the common section once and link it with your other TUs (translation units) later on.
If you know all the possible instantiations of a class you can use this technique to compile all cases in a different translation unit.
For example, in:
enum class PossibleChoices = {Option1, Option2, Option3}
template <PossibleChoices pc>
struct foo { };
We know that this class can have three possible instantiations:
template class foo<PossibleChoices::Option1>;
template class foo<PossibleChoices::Option2>;
template class foo<PossibleChoices::Option3>;
Put the above in a translation unit and use the extern keyword in your header file, below the class definition:
extern template class foo<PossibleChoices::Option1>;
extern template class foo<PossibleChoices::Option2>;
extern template class foo<PossibleChoices::Option3>;
This technique can save you time if you are compiling different tests with a common set of instantiations.
NOTE : MPICH2 ignores the explicit instantiation at this point and always compiles the instantiated classes in all compilation units.
The whole idea behind unity builds is to include all the .cc files that you use in one file and compile that file only once. Using this method, you can avoid reinstantiating common sections of different files and if your project includes a lot of common files, you probably would save on disk accesses as well.
As an example, let's assume you have three files foo1.cc, foo2.cc, foo3.cc and they all include tuple from STL. You can create a foo-all.cc that looks like:
#include "foo1.cc"
#include "foo2.cc"
#include "foo3.cc"
You compile this file only once and potentially reduce the common instantiations among the three files. It is hard to generally predict if the improvement can be significant or not. But one evident fact is that you would lose parallelism in your builds (you can no longer compile the three files at the same time).
Further, if any of these files happen to take a lot of memory, you might actually run out of memory before the compilation is over. On some compilers, such as GCC, this might ICE (Internal Compiler Error) your compiler for lack of memory. So don't use this technique unless you know all the pros and cons.
Precompiled headers (PCHs) can save you a lot of time in compilation by compiling your header files to an intermediate representation recognizable by a compiler. To generate precompiled header files, you only need to compile your header file with your regular compilation command. For example, on GCC:
$ g++ YOUR_HEADER.hpp
This will generate a YOUR_HEADER.hpp.gch file (.gch is the extension for PCH files in GCC) in the same folder. This means that if you include YOUR_HEADER.hpp in some other file, the compiler will use your YOUR_HEADER.hpp.gch instead of YOUR_HEADER.hpp in the same folder before.
There are two issues with this technique:
all-my-headers.hpp). But that means that you have to include the new file in all places. Fortunately, GCC has a solution for this problem. Use -include and give it the new header file. You can comma separate different files using this technique.For example:
g++ foo.cc -include all-my-headers.hpp
Unnamed namespaces (a.k.a. anonymous namespaces) can reduce the generated binary sizes significantly. Unnamed namespaces use internal linkage, meaning that the symbols generated in those namespaces will not be visible to other TU (translation or compilation units). Compilers usually generate unique names for unnamed namespaces. This means that if you have a file foo.hpp:
namespace {
template <typename T>
struct foo { };
} // Anonymous namespace
using A = foo<int>;
And you happen to include this file in two TUs (two .cc files and compile them separately). The two foo template instances will not be the same. This violates the One Definition Rule (ODR). For the same reason, using unnamed namespaces is discouraged in the header files. Feel free to use them in your .cc files to avoid symbols showing up in your binary files. In some cases, changing all the internal details for a .cc file showed a 10% reduction in the generated binary sizes.
In newer compilers you can select your symbols to be either visible or invisible in the Dynamic Shared Objects (DSOs). Ideally, changing the visibility can improve compiler performance, link time optimizations (LTOs), and generated binary sizes. If you look at the STL header files in GCC you can see that it is widely used. To enable visibility choices, you need to change your code per function, per class, per variable and more importantly per compiler.
With the help of visibility you can hide the symbols that you consider them private from the generated shared objects. On GCC you can control the visibility of symbols by passing default or hidden to the -visibility option of your compiler. This is in some sense similar to the unnamed namespace but in a more elaborate and intrusive way.
If you would like to specify the visibilities per case, you have to add the following attributes to your functions, variables, and classes:
__attribute__((visibility("default"))) void foo1() { }
__attribute__((visibility("hidden"))) void foo2() { }
__attribute__((visibility("hidden"))) class foo3 { };
void foo4() { }
The default visibility in GCC is default (public), meaning that if you compile the above as a shared library (-shared) method, foo2 and class foo3 will not be visible in other TUs (foo1 and foo4 will be visible). If you compile with -visibility=hidden then only foo1 will be visible. Even foo4 would be hidden.
You can read more about visibility on GCC wiki.
I'd recommend these articles from "Games from Within, Indie Game Design And Programming":
Granted, they are pretty old - you'll have to re-test everything with the latest versions (or versions available to you), to get realistic results. Either way, it is a good source for ideas.
One technique which worked quite well for me in the past: don't compile multiple C++ source files independently, but rather generate one C++ file which includes all the other files, like this:
// myproject_all.cpp
// Automatically generated file - don't edit this by hand!
#include "main.cpp"
#include "mainwindow.cpp"
#include "filterdialog.cpp"
#include "database.cpp"
Of course this means you have to recompile all of the included source code in case any of the sources changes, so the dependency tree gets worse. However, compiling multiple source files as one translation unit is faster (at least in my experiments with MSVC and GCC) and generates smaller binaries. I also suspect that the compiler is given more potential for optimizations (since it can see more code at once).
This technique breaks in various cases; for instance, the compiler will bail out in case two or more source files declare a global function with the same name. I couldn't find this technique described in any of the other answers though, that's why I'm mentioning it here.
For what it's worth, the KDE Project used this exact same technique since 1999 to build optimized binaries (possibly for a release). The switch to the build configure script was called --enable-final. Out of archaeological interest I dug up the posting which announced this feature: http://lists.kde.org/?l=kde-devel&m=92722836009368&w=2
<core-count> + N sublists that are compiled parallel where N is some suitable integer (depending on system memory and how the machine is otherwise used). –
Wakeless I will just link to my other answer: How do YOU reduce compile time, and linking time for Visual C++ projects (native C++)?. Another point I want to add, but which causes often problems is to use precompiled headers. But please, only use them for parts which hardly ever change (like GUI toolkit headers). Otherwise, they will cost you more time than they save you in the end.
Another option is, when you work with GNU make, to turn on -j<N> option:
-j [N], --jobs[=N] Allow N jobs at once; infinite jobs with no arg.
I usually have it at 3 since I've got a dual core here. It will then run compilers in parallel for different translation units, provided there are no dependencies between them. Linking cannot be done in parallel, since there is only one linker process linking together all object files.
But the linker itself can be threaded, and this is what the GNU gold ELF linker does. It's optimized threaded C++ code which is said to link ELF object files a magnitude faster than the old ld (and was actually included into binutils).
There's an entire book on this topic, which is titled Large-Scale C++ Software Design (written by John Lakos).
The book pre-dates templates, so to the contents of that book add "using templates, too, can make the compiler slower".
Once you have applied all the code tricks above (forward declarations, reducing header inclusion to the minimum in public headers, pushing most details inside the implementation file with Pimpl...) and nothing else can be gained language-wise, consider your build system. If you use Linux, consider using distcc (distributed compiler) and ccache (cache compiler).
The first one, distcc, executes the preprocessor step locally and then sends the output to the first available compiler in the network. It requires the same compiler and library versions in all the configured nodes in the network.
The latter, ccache, is a compiler cache. It again executes the preprocessor and then check with an internal database (held in a local directory) if that preprocessor file has already been compiled with the same compiler parameters. If it does, it just pops up the binary and output from the first run of the compiler.
Both can be used at the same time, so that if ccache does not have a local copy it can send it trough the net to another node with distcc, or else it can just inject the solution without further processing.
Here are some:
make -j2 is a good example).-O1 than -O2 or -O3).-j12 to around -j18 were considerably faster than -j8, just as you suggest. I'm wondering how many cores you can have before memory bandwidth becomes the limiting factor... –
Misadvise -j with 2x the number of actual cores. –
Savannasavannah When I came out of college, the first real production-worthy C++ code I saw had these arcane #ifndef ... #endif directives in between them where the headers were defined. I asked the guy who was writing the code about these overarching things in a very naive fashion and was introduced to world of large-scale programming.
Coming back to the point, using directives to prevent duplicate header definitions was the first thing I learned when it came to reducing compiling times.
More RAM.
Someone talked about RAM drives in another answer. I did this with a 80286 and Turbo C++ (shows age) and the results were phenomenal. As was the loss of data when the machine crashed.
You could use Unity Builds.
Use
#pragma once
at the top of header files, so if they're included more than once in a translation unit, the text of the header will only get included and parsed once.
Use forward declarations where you can. If a class declaration only uses a pointer or reference to a type, you can just forward declare it and include the header for the type in the implementation file.
For example:
// T.h
class Class2; // Forward declaration
class T {
public:
void doSomething(Class2 &c2);
private:
Class2 *m_Class2Ptr;
};
// T.cpp
#include "Class2.h"
void Class2::doSomething(Class2 &c2) {
// Whatever you want here
}
Fewer includes means far less work for the preprocessor if you do it enough.
I had an idea about using a RAM drive. It turned out that for my projects it doesn't make that much of a difference after all. But then they are pretty small still. Try it! I'd be interested in hearing how much it helped.
Upgrade your computer
Then you have all your other typical suggestions
Just for completeness: a build might be slow because the build system is being stupid as well as because the compiler is taking a long time to do its work.
Read Recursive Make Considered Harmful (PDF) for a discussion of this topic in Unix environments.
Not about the compilation time, but about the build time:
Use ccache if you have to rebuild the same files when you are working on your buildfiles
Use ninja-build instead of make. I am currently compiling a project with ~100 source files and everything is cached by ccache. make needs 5 minutes, ninja less than 1.
You can generate your ninja files from cmake with -GNinja.
Starting with Visual Studio 2017 you have the capability to have some compiler metrics about what takes time.
Add those parameters to C/C++ -> Command line (Additional Options) in the project properties window:
/Bt+ /d2cgsummary /d1reportTime
You can have more informations in this post.
Where are you spending your time? Are you CPU bound? Memory bound? Disk bound? Can you use more cores? More RAM? Do you need RAID? Do you simply want to improve the efficiency of your current system?
Under gcc/g++, have you looked at ccache? It can be helpful if you are doing make clean; make a lot.
Dynamic linking (.so) can be much much faster than static linking (.a). Especially when you have a slow network drive. This is since you have all of the code in the .a file which needs to be processed and written out. In addition, a much larger executable file needs to be written out to the disk.
Networks shares will drastically slow down your build, as the seek latency is high. For something like Boost, it made a huge difference for me, even though our network share drive is pretty fast. Time to compile a toy Boost program went from about 1 minute to 1 second when I switched from a network share to a local SSD.
If you have a multicore processor, both Visual Studio (2005 and later) as well as GCC support multi-processor compiles. It is something to enable if you have the hardware, for sure.
On Linux (and maybe some other *NIXes), you can really speed the compilation by NOT STARING at the output and changing to another TTY.
Here is the experiment: printf slows down my program
Faster hard disks.
Compilers write many (and possibly huge) files to disk. Work with SSD instead of typical hard disk and compilation times are much lower.
First of all, we have to understand what so different about C++ that sets it apart from other languages.
Some people say it's that C++ has many too features. But hey, there are languages that have a lot more features and they are nowhere near that slow.
Some people say it's the size of a file that matters. Nope, source lines of code don't correlate with compile times.
But wait, how can it be? More lines of code should mean longer compile times, what's the sorcery?
The trick is that a lot of lines of code is hidden in preprocessor directives. Yes. Just one #include can ruin your module's compilation performance.
You see, C++ doesn't have a module system. All *.cpp files are compiled from scratch. So having 1000 *.cpp files means compiling your project a thousand times. You have more than that? Too bad.
That's why C++ developers hesitate to split classes into multiple files. All those headers are tedious to maintain.
So what can we do other than using precompiled headers, merging all the cpp files into one, and keeping the number of headers minimal?
C++20 brings us preliminary support of modules! Eventually, you'll be able to forget about #include and the horrible compile performance that header files bring with them. Touched one file? Recompile only that file! Need to compile a fresh checkout? Compile in seconds rather than minutes and hours.
The C++ community should move to C++20 as soon as possible. C++ compiler developers should put more focus on this, C++ developers should start testing preliminary support in various compilers and use those compilers that support modules. This is the most important moment in C++ history!
Although not a "technique", I couldn't figure out how Win32 projects with many source files compiled faster than my "Hello World" empty project. Thus, I hope this helps someone like it did me.
In Visual Studio, one option to increase compile times is Incremental Linking (/INCREMENTAL). It's incompatible with Link-time Code Generation (/LTCG) so remember to disable incremental linking when doing release builds.
/INCREMENTAL in debug mode only –
Seep Using dynamic linking instead of static one make you compiler faster that can feel.
If you use t Cmake, active the property:
set(BUILD_SHARED_LIBS ON)
Build Release, using static linking can get more optimize.
I share my experience what additional (sometimes extreme) methods I've tried to reduce compile time even more.
Please note, that updating your compiler, getting a better PC, using parallel compilation, reducing includes gave the most to me as the other answers already mention. Nothing can beat incremental build, but speeding up a clean build is another challenge.
Fixing warnings
The goal was not to speed up compile time, but the end result came with faster build too. Let's say you have enabled a lot of warnings (msvc: /W4 or gcc: -Wall) and you have thousands of warnings. Most of them are easy to fix, especially if they are in header files - fix once and eliminate in each TU.
Why it can improve compilation? Because the compiler needs to report that warning to you. If you use CLI, it needs to print on the console, using an IDE needs to visualize for you.
For our project from ~25 minutes compilation time it was reduced to ~23. Small gain, but recommended.
Not recommended alternative way: You can also switch off all the warnings, probably with the same result.
Running on even more thread
If you have 8 cores it doesn't mean your computer is doing 8 processes. There are way more programs running in the background (depending you usage of course). Because it is constantly swapping between tasks if you launch more threads than cores the compiler gets more CPU time. (Imagine the original cycle is browser-compiler-email, if you launch 3 more compiler threads it might be browser-compiler#1-compiler#2-email-comiler#3-compiler#4 on repeat).
It highly depends on your usage and your codebase. If you have less source files to compile than cores, it adds no benefit. But if you have 1000+ source files that can be compiled parallel and using your computer for less relevant task (or sharing a remote with other projects but you need more resources) you might give it a try.
Blog post for MsBuild, and relevant CMake solution
Messing with thread affinity
Note! As the task manager says:
Changing the priority of certain processes could cause system instability.
Use it on your on risk and never change the process priority on a build agent.
You can either set the priority to the max or to the lowest. Initially I launched the build process with the highest priority, causing my system to lag for hopefully some saved seconds. During the build the computer was unusable, so I cannot recommend this anymore.
So instead, I made a 'background' build script - that launched the build with 'lower than normal' priority. Because usually the developers don't have to focus on multiple things, I cloned the project twice on different branches, and always launched a background build on one of them while working on the other. So when I was able to switch context, a fresh build was waiting for me. Combined with incremental build was a huge benefit for me (saving myself from going insane of the wait).
This is the batch script:
build_low.bat
start /belownormal /d "C:\my_workspace" build.bat
Where build.bat can build my application. See the documentation of starting a program here.
Different build types
As others already said, different optimization levels have huge impact on the build time.
Rule of thumb: by default Debug build configuration is faster to build than a
Release build config.
That means most of the time you would choose for the Debug mode to save time and delay the release build as late as you can. (So first make sure unit tests are in place, the feature works correctly, and not introducing new bugs. Then you switch to release mode.)
On the contrary, if you can live without debug symbols in release mode (never planed to collect crash dump from users and analyze it) disable the debug info generation to speed up the build. From MSVC debug information flags:
By default, if no debug information format option is specified, the compiler produces no debugging information, so compilation is faster.
From Microsoft: https://devblogs.microsoft.com/cppblog/recommendations-to-speed-c-builds-in-visual-studio/
Specific recommendations include:
DO USE PCH for projects
DO include commonly used system, runtime and third party headers in PCH
DO include rarely changing project specific headers in PCH
DO NOT include headers that change frequently
DO audit PCH regularly to keep it up to date with product churn
DO USE /MP
DO Remove /Gm in favor of /MP
DO resolve conflict with #import and use /MP
DO USE linker switch /incremental
DO USE linker switch /debug:fastlink
DO consider using a third party build accelerator
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