Well... When i was searching for a good compiler I came across clang/LLVM. This compiler gives me same result as other compilers like icc, pgi. But the problem is there are very few tutorials on this compiler... Kindly let me know where can I find the tutorials on the clang compiler.
Note by:
I have compiled my c code using the following flags clang -O3 -mfpmath=sse file.c
Clang (the command line compiler) takes gcc-compatible options, but accepts and ignores a lot of flags that GCC takes (like -mfpmath=sse). We aim to generate good code out of the box. There are some flags that allow clang to violate the language standards that can be useful in some scenarios, like -ffast-math though.
If you're looking for good performance, I highly recommend experimenting with link-time-optimization, which allows clang to optimize across source files in your application. Depending on what platform you're on, this is enabled by passing -O4 to the compiler. If you're on linux, you need to use the "gold" linker (see http://llvm.org/docs/GoldPlugin.html). If you're on the mac, it should "just work" with any recent version of Xcode.
The clang is not a compiler, it is just frontend of LLVM compiler. So, when you calls clang, it parses c/c++ file but the optimization and code generation is handled in LLVM itself.
Here you can found a documentation of LLVM optimization and analysis options: http://llvm.org/docs/Passes.html
The full documentation is here http://llvm.org/docs/
Also useful options are listed here http://linux.die.net/man/1/llvmc (I suggest clang will accept most of them too)
Related
I read about tool called "include-what-you-use" which can help clean superfluous includes
from source code. I understood that there is a version for compiler LLVM(clang) and version for GCC.
My questions are:
Why this tool is compiler-dependent and not "cross-platform" for compilers. Why from the beginning the creators of the tool didn't make it compiler-independent? Is it related to the special implementation it has or something like that?
If I want to take a version of the tool compatible for LLVM and I want to make it compatible with GCC (since I'm working with GCC), what do I have to do for that?
For the most part, Include-What-You-Use should be able to handle any valid C++ codebase, regardless of whether that codebase was written with gcc or clang in mind. I recently had the occasion to run Include-What-You-Use on a very large codebase that was usually compiled with gcc and it worked fine. So in that sense it is already compatible.
That said, it might not work perfectly. It's likely that some of the information it provides will be wrong, even if it's a clang codebase. So always verify the output manually.
why this tool is compiler-dependent and not "cross-platform" for compilers. Why from the beginning the creaters of the tool didn't make
it compiler-independent ? is it related to the special implementation
it has or something like that ?
Reason is simple, clang has is more modern fresh and has better modular architecture. As a result is is much easier to create tools using clang modules.
That is why clang was first which had address sanitizer and have more cool sanitizers. And this is why when someone creates new tool for C++ stars from clang.
If i want to take a version of the tool compatible for llvm and i want to make it compatible with gcc(since i'm working with gcc). What
i have to do for that ?
clang was created since Apple was not happy with gcc. So when it was written it supposed to be as much compatible with gcc as possible, since there was lots of code which was verified with gcc.
Since clang is mature now and provides new own features, there might be small differences with gcc (for example different bugs in implementations of new C++ standards), but in most common code there should be no problem.
IWYU should work without problems on both compilers. My coworker used it on large project build with 3 compilers (clang, gcc and VS) and it worked like a charm.
The tool itself needs parts of the compiler! It is sitting somewhere between reading the source and parsing it. LLVM provides an API which is used for the tool. The tool itself is not standalone but a plugin to the clang/llvm. As this, it needs the clang/llvm.
The modifications which will be done by the tool are fully compatible to every c++ compiler. And also the plugin in combination with clang/llvm should be able to parse more or less every code base independent of used other compilers. There might be some strange macros which are supported by other tool chains, which llvm might be struggle with. But that should be a rare case at all.
I was searching online to see if clang supported reproducible builds. I read that GCC guaranteed reproducible builds using the -frandom-seed flag. I wanted to know if clang supports that flag and I could not find anything regarding that.I then came here which had a statement such as:
...two consecutive builds of (GCC-built) Clang
My question is what is GCC built clang ? I am currently aware of only 2 compilers (Microsoft , GCC (Coudl be Cygwin/Mingw flavor) ) and the third one was suppose to be clang. My question is what does clang (GCC built) mean ? Built from source ? I would like to think that clang is a totally different compiler from GCC and Windows. Also this documentation here states
Clang has experimental support for targeting “Cygming” (Cygwin /
MinGW) platforms.
What does that mean ? Does clang mean that it uses Mingw GCC as a compiler ? What does targeting mean here ?
To my mind, this phrase means clang was built from source using GCC as a compiler. Then, clang is a compiler, so it can't use GCC as a compiler.
Compilers are written in programming languages to be able to compile code written in a programming language. This means, a compiler can compile a compiler or even itself.
If you don't know is feature X supported in product Y, please, read the docs on product Y. If this feature isn't mentioned, it's not supported and vice versa.
Is there a website I can go to to run C++ code online? I have seen a few compilers, but is there one that can also receive input from cin? I want one that can basically host a C++ .exe online, and even more preferable is if it is like the VC++ platform.
You may want to give rextester a try. Currently g++ 4.7.2 with boost libraries and ability to supply compiler args.
Edit
Visual C++ is also supported now.
http://ideone.com/ Can compile several languages, and supports stdin
Yes, Ideone can do that. It's possible to specify stdin there.
Edit: Microsoft's online compiler seems to no longer be active :(
Microsoft research have made an online version of Visual C++ here: http://webcompiler.cloudapp.net/.
It (currently?) only compiles the code and does not run it so you won't be able to check cin though :(
Running code is now supported.
Currently the best online tools seem to be
Special Online Compilers
https://cppinsights.io shows, how C++ introduces conversions, etc. and interprets the source code (current clang based)
https://godbolt.org compare C++ compilers, show disassembly, run (many current compilers including Microsoft VCC and embedded compilers)
http://quick-bench.com create and compare benchmarks (current g++ and clang)
https://www.onlinegdb.com, includes gdb debugger (g++ 7.4.0)
Online Compilers that let you set compile flags
https://wandbox.org (current g++ and clang)
http://coliru.stacked-crooked.com (g++ 9.2.0)
https://repl.it/languages/cpp11 (clang 7.0.0)
https://www.tutorialspoint.com/compile_cpp11_online.php = codingground (g++ 7.1.1)
https://tio.run/#cpp-clang (g++ 8.3.1 and clang 7.0.1)
https://rextester.com/l/cpp_online_compiler_gcc (g++ 7.4.0, clang 6.0.0, VS C++ 2015 Update 1)
Online Compilers that do not let you set compile flags
https://www.jdoodle.com (g++ 9.2.0)
https://www.codechef.com/ide (g++ 9.1.0)
https://ideone.com (g++ 8.3.0 and clang 8.0.0)
https://paiza.io (clang 8.0.0)
https://www.codiva.io/c (clang 7.1.0)
https://code.labstack.com/cpp (g++ 8.3.0)
All listed compilers support C++17 code
Perhaps the following site is what you're aiming for: http://codepad.org/
There is Wandbox.org, which offers GCC and Clang to its latest versions (as of Nov 2018). It has stdin.
I wouldn't say it's a VC++ platform though.
TutorialsPoint's C++ Coding Ground lets you save and compile multiple source files at a time, which can be useful at times, and gives you a GNU bash console to play around with. It's not as convenient for sharing code as sites like Ideone, however.
[Note: While it only has links to C++, C++0x, and C++11 environments, you can compile code for C++14 by specifying the compiler option -std=c++1y. I'm not sure if it has any experimental C++17 support, however.]
repl.it can compile, run, and execute C++ and C++11. It's also interactive and you can ask for user input in realtime.
I have a question concerning llvm, clang, and gcc on OS X.
What is the difference between the llvm-gcc 4.2, llvm 2.0 and clang? I know that they all build on llvm but how are they different?
Besides faster compiling, what is the advantage of llvm over gcc?
LLVM originally stood for "low-level virtual machine", though it now just stands for itself as it has grown to be something other than a traditional virtual machine. It is a set of libraries and tools, as well as a standardized intermediate representation, that can be used to help build compilers and just-in-time compilers. It cannot compile anything other than its own intermediate representation on its own; it needs a language-specific frontend in order to do so. If people just refer to LLVM, they probably mean just the low-level library and tools. Some people might refer to Clang or llvm-gcc incorrectly as "LLVM", which may cause some confusion.
llvm-gcc is a modified version of GCC, which uses LLVM as its backend instead of GCC's own. It is now deprecated, in favor of DragonEgg, which uses GCC's new plugin system to do the same thing without forking GCC.
Clang is a whole new C/C++/Objective-C compiler, which uses its own frontend, and LLVM as the backend. The advantages it provides are better error messages, faster compile time, and an easier way for other tools to hook into the compilation process (like the LLDB debugger and Clang static analyzer). It's also reasonably modular, and so can be used as a library for other software that needs to analyze C, C++, or Objective-C code.
Each of these approaches (plain GCC, GCC + LLVM, and Clang) have their advantages and disadvantages. The last few sets of benchmarks I've seen showed GCC to produce slightly faster code in most test cases (though LLVM had a slight edge in a few), while LLVM and Clang gave significantly better compile times. GCC and the GCC/LLVM combos have the advantage that a lot more code has been tested and works on the GCC flavor of C; there are some compiler specific extensions that only GCC has, and some places where the standard allows the implementation to vary but code depends on one particular implementation. It is a lot more likely if you get a large amount of legacy C code that it will work in GCC than that it will work in Clang, though this is improving over time.
There are 2 different things here.
LLVM is a backend compiler meant to build compilers on top of it. It deals with optimizations and production of code adapted to the target architecture.
CLang is a front end which parses C, C++ and Objective C code and translates it into a representation suitable for LLVM.
llvm gcc was an initial version of a llvm based C++ compiler based on gcc 4.2, which is now deprecated since CLang can parse everything it could parse, and more.
Finally, the main difference between CLang and gcc does not lie in the produced code but in the approach. While gcc is monolithic, CLang has been built as a suite of libraries. This modular design allow great reuse opportunities for IDE or completion tools for example.
At the moment, the code produced by gcc 4.6 is generally a bit faster, but CLang is closing the gap.
llvm-gcc-4.2 uses the GCC front-end to parse your code, then generates the compiled output using LLVM.
The "llvm compiler 2.0" uses the clang front-end to parse your code, and generates the compiled output using LLVM. "clang" is actually just the name for this front-end, but it is often used casually as a name for the compiler as a whole.
I have a question concerning the LLVM compiler:
I would like to use it to compile my Objective-C source code for Mac and iOS and from their release notes it seems that LLVM is stable enough for using this.
Having made good experiences with the LLVM I would also like to use it to compile C++ or Objective-C++. However it is not clear to me if I should still use the hybrid LLVM-GCC compiler (the GCC parser and the LLVM code generator) or the pure LLVM compiler.
I am also unsure about the new C++ standard library and if I should use it and how I would make the transition from GNU's libstdc++.
The Questions
Which compiler would one use today to generate fast production quality code from C++: the LLVM-GCC hybrid compiler or pure LLVM?
Should one migrate the C++ standard library from GNU's libstdc++ to the new libc++ library created by the LLVM project?
Any comments and hints are appreciated.
Several questions asked here, I'll try to answer all of them.
There is no "pure LLVM compiler". LLVM is a set of libraries which do code optimization and code generation . There are several C/C++ frontends which can be hooked to LLVM. Among them are clang and llvm-gcc. See http://llvm.org/ for more information about various components of LLVM Compiler Infrastructure. As written at http://llvm.org/docs/ReleaseNotes.html, llvm-gcc is EOL since LLVM 2.9 release, so you'd better use clang, because it will certainly be developed and maintained in the future.
libc++ is still in development, so for production you should use vendor-provided C++ (libstdc++ in your case).
Remember, all this stuff is changing, so benchmarks gets easily outdated.
I've found following report interesting, not only as a kind of benchmark, but it seems showing some LLVM vs GCC compiler differences :
Clang/LLVM Maturity Evaluation Report by Dominic Fandrey