why they use this before#include<bits/stdc++.h>
mainly I've been using #include ...
Now I'm seeing these lines on a cpp program so I became curious.
#pragma optimization_level 3
#pragma GCC optimize("Ofast,no-stack-protector,unroll-loops,fast-math,O3")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx")
#pragma GCC optimize("Ofast")//Comment optimisations for interactive problems (use endl)
#pragma GCC target("avx,avx2,fma")
#pragma GCC optimization ("unroll-loops")
These are indications to the compiler to change its behaviour as if you had passed the equivalent command line flags. For programming competitions, you often submit source code rather than a binary - it's then built and tested using a system you don't control (and can't set the command line on). Putting these settings in as #pragma lines lets you control settings you might not otherwise be able to do in the competition environment.
Related
I have a C single-header library that I would like to use in my C++ project. Normally, I would just include the file and that would be fine because C++ is almost a superset of C. However, this library has a goto that jumps over an initialization, violating the C++ standard.
I can get around this be enabling the -fpermissive compiler flag on GCC, but I want the errors to trigger properly for the rest of my code.
Is there a way I can enable it just for this one header file (perhaps similar to #pragma GCC diagnostic XXX).
There's #pragma GCC optimize "blah" or the function attribute __attribute__((optimize("blah"))) that act like the argument -fblah was given for the rest of that file/that specific function, but it doesn't seem to work with -fpermissive:
$ cat foo.cpp
#pragma GCC optimize "permissive"
void foo(int x) {
}
$ g++-8 -c -Wall -Wextra foo.cpp
foo.cpp:1:22: warning: bad option ‘-fpermissive’ to pragma ‘optimize’ [-Wpragmas]
#pragma GCC optimize "permissive"
^~~~~~~~~~~~
foo.cpp:3:16: warning: bad option ‘-fpermissive’ to attribute ‘optimize’ [-Wattributes]
void foo(int x) {
One option would be to put the function that needs this in a file by itself, and configure your build system to compile just that file with -fpermissive, though that breaks the header-only model. Or fix the code so it doesn't need that option at all.
Per gcc man page:
-fpermissive
Downgrade some diagnostics about nonconformant code from errors to warnings. Thus, using -fpermissive allows some nonconforming code to compile.
So in theory, one can allow a section to compile as permissive using a sequence of gcc warning pragmas:
// Start permissive code
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wunused-result"
#pragma GCC diagnostic ignored "-Wuninitialized"
// .. Other #pragmas
// Permissive Code here.
// Restore normal processing.
#pragma GCC diagnostic pop
One challenge is that there is no published list of errors that will be ignored with -fpermissive (At least I could not find it). One possible approach will be to compile the code, and enter the '#pragma's one at at a time, until the code compiles cleanly.
If one can identify all (or most) of the rules, possible to put them into #include file.
#pragma GCC diagostic push
#include "permissive.h"
// Permissive Code here
#pragma GCC diagostic pop
At a company I used to work at, the build system was set up to turn warnings into errors only within changed code. It worked by supplying generating a diff (typically between the branch you were trying to merge and master and then supplying that diff to some compilation tool, and the tool would produce warnings only within the supplied diff.
This was great as it allowed you to e.g. deprecate some function, and have the build system prevent people from introducing new uses of that function, and then remove old usages of that function later.
Unfortunately, I didn't look at the setup closely enough before I left the company, and don't know how to replicate it. My question: How can I replicate this setup?
Question is tagged Clang but I would also be interested in answers that use tooling from other compilers.
If I had to implement that, my first idea would be:
Get merged file.
Analyze diff to figure out which regions were changed.
Generate a new file and inject #pragma directives1 that locally enable/disable warnings around the changed regions.
Also inject #line directives to make it look like warnings/errors are coming from the original file.
Compile modified file and save compiler warnings/errors.
Delete modified file.
Present compiler diagnostics to the user.
1 E.g. https://gcc.gnu.org/onlinedocs/gcc/Diagnostic-Pragmas.html#Diagnostic-Pragmas for GCC.
Clang supports GCC's #pragma diagnostic.
For example:
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wuninitialized"
// ... changed code here ...
#pragma GCC diagnostic pop
MSVC also has something similar:
#pragma warning(push, 3)
// ... changed code here ...
#pragma warning(pop)
In existing code I saw #pragma once be used after header #includes
//Some_Header.h
#include "header1.h"
#include "header2.h"
#pragma once
//implementations
Instead of
//Some_Header.h
#pragma once
#include "header1.h"
#include "header2.h"
//implementations
I thought it always needed to be like the second example, does it matter where your #pragma once is defined or does the preprocessor pick it up anywhere in your file?
Edit
I know #pragma once is not part of the standard and include guards are but that is not my question.
#pragma once should be placed before any headers are included. Argument of #pragma directive is a subject to macro expansion. So content of included headers can alter the pragma behavior:
// whatever.hpp
...
#define once lol_no
// your_header.hpp
#include "whatever.hpp"
#pragma once // warning C4068: unknown pragma
#pragma once relates only to a file where it is placed. It matters to the compiler whether the file contains this pragma or not, and the position of it is unimportant. Therefore, the line with #pragma once can be placed anywhere in the file except for chunk of code that is excluded from compilation by the conditional preprocessor directives like #if, #ifdef and #ifndef. The excluded chunk of code is not parsed by the compiler and if it contains the preprocessor directive it has no effect.
Despite the fact that #pragma once can be placed at any line that is parsed by the compiler, I strongly recommend to follow the common practice and put #pragma once at the beginning of a header file.
Also, as #user7860670 mentioned, arguments of #pragma directive is a subject to macro expansion for MSVC compiler. But neither gcc nor clang support it:
example for msvc
example for gcc
example for clang
There isn't really a complete answer on this question covering more than the big 3 compilers, so here's my attempt at a more complete answer.
Summary
TL;DR: If you care for portability, put it before any #include or #define statements that may conflict with it (e.g. put it first in the header).
Supported by all major compilers ("the big 3" x86_64 compilers as well as intel, and embedded compilers)
Placement generally does not matter as long as it reaches the preprocessor (e.g. not blocked by an #if-branch)
Different compilers are opinionated on whether it should be first, without documenting what happens if not.
Most compilers already detect include guards and treat it as a pragma once anyway, making the benefit largely just not creating a unique guard name.
Below is a quick, summarized guide:
Compiler
Support
Documentation
Clang
Supported
GNU-compatible. Not documented, but code shows it as normal preprocessing
GCC
Supported
gcc pragmas
MSVC
Supported (1)
MSVC one pragma
Intel (ICC)
Supported (1)
Intel Compiler Reference Manual - Supported Pragmas
Intel (ICL)
Supported (1)
ICL uses MSVC front-end
Intel (ICX)
Supported
ICX is based on Clang
Texas Instruments
Supported (2)
Reference Manual 5.11.23
Texas Instruments (Clang)
Supported
This is a fork of Clang with all major features still in effect
ArmCC
Supported (3)
Compiler Docs for #pragma once
(1) - Supported, but is subject to macro expansion
(2) - Supported, but is documented to be expected at the beginning of the header.
(3) - Supported, but not recommended.
Details
GCC
From the GCC reference manual:
If #pragma once is seen when scanning a header file, that file will never be read again, no matter what. It is a less-portable alternative to using ‘#ifndef’ to guard the contents of header files against multiple inclusions.
(emphasis mine)
Scanning is done at preprocessing time, and so as long as the #pragma statement is visible to the preprocessor (not in an unreachable conditional block from #if), then it will take effect.
GCC's #pragma once is not affected by preprocessor substitution.
Live Example
Clang
Clang's reference manual doesn't actually specify #pragma once so far as I can tell, however Clang is meant to be compatible with most, if not all, GCC builtins and features.
Viewing the source code for Clang's preprocessor phase indicates what you would expect; it handles #pragma once during preprocessing (source)
void Preprocessor::HandlePragmaOnce(Token &OnceTok) {
...
// Mark the file as a once-only file now.
HeaderInfo.MarkFileIncludeOnce(getCurrentFileLexer()->getFileEntry());
}
Like GCC, the placement of the #pragma once does not matter if it's first, and is not affected by preprocessor substitution.
Live Example
MSVC
MSVC's documentation on #pragma once does not indicate where it belongs, just that it should be in a source (and has an example of it at the top).
As mentioned by others, when using #pragma once in MSVC, it is subject to preprocessor-expansion.
With Substitution
Live Example
Without Substitution
Live Example
Intel (CL-Based)
When using the Intel compiler on Windows, the compiler uses an MSVC Compatibility mode (ICL). Although it's not documented in the Supported Pragma, though it does appear to be supported.
The placement does not appear to matter as well so long as the preprocessor reaches it.
ICL's #pragma once is subject to the preprocessor-expansion issue that MSVC experiences.
Note: icl is not supported on compiler-explorer, so no example is available.
Intel (GNU-Based)
When using the Intel compiler on Linux or older macOS versions (ICC), the compiler uses a GNU compatibility mode.
Like above, it's not explicitly listed as a Supported Pragma, though it does appear to be supported in practice. The placement does not appear to matter as well so long as the preprocessor reaches it.
ICC's #pragma once is subject to the preprocessor-expansion issue that MSVC experiences.
With Substitution
Live Example
Without Substitution
Live Example
Intel (Clang-Based)
The newer Intel ICX NextGen compiler is based off of Clang / LLVM technology. Behaviourally, this matches what Clang does.
Unlike other Intel compilers, but like Clang, thisdoes not suffer from the preprocessor-expansion issue.
Live Example
Arm (armcc)
The armcc compiler advises against #pragma once, but does also provide an example of it existing after #define statements as an optional feature to work with inclusion guards.
Given the example, placement should likely not be an issue.
It's unclear where this will experience any preprocessor expansion.
Note: armcc is not supported on compiler-explorer, so no example is available.
Texas Instruments (TI ArmCL)
As mentioned in the reference manual, section 5.11.23:
This pragma should be used at the beginning of a header file that should only be included once. For example:
// hdr.h
#pragma once
#warn You will only see this message one time
struct foo
{
int member;
};
(Emphasis mine)
I haven't tested what happens if it's moved anywhere lower than the comment header, but the compiler only officially supports it at the beginning of the file.
I would suspect this should not matter, but cannot confirm.
It's unclear where this will experience any preprocessor expansion.
Note: tiarmcl (and other similar ti compilers) are not supported on compiler-explorer, so no example is available.
Texas Instruments (tiarmclang)
This is a fork of clang, so it behaves the same as clang does.
The #pragma once in this implementation can be effectively placed anywhere the preprocessor reaches, and does not deal with preprocessor substitution.
Note: tiarmclang is not supported on compiler-explorer, so no example is available.
I have C++ code with OpenMP pragmas inside. I want to test this code both for multithread mode (with OpenMP) and in single thread mode (no OpenMP).
For now, to switch between modes I need to comment #pragma omp (or at least parallel).
What is the cleanest, or default, way to enable / disable OpenMP?
If you do not compile with -fopenmp option, you won't get the parallel code. You can do it with an appropiate define and makefile that generates all codes.
The OpenMP documentation says (only an example):
#ifdef _OPENMP
#include <omp.h>
#else
#define omp_get_thread_num() 0
#endif
See http://www.openmp.org/mp-documents/spec30.pdf (conditional compilation).
Look into the compiler manual for the switch that disables OpenMP. For GCC, OpenMP is disabled by default and enabled with the -fopenmp option.
Another option would be to run the code with the OMP_NUM_THREADS environment variable set to 1, though that is not exactly the same as compiling without OpenMP in the first place.
The way such things are usually handled (the general case) is with #defines and #ifdef:
In your header file:
#ifndef SINGLETHREADED
#pragma omp
#endif
When you compile, add -DSINGLETHREADED to disable OpenMP:
cc -DSINGLETHREADED <other flags go here> code.c
I'm wondering if there's a way to disable all warnings on a specific file (for example, using a preprocessor directive).
I'm using CImg.h and I want to get rid of the warnings involving that code.
I'm compiling both with VS (the version for Windows) and gcc (the Linux one), so I would like to have a generic way...
Thanks!
You can do it using #pragma in Microsoft compiler:
http://msdn.microsoft.com/en-us/library/2c8f766e%28VS.80%29.aspx
Something like this:
#pragma warning (push, 0)
//....header file
#pragma warning (pop)
Can't help you with gcc compiler, some info here: Selectively disable GCC warnings for only part of a translation unit?
EDIT EDIT Try push, 0.
Look into #pragma warning.