I am writing a LLVM code generator for the language Timber, the current compiler emits C-code. My problem is that I need to call C functions from the generated LLVM files, for example the compiler has a real-time garbage collector and i need to call functions to notify when new objects are allocated on the heap. I have no idea on how to link these functions with my generated LLVM files.
The code generation is made by generate .ll-files and then manually compile these.
I'm trying to call an external function from LLVM but i have no luck. In the examples I've >found only C standard functions like "puts" and "printf" are called, but I want to call a >homemade function. I'm stuck.
I'm assuming you're writing an LLVM transformation, and you want to add calls to external functions into transformed code. If this is not the case, edit your question and include more information.
Before you can call an external function from LLVM code, you need to insert a declaration for it. For example:
virtual bool runOnModule(Module &m) {
Constant *log_func = m.getOrInsertFunction("log_func",
Type::VoidTy,
PointerType::getUnqual(Type::Int8Ty),
Type::Int32Ty,
Type::Int32Ty,
NULL);
...
}
The code above declares a function log_func which returns void and takes three arguments: a byte pointer (string), and two 32-bit integers. getOrInsertFunction is a method of Module.
To actually call the function, you have to insert a CallInst. There are several static Create methods for this.
Compile your LLVM assembly files normally with llvm-as:
llvm-as *.ll
Compile the bitcode files to .s assembly language files:
llc *.bc
GCC them in with the runtime library:
gcc *.s runtime.c -o executable
Substitute in real makefiles, shared libraries, etc. if necessary. You get the idea.
I'm interpreting your question as being "how do I implement a runtime library in C or C++ for my language that gets compiled to LLVM?"
One approach is, as detailed by Jonathan Tang, to transform the output of your compiler from LLVM IR to bitcode to assembly, and have vanilla gcc link the assembly against the runtime source (or object files).
An alternative, possibly more flexible approach is to use llvm-gcc to compile the runtime itself into LLVM bitcode, and then use llvm-ld to link the bitcode from your compiler with the bitcode of your runtime. This bitcode can then be re-optimized with opt, converted back to IR with llvm-dis, interpreted directly with lli (this will, afaik, only work if LLVM was built against libffi), or compiled to assembly with llc (and then to a native binary with vanilla gcc).
Related
I'm currently trying to figure out the way to produce equivalent assembly code from corresponding C source file.
I've been using the C language for several years, but have little experience with assembly language.
I was able to output the assembly code using the -S option in gcc. However, the resulting assembly code contained call instructions which in turn make a jump to another function like _exp. This is not what I wanted, I needed a fully functional assembly code in a single file, with no dependency to other code.
Is it possible to achieve what I'm looking for?
To better describe the problem, I'm showing you my code here:
#include <math.h>
float sigmoid(float i){
return 1/(1+exp(-i));
}
The platform I am working on is Windows 10 64-bit, the compiler I'm using is cl.exe from MSbuild.
My initial objective was to see, at a lowest level possible, how computers calculate mathematical functions. The level where I decided to observe the calculation process is assembly code, and the mathematical function I've chosen was sigmoid defined as above.
_exp is the standard math library function double exp(double); apparently you're on a platform that prepends a leading underscore to C symbol names.
Given a .s that calls some library functions, build it the same way you would a .c file that calls library functions:
gcc foo.S -o foo -lm
You'll get a dynamic executable by default.
But if you really want all the code in one file with no external dependencies, you can link your .c into a static executable and disassemble that.
gcc -O3 -march=native foo.c -o foo -static -lm
objdump -drwC -Mintel foo > foo.s
There's no guarantee that the _exp implementation in libm.a (static library) is identical to the one you'd get in libm.so or libm.dll or whatever, because it's a different file. This is especially true for a function like memcpy where dynamic-linker tricks are often used to select an optimal version (for your CPU) at run-time.
It is not possible in general, there are exceptions sure, I could craft one so that means other folks can too, but it isnt an interesting program.
Normally your C program, your main() entry point is only a percentage of the code. There is a bootstrap that contains the actual entry point for the operating system to launch your program, this does some things that prepare your virtual memory space so that your program can run. Zeros .bss and other such things. that is often and or should be written in assembly language (otherwise you get a chicken and egg problem) but not an assembly language file you will see unless you go find the sources for the C library, you will often get an object as part of the toolchain along with other compiler libraries, etc.
Then if you make any C calls or create code that results in a compiler library call (perform a divide on a platform that doesnt support divide, perform floating point on a platform that doesnt have floating point, etc) that is another object that came from some other C or assembly that is part of the library or compiler sources and is not something you will see during the compile/assemble/link (the chain in toolchain) process.
So except for specifically crafted trivial programs or specifically crafted tools for this purpose (for specific likely baremetal platforms), you will not see your whole program turn into one big assembly source file before it gets assembled then linked.
If not baremetal then there is of course the operating system layer which you certainly would not get to see as part of your source code, ultimately the C library calls that need the system will have a place where they do that, all compiled to object/lib before you use them, and the assembly sources for the operating system side is part of some other source and build process somewhere else.
new to using C
Header files for libraries like stdlib do not contain the actual implementation code for the functions they provide access to. I understand that the actual source text for libraries like this aren't needed to compile, but how does this work specifically? Are the implementation details for these libraries contained within the compiler?
When you use a function like printf(), including the header file essentially pastes in code for the declaration of the function, but normally the implementation code would need to be available as well.
What form is it stored in? (and where?) Is this compiler specific? Would it be possible to write custom code and reference it in this way without modifying the behavior of the compiler?
I've been searching around and found some info that is relevant but nothing specific. This could be related to not formulating the question well. Thanks.
When you link a program, the compiler will implicitly add some extra libraries to your program:
$ ls
main.c
$ cc -c main.c
$ cc main.o
$ ls
main.c main.o a.out
You can discover the extra libraries a program uses with ldd. Here, there are three libraries linked into the program, and I didn't ask for any of them:
$ ldd a.out
linux-vdso.so => (0x00...)
libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00...)
/lib64/ld-linux-x86-64.so.2 (0x00...)
So, what happens if we link without these libraries? That's easy enough, just use the linker (ld) directly, instead of calling it through cc. When you use ld, it doesn't give you these extra libraries, so you get an error:
$ ld main.o
Undefined symbols:
"_printf", referenced from:
_main in main.o
The implementation for printf() is stored in the standard C library, which is usually just another library on your system... the only difference is that it gets automatically included into your program when you compile C.
You can use nm to find out what symbols are in a library, so I can use it to find printf() in libc:
$ nm -D /lib/x86_64-linux-gnu/libc-2.13.so | grep printf
...
000000000004e4b0 T printf
...
So, now that we know that libc has printf(), we can use -lc to tell the linker to include libc, and that will get rid of the errors about printf() being missing:
$ ld main.o -lc
There might be some other bits missing, and that's why we use cc to link our programs instead of ld: cc gives us all the default libraries.
When you compile a file you only need to promise the compiler that you have certain functions and symbols. A function call is in the compiled into a call [some_address]
The compiler will compile each C-file into object files that just have place holders for calls to functions declared in the headers. That is [some_address] does not need to be known at this point.
A number of oject files can be collected into what is known as a library.
After that it is the linkers job to look through all object files and libraries it know of and find out what the real value of all unknown [some_address] is and translate the call to, e.g. call 0x1234 if the particular function you are calling starts at 0x1234 (or it might be a relative offset from the current program pointer.
Stdlib and other library functions are implemented in an object library. A library is a collection of code that is linked with your program. By default C programs are linked against the stdlib library, which is usually provided by the operating system. Most modern operating systems use a dynamical linker. That is, your program is not linked against the library until it is executed. When it is being loaded, the linker-loader combines your code and the library code in your program's address space. You code and then make a call to the printf() code that is located in that library.
Usually a header file contains only a function prototype while the implementation is either in a separate source file or a precompiled library in the case of stdlib (and other libraries, both shipped with a compiler or available separately) the precompiled library gets linked at the end of the compilation process. (There's also a distinction between static and dynamic libraries, but I won't go into detail about that)
The implementation of standard libraries (which are shipped with a compiler) are usually compiler specific (there is a standard describing which functions have to be in a library, but the compiler programmer can decide how exactly he implements them) and it is (in theory) possible to exchange these libraries with some of your own without modifying the behaviour of the compiler (though not recommended as you would have to rewrite the entire library in order to ensure that all functions are contained).
How does a compiler find out which dynamic link library will be used in my code, if I only include headers-files, where is not describe it?
#include <stdio.h>
void main()
{
printf("Hello world\n");
}
There is I only include
stdio.h
and my code is used
printf function
How it is known, in headers-files prototypes , macros and constant are described, but nothing about in which file "printf" is implement. How does then it works?
When you compile a runnable executable, you don't just specify the source code, but also a list of libraries from which undefined references are looked up. With the C standard library, this happens implicitly (unless you tell GCC -nostdinc), so you may not have been consciously aware of this.
The libraries are only consumed by the linker, not the compiler. The linker locates all the undefined references in the libraries. If the library is a static one, the linker just adds the actual machine code to your final executable. On the other hand, if the library is a shared one, the linker only records the name (and version?) of the library in the executable's header. It is then the job of the loader to find appropriate libraries at load time and resolve the missing dependencies on the fly.
On Linux, you can use ldd to list the load-time dependencies of a dynamically linked executable, e.g. try ldd /bin/ls. (On MacOS, you can use otool -L for the same purpose.)
As others have answered, the standard c library is implicitly linked. If you are using gcc you can use the -Wl,--trace option to see what the linker is doing.
I tested your example code:
gcc -Wl,--trace main.c
Gives:
/usr/bin/ld: mode elf_x86_64
/usr/lib/gcc/x86_64-linux-gnu/4.6/../../../x86_64-linux-gnu/crt1.o
/usr/lib/gcc/x86_64-linux-gnu/4.6/../../../x86_64-linux-gnu/crti.o
/usr/lib/gcc/x86_64-linux-gnu/4.6/crtbegin.o
/tmp/ccCjfUFN.o
-lgcc_s (/usr/lib/gcc/x86_64-linux-gnu/4.6/libgcc_s.so)
/lib/x86_64-linux-gnu/libc.so.6
(/usr/lib/x86_64-linux-gnu/libc_nonshared.a)elf-init.oS
/lib/x86_64-linux-gnu/ld-linux-x86-64.so.2
-lgcc_s (/usr/lib/gcc/x86_64-linux-gnu/4.6/libgcc_s.so)
/usr/lib/gcc/x86_64-linux-gnu/4.6/crtend.o
/usr/lib/gcc/x86_64-linux-gnu/4.6/../../../x86_64-linux-gnu/crtn.o
This shows that the linker is using libc.so (and also ld-linux.so).
The library glibc is linked by default by GCC. There is no need to mention -l library when you are building your executable. Hence you find that the functions printf and others which are a part of glibc do not need any linking exclusively.
Technically your compiler does not figure out which libraries will be used. The linker (commonly ld) does this. The header files only tell the compiler what interface your library functions use and leaves it up to the linker to figure out where they are.
A source file goes a long path until it becomes an executable. Commonly
source.c -[preprocess]> source.i -[compile]> source.s -[assemble]> source.o -[link]> a.out
When you invoke cc source.c all those steps are done transparently for you in one go and the standard libraries (commonly libc.so) and executable loader (commonly crt0.o) are linked together.
Any additional libraries have to be passed as additional linker flags i.e. -lpthread.
I would say that depends on IDE or the compiler and system. Header file just contains interface information like name of function parameters it expects any attributes others and that's how compiler first convert your code to an intermediate object file.
After that comes linking where in code for printf is getting added to the executable either through static library or dynamic library.
Functions and other facilities like STL are part of C/C++ so they are either delivered by compiler or system. e.g on Solaris there is no debug version of C library unless you are using gcc. But on Visual Studio you have debug version msvcrt.dll and you can also link C library statically.
In short the answer is that code for printf and other functions in C library are added by compiler at link time.
What is the difference between a compiler and a linker in C?
The compiler converts code written in a human-readable programming language into a machine code representation which is understood by your processor. This step creates object files.
Once this step is done by the compiler, another step is needed to create a working executable that can be invoked and run, that is, associate the function calls (for example) that your compiled code needs to invoke in order to work. For example, your code could call sprintf, which is a routine in the C standard library. Your code has nothing that does the actual service provided by sprintf, it just reports that it must be called, but the actual code resides somewhere in the common C library. To perform this (and many others) linkages, the linker must be invoked. After linking, you obtain the actual executable that can run.
A compiler generates object code files (machine language) from source code.
A linker combines these object code files into an executable.
Many IDEs invoke them in succession, so you never actually see the linker at work. Some languages/compilers do not have a distinct linker and linking is done by the compiler as part of its work.
In Simple words -> Linker comes into act whenever a '.obj' file needs to be linked with its library functions as compiler doesn't understand what is (scanf or printf..etc) , compiler just converts '.c' file to '.obj' file if there's no error without understanding library functions we used. So To make 'obj' file to 'exe'(executable file) we need linker because it makes compiler understand of library functions.
I am trying to make LLVM inline a function from a library.
I have LLVM bitcode files (manually generated) that I linked together with llvm-link, and I also have a library (written in C) compiled into bitcode by clang and archived with llvm-ar. I manage to link everything together and to execute but I can't manage to get LLVM to inline a function from the library. Any clue about how this should be done?
After you link the bitcode files together with the library, do you run an Internalize pass on the linked bitcode? The internalize pass makes all functions (besides main()) static and tells optimizer/code generator that the functions can be safely inlined without keeping a copy available for some (non-existent) external reference.
I manually link my bitcode files and bitcode libraries together using code borrowed from llvm-ld and I do the internalize pass, but I'm not sure if llvm-link does the internalize pass or not.