I am trying to do some analysis on Linux kernel source code using clang AST representation. But I found some statements in source are missing in clang AST.
The first picture is a snippet of the kernel source code. As we can see, there are 7 functions in the IfStmt.
Linux Source snippet
The second picture shows the result of clang -Xclang -ast-dump ~/mm/mmap.c, but there are only four functions. Three functions and four statements in yellow boxes are missing in AST representation. result of clang AST
I am running on Ubuntu 18.04, with clang9.0.0. Does anyone know what's going on?
Related
Is there any frontend that will generate MLIR (not LLVM) code currently? I am interested in parsing C/C++ or Java code in particular. Does clang support this now? This page doesn't list any at the moment.
As of Oct 2020, compiling C++ into CIL (C intermediate language) mlir dialect is not public yet. But they will be making it available "soon".
This was hinted on this year on LLVM developers meeting (http://llvm.org/devmtg/2020-09/program/) on the following talk:
CIL : Common MLIR Dialect for C/C++ and Fortran - P. NR; V. M; Ranjith; Srihari
In case people are not aware, as an alternative if you just want your C/C++ code in the mlir environment is to compile your program into LLVM with clang -S -fno-discard-value-names -emit-llvm and then later use mlir-translate --import-llvm to transform your .ll file into a .mlir file. But you do lose some higher level information and the opportunity for higher level optimizations.
I have using function ParserAST() to get a AST ,but I don't know how to display the ast on my console(i am using vs 2017). And how can i using llvm to run the ast and get the information about the var value.
Try clang -Xclang -ast-dump -fsyntax-only test.c, if want to print AST.
And,
You cannot run your own AST on llvm. Instead, see LLVM-JIT.
I'm using GCC for make some powerpc64 executable, but sometimes between functions i have the following mistakes: Screenshot
Powerpc instructions format are still in 4 bytes, i tried some gcc commands (-fno-align-functions) but the compiler still fill bytes between functions.
I want my functions start directly after the end of the previous functions, without any values/zero filled (in the case of the screenshots the functions should start at 0x124).
Thanks.
The PPC64 ABI specifies a traceback table appended to functions. The zeroes may be due to the traceback table and not related to alignment. Try using the
-mtraceback=no command line option.
In addition to the traceback table issue noted in the previous answer, functions are normally aligned on a 16-byte boundary. This is important for various reasons, including so the compiler can align hot loops on a 16-byte boundary for improved icache performance. Assembly code from GCC will have a directive like:
.p2align 4,,15
before each function definition to enforce this. So even without the traceback table your function will not start at address 0x124 without more effort.
This behavior can be overridden using -fno-align-functions, or using optimization level -Os (optimize for size). I've tried both methods, and they both remove the .p2align directive. Using -fno-align-functions is preferable unless you really want smaller and potentially slower code.
(If you are compiling with -O0 or -O1, you won't see the directive either, but we do not recommend compiling at such low optimization levels for either size or speed.)
I'm dumping the AST of some headers like this:
clang -cc1 -ast-dump -fblocks header.h
However, any #defines on the header are not showing on the dump. Is there a way of adding them?
It's true, #defines are handled by the preprocessor, not the compiler. So you need a preprocessor parser stage. I know of two:
Boost Wave can preprocess the input for you, and/or give you hooks to trigger on macro definitions or uses.
The Clang tool pp-trace uses a Clang library that can do callbacks on many preprocessor events, including macro definitions.
By concept/function/implementation, what are the differences between compilers and parsers?
A compiler is often made up of several components, one of which is a parser.
A common set of components in a compiler is:
Lexer - break the program up into words.
Parser - check that the syntax of the sentences are correct.
Semantic Analysis - check that the sentences make sense.
Optimizer - edit the sentences for brevity.
Code generator - output something with equivalent semantic meaning using another vocabulary.
To add a little bit:
As mentioned elsewhere, small C is a recursive decent compiler that generated code as it parsed. Basically syntactical analysis, semantic analysis, and code generation in one pass. As I recall, it also lexed in the parser.
A long time ago, I wrote a C compiler (actually several: the Introl-C family for microcontrollers) that used recursive descent and did syntax and semantic checking during the parse and produced a tree representation of the program from which code was generated.
Today, I'm working on a compiler that does source -> tokens -> AST -> IR -> code, pretty much as I described above.
A parser just reads a text into an internal, more abstract representation, often a tree or graph of some sort.
A compiler translates such an internal representation into another format. Most often this means converting source code into executable programs. But the target doesn't have to be machine code. It can be another programming language as well; the compiler would still be a compiler. Obviously a compiler needs a parser to actually read its input.
Compiler always have a parser inside. Parser just process the language and return the tree representation of it, compiler generate something from that tree, actual machine codes or another language.
A parser is one element of a compiler.
Are you looking for the differences between an interpreter and a compiler?
A parser takes in raw-data and parses it into a tree structure. This syntax-tree is then passed on to generator, which will turn it into whatever it is supposed to generate.
So, a parser is a part of a compiler.
In general, parser is a part of the compiler, but compiler is designed to convert the received script generally into machine-readable code or sometimes into another language.
A compiler is a special type of computer program that translates a human readable text file into a form that the computer can more easily understand. At its most basic level, a computer can only understand two things, a 1 and a 0. At this level, a human will operate very slowly and find the information contained in the long string of 1s and 0s incomprehensible. A compiler is a computer program that bridges this gap.
A parser is a piece of software that evaluates the syntax of a script when it is executed on a web server. For scripting languages used on the web, the parser works like a compiler might work in other types of application development environments.Parsers are commonly used in script development because they can evaluate code when the script is executed and do not require that the code be compiled first.