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I just started to learn about compilers, and I wondered - is there a way to tell GCC (or any other compiler) to stop in the middle of the process, and give me a file with the current state (for example - a token tree)?
In general, yes!
When building a compiler, often one of your most important debugging tools is to see the current representation of the program, or even better, the states throughout compilation, so that when a transformation goes wrong -- i.e. a compiler bug -- it can be identified.
Dumping all the state of a compiler can be a lot however; Even just the intermediate representation for a large function can be tens-of-thousands of lines of plain-text. Then, there's also all the associated data structures: Control-flow graphs, dependency information, symbols etc.
See this question for some more detail on answers for LLVM and GCC.
I just realized that binary compilers convert source code to the binary of the destination platform. Kind of obvious... but if a compiler works such way, then how can the same compiler be used for different systems like x86, ARM, MIPS, etc?
Shouldn't they be supposed to "know" the machine-language of the hardware platform to be able to know how to build the binary? Does a compiler(like gcc) knows the machine language of every single platform that is supported?
How is that system possible, and how can a compiler be optimized for that many platforms at the same time?
Yes, they have to "know" the machine language for every single platform they support. This is a required to generate machine code. However, compilation is a multi-step process. Usually, the first steps of the compilation are common to most architectures.
Taken from wikipedia
Structure of a compiler
Compilers bridge source programs in high-level
languages with the underlying hardware.
A compiler requires
determining the correctness of the syntax of programs,
generating correct and efficient object code,
run-time organization, and
formatting output according to assembler and/or linker conventions.
A
compiler consists of three main parts: the frontend, the middle-end,
and the backend.
The front end
checks whether the program is correctly
written in terms of the programming language syntax and semantics.
Here legal and illegal programs are recognized. Errors are reported,
if any, in a useful way. Type checking is also performed by collecting
type information. The frontend then generates an intermediate
representation or IR of the source code for processing by the
middle-end.
The middle end
is where optimization takes place. Typical
transformations for optimization are removal of useless or unreachable
code, discovery and propagation of constant values, relocation of
computation to a less frequently executed place (e.g., out of a loop),
or specialization of computation based on the context. The middle-end
generates another IR for the following backend. Most optimization
efforts are focused on this part.
The back end
is responsible for translating the IR from the middle-end into assembly code. The target
instruction(s) are chosen for each IR instruction. Register allocation
assigns processor registers for the program variables where possible.
The backend utilizes the hardware by figuring out how to keep parallel
execution units busy, filling delay slots, and so on. Although most
algorithms for optimization are in NP, heuristic techniques are
well-developed.
More this article which describes the structure of a compiler and on this one which deals with Cross compilers.
The http://llvm.org/ project will answer all of your questions in this regard :)
In a nutshell, cross HW compilers emit "intermediate representation" of the code , which is HW agnostic and then its being customized via the native tool chain
Yes it is possible, it's called Cross Compiler. Compilers usually first they generate the object code which is not understanable by the current machine but it can be migrated to the destiny machine with another compiler. Next, object code is "compiled" again and linked with external libraries of the target machines.
TL;DR: Yes, the compilers knows the target code, but you can compile in another hardware.
I recommend you to read attached links for information.
Every platform has its own toolchain, toolchain includes gcc,gdb,ld,nm etc.
Let's take specific example of gcc as of now. GCC source code has many layers including architecture dependent and independent part. Architecture dependent part contains procedures to handle architecture specific things like their stack, function calls, floating point operations. We need to cross compile the gcc source code for a specific architecture like for ARM. You can see its steps here for reference:- http://www.ailis.de/~k/archives/19-arm-cross-compiling-howto.html#toolchain.
This architecture dependent part is responsible for handling machine language operations.
From Thinking in C++ - Vol 1:
In the second pass, the code generator walks through the parse tree
and generates either assembly language code or machine code for the
nodes of the tree.
Well at least in GCC if we give the option of generating the assembly code, the compiler obeys by creating a file containing assembly code. But, when we simply run the command gcc without any options does it not produce the assembly code internally?
If yes, then why does it need to first produce an assembly code and then translate it to machine language?
TL:DR different object file formats / easier portability to new Unix platforms (historically) is one of the main reasons for gcc keeping the assembler separate from the compiler, I think. Outside of gcc, the mainstream x86 C and C++ compilers (clang/LLVM, MSVC, ICC) go straight to machine code, with the option of printing asm text if you ask them to.
LLVM and MSVC are / come with complete toolchains, not just compilers. (Also come with assembler and linker). LLVM already has object-file handling as a library function, so it can use that instead of writing out asm text to feed to a separate program.
Smaller projects often choose to leave object-file format details to the assembler. e.g. FreePascal can go straight to an object file on a few of its target platforms, but otherwise only to asm. There are many claims (1, 2, 3, 4) that almost all compilers go through asm text, but that's not true for many of the biggest most-widely-used compilers (except GCC) that have lots of developers working on them.
C compilers tend to either target a single platform only (like a vendor's compiler for a microcontroller) and were written as "the/a C implementation for this platform", or be very large projects like LLVM where including machine code generation isn't a big fraction of the compiler's own code size. Compilers for less widely used languages are more usually portable, but without wanting to write their own machine-code / object-file handling. (Many compilers these days are front-ends for LLVM, so get .o output for free, like rustc, but older compilers didn't have that option.)
Out of all compilers ever, most do go to asm. But if you weight by how often each one is used every day, going straight to a relocatable object file (.o / .obj) is significant fraction of the total builds done on any given day worldwide. i.e. the compiler you care about if you're reading this might well work this way.
Also, compilers like javac that target a portable bytecode format have less reason to use asm; the same output file and bytecode format work across every platform they have to run on.
Related:
https://retrocomputing.stackexchange.com/questions/14927/when-and-why-did-high-level-language-compilers-start-targeting-assembly-language on retrocomputing has some other answers about advantages of keeping as separate.
What is the need to generate ASM code in gcc, g++
What do C and Assembler actually compile to? - even compilers that go straight to machine code don't produce linked executables directly, they produce relocatable object files (.o or .obj). Except for tcc, the Tiny C Compiler, intended for use on the fly for one-file C programs.
Semi-related: Why do we even need assembler when we have compiler? asm is useful for humans to look at machine code, not as a necessary part of C -> machine code.
Why GCC does what it does
Yes, as is a separate program that the gcc front-end actually runs separately from cc1 (the C preprocessor+compiler that produces text asm).
This makes gcc slightly more modular, making the compiler itself a text -> text program.
GCC internally uses some binary data structures for GIMPLE and RTL internal representations, but it doesn't write (text representations of) those IR formats to files unless you use a special option for debugging.
So why stop at assembly? This means GCC doesn't need to know about different object file formats for the same target. For example, different x86-64 OSes use ELF, PE/COFF, MachO64 object files, and historically a.out. as assembles the same text asm into the same machine code surrounded by different object file metadata on different targets. (There are minor differences gcc has to know about, like whether to prepend an _ to symbol names or not, and whether 32-bit absolute addresses can be used, and whether code has to be PIC.)
Any platform-specific quirks can be left to GNU binutils as (aka GAS), or gcc can use the vendor-supplied assembler that comes with a system.
Historically, there were many different Unix systems with different CPUs, or especially the same CPU but different quirks in their object file formats. And more importantly, a fairly compatible set of assembler directives like .globl main, .asciiz "Hello World!\n", and similar. GAS syntax comes from Unix assemblers.
It really was possible in the past to port GCC to a new Unix platform without porting as, just using the assembler that comes with the OS.
Nobody has ever gotten around to integrating an assembler as a library into GCC's cc1 compiler. That's been done for the C preprocessor (which historically was also done in a separate process), but not the assembler.
Most other compilers do produce object files directly from the compiler, without a text asm temporary file / pipe. Often because the compiler was only designed for one or a couple targets, like MSVC or ICC or various compilers that started out as x86-only, or many vendor-supplied compilers for embedded chips.
clang/LLVM was designed much more recently than GCC. It was designed to work as an optimizing JIT back-end, so it needed a built-in assembler to make it fast to generate machine code. To work as an ahead-of-time compiler, adding support for different object-file formats was presumably a minor thing since the internal software architecture was there to go straight to binary machine code.
LLVM of course uses LLVM-IR internally for target-independent optimizations before looking for back-end-specific optimizations, but again it only writes out this format as text if you ask it to.
The assembler stage can be justified by two reasons:
it allows c/c++ code to be translated to a machine independent abstract assembler, from which there exists easy conversions to a multitude of different instruction set architectures
it takes out the burden of validating correct opcode, prefix, r/m, etc. instruction encoding for CISC architectures, when one can utilize an existing software [component].
The 1st edition of that book is from 2000, but is may as well talk about the early 90's, when c++ itself was translated to c and when the gnu/free software idea (including source code for compilers) was not really known.
EDIT: One of several nonsensical abstract machine independent languages used by GCC is RTL -- Register Transfer Language.
It's a matter of compiler implementation. Assembly code is an intermediate step between higher-level language (the one being compiled) and the resulting binary output. In general it's easier first to convert to assembly and after that to binary code instead of directly creating the binary code.
Gcc does create the assembly code as a temporary file, calls the assembler, and maybe the linker depending on what you do or dont add on the command line. That makes an object and then if enabled the binary, then all the temporary files are cleaned up. Use -save-temps to see what is really going on (there are a number of temporary files).
Running gcc without any options absolutely creates an asm file.
There is no "need" for this, it is simply how they happened to design it. I assume for multiple reasons, you will already want/need an assembler and linker before you start on a compiler (cart before the horse, asm on a processor before some other language). "The unix way" is to not re-invent tools or libraries, but just add a little on top, so that would imply going to asm then letting the assembler and linker do the rest. You dont have to re-invent so much of the assemblers job that way (multiple passes, resolving labels, etc). It is easier for a developer to debug ascii asm than bits. Folks have been doing it this way for generations of compilers. Just in time compilers are the primary exception to this habit, by definition they have to be able to go to machine code, so they do or can. Only recently though did llvm provide a way for the command line tools (llc) to go straight to object without stopping at asm (or at least it appears that way to the user).
I have few programs (written in C) implementing some algorithms, that I use to measure computation time. Whole data is implemented as static libraries directly in code, there's no input and output from these programs. There's also no C library calls (no printfs etc.).
I want to build fully independent and minimal executable. I don't want to link my program with libgcc (target CPU has coprocessor, so I don't need to emulate floating point arithmetic), C library or any other. Actually I want to make my program as independent as it's possible. On Linux ELF program has to be linked only with crt0.o to run properly, right?
I'm mostly asking because I'm curious ;)
Link with gcc -nostdlib, then use objdump -h and strip --remove-section=... to really make it small by getting rid of silly things like the comment section and the exception handling frame information sections. Keep removing sections until it stops working.
And compile with -Os of course
I've Googled and poked around the Go website, but I can't find an explanation for Go's extraordinary build times. Are they products of the language features (or lack thereof), a highly optimized compiler, or something else? I'm not trying to promote Go; I'm just curious.
Dependency analysis.
The Go FAQ used to contain the following sentence:
Go provides a model for software
construction that makes dependency
analysis easy and avoids much of the
overhead of C-style include files and
libraries.
While the phrase is not in the FAQ anymore, this topic is elaborated upon in the talk Go at Google, which compares the dependency analysis approach of C/C++ and Go.
That is the main reason for fast compilation. And this is by design.
I think it's not that Go compilers are fast, it's that other compilers are slow.
C and C++ compilers have to parse enormous amounts of headers - for example, compiling C++ "hello world" requires compiling 18k lines of code, which is almost half a megabyte of sources!
$ cpp hello.cpp | wc
18364 40513 433334
Java and C# compilers run in a VM, which means that before they can compile anything, the operating system has to load the whole VM, then they have to be JIT-compiled from bytecode to native code, all of which takes some time.
Speed of compilation depends on several factors.
Some languages are designed to be compiled fast. For example, Pascal was designed to be compiled using a single-pass compiler.
Compilers itself can be optimized too. For example, the Turbo Pascal compiler was written in hand-optimized assembler, which, combined with the language design, resulted in a really fast compiler working on 286-class hardware. I think that even now, modern Pascal compilers (e.g. FreePascal) are faster than Go compilers.
There are multiple reasons why the Go compiler is much faster than most C/C++ compilers:
Top reason: Most C/C++ compilers exhibit exceptionally bad designs (from compilation speed perspective). Also, from compilation speed perspective, some parts of the C/C++ ecosystem (such as editors in which programmers are writing their code) aren't designed with speed-of-compilation in mind.
Top reason: Fast compilation speed was a conscious choice in the Go compiler and also in the Go language
The Go compiler has a simpler optimizer than C/C++ compilers
Unlike C++, Go has no templates and no inline functions. This means that Go doesn't need to perform any template or function instantiation.
The Go compiler generates low-level assembly code sooner and the optimizer works on the assembly code, while in a typical C/C++ compiler the optimization passes work on an internal representation of the original source code. The extra overhead in the C/C++ compiler comes from the fact that the internal representation needs to be generated.
Final linking (5l/6l/8l) of a Go program can be slower than linking a C/C++ program, because the Go compiler is going through all of the used assembly code and maybe it is also doing other extra actions that C/C++ linkers aren't doing
Some C/C++ compilers (GCC) generate instructions in text form (to be passed to the assembler), while the Go compiler generates instructions in binary form. Extra work (but not much) needs to be done in order to transform the text into binary.
The Go compiler targets only a small number of CPU architectures, while the GCC compiler targets a large number of CPUs
Compilers which were designed with the goal of high compilation speed, such as Jikes, are fast. On a 2GHz CPU, Jikes can compile 20000+ lines of Java code per second (and the incremental mode of compilation is even more efficient).
Compilation efficiency was a major design goal:
Finally, it is intended to be fast: it should take at most a few seconds to build a large executable on a single computer. To meet these goals required addressing a number of linguistic issues: an expressive but lightweight type system; concurrency and garbage collection; rigid dependency specification; and so on. FAQ
The language FAQ is pretty interesting in regards to specific language features relating to parsing:
Second, the language has been designed to be easy to analyze and can be parsed without a symbol table.
While most of the above is true, there is one very important point that was not really mentionend: Dependency management.
Go only needs to include the packages that you are importing directly (as those already imported what they need). This is in stark contrast to C/C++, where every single file starts including x headers, which include y headers etc. Bottom line: Go's compiling takes linear time w.r.t to the number of imported packages, where C/C++ take exponential time.
A good test for the translation efficiency of a compiler is self-compilation: how long does it take a given compiler to compile itself? For C++ it takes a very long time (hours?). By comparison, a Pascal/Modula-2/Oberon compiler would compile itself in less than one second on a modern machine [1].
Go has been inspired by these languages, but some of the main reasons for this efficiency include:
A clearly defined syntax that is mathematically sound, for efficient scanning and parsing.
A type-safe and statically-compiled language that uses separate compilation with dependency and type checking across module boundaries, to avoid unnecessary re-reading of header files and re-compiling of other modules - as opposed to independent compilation like in C/C++ where no such cross-module checks are performed by the compiler (hence the need to re-read all those header files over and over again, even for a simple one-line "hello world" program).
An efficient compiler implementation (e.g. single-pass, recursive-descent top-down parsing) - which of course is greatly helped by points 1 and 2 above.
These principles have already been known and fully implemented in the 1970s and 1980s in languages like Mesa, Ada, Modula-2/Oberon and several others, and are only now (in the 2010s) finding their way into modern languages like Go (Google), Swift (Apple), C# (Microsoft) and several others.
Let's hope that this will soon be the norm and not the exception. To get there, two things need to happen:
First, software platform providers such as Google, Microsoft and Apple should start by encouraging application developers to use the new compilation methodology, while enabling them to re-use their existing code base. This is what Apple is now trying to do with the Swift programming language, which can co-exist with Objective-C (since it uses the same runtime environment).
Second, the underlying software platforms themselves should eventually be re-written over time using these principles, while simultaneously redesigning the module hierarchy in the process to make them less monolithic. This is of course a mammoth task and may well take the better part of a decade (if they are courageous enough to actually do it - which I am not at all sure in the case of Google).
In any case, it's the platform that drives language adoption, and not the other way around.
References:
[1] http://www.inf.ethz.ch/personal/wirth/ProjectOberon/PO.System.pdf, page 6: "The compiler compiles itself in about 3 seconds". This quote is for a low cost Xilinx Spartan-3 FPGA development board running at a clock frequency of 25 MHz and featuring 1 MByte of main memory. From this one can easily extrapolate to "less than 1 second" for a modern processor running at a clock frequency well above 1 GHz and several GBytes of main memory (i.e. several orders of magnitude more powerful than the Xilinx Spartan-3 FPGA board), even when taking I/O speeds into account. Already back in 1990 when Oberon was run on a 25MHz NS32X32 processor with 2-4 MBytes of main memory, the compiler compiled itself in just a few seconds. The notion of actually waiting for the compiler to finish a compilation cycle was completely unknown to Oberon programmers even back then. For typical programs, it always took more time to remove the finger from the mouse button that triggered the compile command than to wait for the compiler to complete the compilation just triggered. It was truly instant gratification, with near-zero wait times. And the quality of the produced code, even though not always completely on par with the best compilers available back then, was remarkably good for most tasks and quite acceptable in general.
Go was designed to be fast, and it shows.
Dependency Management: no header file, you just need to look at the packages that are directly imported (no need to worry about what they import) thus you have linear dependencies.
Grammar: the grammar of the language is simple, thus easily parsed. Although the number of features is reduced, thus the compiler code itself is tight (few paths).
No overload allowed: you see a symbol, you know which method it refers to.
It's trivially possible to compile Go in parallel because each package can be compiled independently.
Note that Go isn't the only language with such features (modules are the norm in modern languages), but they did it well.
Quoting from the book "The Go Programming Language" by Alan Donovan and Brian Kernighan:
Go compilation is notably faster than most other compiled languages, even when building from scratch. There are three main reasons for the compiler’s speed. First, all imports must be explicitly listed at the beginning of each source file, so the compiler does not have to read and process an entire file to determine its dependencies. Second, the dependencies of a package form a directed acyclic graph, and because there are no cycles, packages can be compiled separately and perhaps in parallel. Finally, the object file for a compiled Go package records export information not just for the package itself, but for its dependencies too. When compiling a package, the compiler must read one object file for each import but need not look beyond these files.
The basic idea of compilation is actually very simple. A recursive-descent parser, in principle, can run at I/O bound speed. Code generation is basically a very simple process. A symbol table and basic type system is not something that requires a lot of computation.
However, it is not hard to slow down a compiler.
If there is a preprocessor phase, with multi-level include directives, macro definitions, and conditional compilation, as useful as those things are, it is not hard to load it down. (For one example, I'm thinking of the Windows and MFC header files.) That is why precompiled headers are necessary.
In terms of optimizing the generated code, there is no limit to how much processing can be added to that phase.
Simply ( in my own words ), because the syntax is very easy ( to analyze and to parse )
For instance, no type inheritance means, not problematic analysis to find out if the new type follows the rules imposed by the base type.
For instance in this code example: "interfaces" the compiler doesn't go and check if the intended type implement the given interface while analyzing that type. Only until it's used ( and IF it is used ) the check is performed.
Other example, the compiler tells you if you're declaring a variable and not using it ( or if you are supposed to hold a return value and you're not )
The following doesn't compile:
package main
func main() {
var a int
a = 0
}
notused.go:3: a declared and not used
This kinds of enforcements and principles make the resulting code safer, and the compiler doesn't have to perform extra validations that the programmer can do.
At large all these details make a language easier to parse which result in fast compilations.
Again, in my own words.
Go imports dependencies once for all files, so the import time doesn't increase exponentially with project size.
Simpler linguistics means interpreting them takes less computing.
What else?