You write a function and, looking at the resulting assembly, you see it can be improved.
You would like to keep the function you wrote, for readability, but you would like to substitute your own assembly for the compiler's. Is there any way to establish a relationship between your high-livel language function and the new assembly?
If you are looking at the assembly, then its fair to assume that you have a good understanding about how code gets compiled down. If you have this knowledge, then its sometimes possible to 'reverse enginer' the changes back up into the original language but its often better not to bother.
The optimisations that you make are likely to be very small in comparison to the time and effort required in first making these changes. I would suggest that you leave this kind of work to the compiler and go have a cup of tea. If the changes are significant, and the performance is critical, (as say in the embedded world) then you might want to mix the normal code with the assemblar in some fashion, however, on most computers and chips the performance is usually sufficient to avoid this headache.
If you really need more performance, then optimise the code not the assembly.
None, I suppose. You've rejected the compiler's work in favor of your own. You might as well throw out the function you wrote in the compiled language, because now all you have is your assembler in that platform.
I would highly advise against engaging in this kind of optimization because unless you're sure, via profiling and analysis, that you truly are making a difference.
It depends on the language you wrote your function in. Some languages like C are very low-level, translating each function call or statement to specific assembly statements. If you did use C, you can replace your function with inline assembly to improve performance.
Other high-level languages may convert each statement into macro routines or other more complex calls on the assembly side. Certain optimizations (like tail recursion, loop unrolling, etc) can be implemented easily on the source side, but others (like making more efficient use of the register file) may be impossible (again, depending on the language and the compiler you're using).
Its tough to say there is any relationship between modified assembly and the source which generated the unmodified version. It will certainly confuse debugging tools: register contents will no longer match the source variables they were supposed to correspond to.
There are a number of places in packet processing code where I've examined the generated assembly and gone back to change the original source code in order to improve the result. Re-arranging source can reduce the number of branches, __attribute__ and compiler arguments can align branch points and functions to reduce I$ misses. In desperate cases a little inline assembly can be used, so that the binary can still be compiled from source.
Something you could try is to separate your original function into its own file, and provide a make rule to build the assembler from there. Then update the assembler file with your improved version, and provide a make rule to build an object file from the assembler file. Then change your link rules to include that object file.
If you only ever change the assembler file, that will keep on being used. If you ever change the original higher-level language file, the assembler file will be rebuilt and the object file built from the new (unimproved) version.
This gives you a relationship between the two; you probably want to add a warning comment at the top of the higher-level language file to warn about the behaviour. Using some form of VCS will give you the ability to recover the improved assembler file if you make a mistake here.
If you're writing a native compiled app in Visual C++, there are two methods:
Use the __asm { } block and write your assembler in there.
Write your functions in MASM assembler, assemble to .obj, and link it as an static library. In your C/C++ code, declare the function with an extern "C" declaration.
Other C/C++ compilers have similar approaches.
In this situation, you generally have two options: optimize the code or rewrite the compiler. I can't see where breaking the link between source and op is ever going to be the correct solution.
Related
i am new to java.
i wanted to know this.
what is the need to create the .class file in java ?
can't we just pass the source code to every machine so that each machine can compile it according to the OS and the hardware ?
I believe it's mostly for efficiency reasons.
From wikipedia http://en.wikipedia.org/wiki/Bytecode:
Bytecode, also known as p-code (portable code), is a form of
instruction set designed for efficient execution by a software
interpreter. Unlike human-readable source code, bytecodes are compact
numeric codes, constants, and references (normally numeric addresses)
which encode the result of parsing and semantic analysis of things
like type, scope, and nesting depths of program objects. They
therefore allow much better performance than direct interpretation of
source code.
(my emphasis)
And as others have mentioned possible weak obfuscation of the source code.
The main reason for the compilation is that the Virtual Machines which are used to host java classes and run them only understands bytecode
And since compiling a class each time to the language the virtual machine understands is expensive. That's the only reason why the source code is compiled into bytecode.
But we can also use some compilers which compiles source code directly into machine code.But that's a different story which I don't know about much.
I need a parser (mainly for the "select" type of queries) and avoid the hassle of doing it from scratch. Does anybody know how to use the scan.l/gram.y of pgsql for this purpose? I've looked up pgpool too, but it seems similar. Currently, it might be very helpful if someone could give instructions to compile the parser (using the makefile provided maybe) without errors so that it can be supplied (valid?) queries and outputs the parse tree (in whatever form)!
You probably cannot take any file from postgres source tarball and compile it separately. Parser use internal OOP structures (implemented in C). But there is some possibility (not simple) - ecpg preprocessor try to transform PostgreSQL gram file to secondary gram file - and you can use same mechanism. It use a small utility parse.pl (it is part of PostgreSQL source code (src/postgresql/src/interfaces/ecpg/preproc))
PostgreSQL compiles the language parser using yacc. Presumably you could take the yacc files and create a compatible parser with very little effort. Note you must have flex and yacc installed to do this.
Note this is not taking a .c file from source and transplanting it into your system. All you are getting is the parser, not the planner or anything else.
Given the level of detail in the question no more detail can be possible. Perhaps you could start there and post another question when you get stuck.
I would like to know in which language the "proto compiler" (the compiler used to generate source files from Java, Python or c++) is written? Is it maybe a mix of languages?
Any help would be appreciated.
Thanks in Advance
Horace
It appears to be written in C++. There's also documentation on Java and Python APIs, but those don't appear to contain the compiler itself (at least I don't see anything that's obviously the compiler in either case, though I didn't spend a whole lot of time looking for it either).
That said, I'm almost tempted to vote to close -- for most practical purposes, the language used to implement the compiler is basically a trivia question, irrelevant to actual use. There is, however, an entirely legitimate exception: if you're going to download and modify the compiler, knowing the language you'd need to work with could be quite useful.
The protoc compiler is written in C or C++ (its a native program anyway).
When I want to process proto files in java files, I
I use the protoc command to convert them to a Protocol Buffer File ie
protoc protofile.proto --descriptor_set_out=OutputFile
Read the new protocol buffer file (its a FileDescriptorSet) and use it
An over complicated example is example, is compileProto method in
http://code.google.com/p/protobufeditor/source/browse/trunk/%20protobufeditor/Source/ProtoBufEditor/src/net/sf/RecordEditor/ProtoBuf/re/display/ProtoLayoutSelection.java
its compilcated because options because the protoc command and options can be stored in a properties file.
Note: The getFileDescriptor method reads the newly created protocol buffer
To support multiple platforms in C/C++, one would use the preprocessor to enable conditional compiles. E.g.,
#ifdef _WIN32
#include <windows.h>
#endif
How can you do this in Ada? Does Ada have a preprocessor?
The answer to your question is no, Ada does not have a pre-processor that is built into the language. That means each compiler may or may not have one and there is not "uniform" syntax for pre-processing and things like conditional compilation. This was intentional: it's considered "harmful" to the Ada ethos.
There are almost always ways around a lack of a preprocessor but often times the solution can be a little cumbersome. For example, you can declare the platform specific functions as 'separate' and then use build-tools to compile the correct one (either a project system, using pragma body replacement, or a very simple directory system... put all the windows files in /windows/ and all the linux files in /linux/ and include the appropriate directory for the platform).
All that being said, GNAT realized that sometimes you need a preprocessor and has created gnatprep. It should work regardless of the compiler (but you will need to insert it into your build process). Similarly, for simple things (like conditional compilation) you can probably just use the c pre-processor or even roll your own very simple one.
AdaCore provides the gnatprep preprocessor, which is specialized for Ada. They state that gnatprep "does not depend on any special GNAT features", so it sounds as though it should work with non-GNAT Ada compilers. Their User Guide also provides some conditional compilation advice.
I have been on a project where m4 was used as well, with the Ada spec and body files suffixed as ".m4s" and ".m4b", respectively.
My preference is really to avoid preprocessing altogether, and just use specialized bodies, setting up CM and the build process to manage them.
No but the CPP preprocessor or m4 can be called on any file on the command line or using a building tool like make or ant. I suggest calling your .ada file something else. I have done this for some time on java files. I call the java file .m4 and use a make rule to create the .java and then build it in the normal way.
I hope that helps.
Yes, it has.
If you are using GNAT compiler, you can use gnatprep for doing the preprocessing, or if you use GNAT Programming Studio you can configure your project file to define some conditional compilation switches like
#if SOMESWITCH then
-- Your code here is executed only if the switch SOMESWITCH is active in your build configuration
#end if;
In this case you can use gnatmake or gprbuild so you don't have to run gnatprep by hand.
That's very useful, for example, when you need to compile the same code for several different OS's using even different cross-compilers.
Some old Ada1983-era compilers have a package called a.app that utilized a #-prefixed subset of Ada (interpreted at build-time) as a preprocessing language for generating Ada (to be then translated to machine code at compile-time). Rational's Verdix Ada Development System (VADS) appears to be the progenitor of a.app among several Ada compilers. Sun Microsystems, for example, derived the Ada SPARCompiler from VADS and thus also had a.app. This is not unlike the use of PL/I as the preprocessor of PL/I, which IBM did.
Chapter 2 is some documentation of what a.app looks like: http://dlc.sun.com/pdf/802-3641/802-3641.pdf
No, it does not.
If you really want one, there are ways to get one (Use C's, use a stand-alone one, etc.) However I'd argue against it. It was a purposeful design decision to not have one. The whole idea of a preprocessor is very un-Ada.
Most of what C's preprocessor is used for can be accomplished in Ada in other more reliable ways. The only major exception is in making minor changes to a source file for cross-platform support. Given how much this gets abused in a typical cross-platform C program, I'm still happy there's no support for it in Ada. Very few C/C++ developers can control themselves enough to keep the changes "minor". The result may work, but is often nearly impossible for a human to read.
The typical Ada way to accomplish this would be to put the different code in different files and use your build system to somehow choose between them at compile time. Make is plenty powerful enough to help you do this.
I'm producing a hex file to run on an ARM processor which I want to keep below 32K. It's currently a lot larger than that and I wondered if someone might have some advice on what's the best approach to slim it down?
Here's what I've done so far
So I've run 'size' on it to determine how big the hex file is.
Then 'size' again to see how big each of the object files are that link to create the hex files. It seems the majority of the size comes from external libraries.
Then I used 'readelf' to see which functions take up the most memory.
I searched through the code to see if I could eliminate calls to those functions.
Here's where I get stuck, there's some functions which I don't call directly (e.g. _vfprintf) and I can't find what calls it so I can remove the call (as I think I don't need it).
So what are the next steps?
Response to answers:
As I can see there are functions being called which take up a lot of memory. I cannot however find what is calling it.
I want to omit those functions (if possible) but I can't find what's calling them! Could be called from any number of library functions I guess.
The linker is working as desired, I think, it only includes the relevant library files. How do you know if only the relevant functions are being included? Can you set a flag or something for that?
I'm using GCC
General list:
Make sure that you have the compiler and linker debug options disabled
Compile and link with all size options turned on (-Os in gcc)
Run strip on the executable
Generate a map file and check your function sizes. You can either get your linker to generate your map file (-M when using ld), or you can use objdump on the final executable (note that this will only work on an unstripped executable!) This won't actually fix the problem, but it will let you know of the worst offenders.
Use nm to investigate the symbols that are called from each of your object files. This should help in finding who's calling functions that you don't want called.
In the original question was a sub-question about including only relevant functions. gcc will include all functions within every object file that is used. To put that another way, if you have an object file that contains 10 functions, all 10 functions are included in your executable even if one 1 is actually called.
The standard libraries (eg. libc) will split functions into many separate object files, which are then archived. The executable is then linked against the archive.
By splitting into many object files the linker is able to include only the functions that are actually called. (this assumes that you're statically linking)
There is no reason why you can't do the same trick. Of course, you could argue that if the functions aren't called the you can probably remove them yourself.
If you're statically linking against other libraries you can run the tools listed above over them too to make sure that they're following similar rules.
Another optimization that might save you work is -ffunction-sections, -Wl,--gc-sections, assuming you're using GCC. A good toolchain will not need to be told that, though.
Explanation: GNU ld links sections, and GCC emits one section per translation unit unless you tell it otherwise. But in C++, the nodes in the dependecy graph are objects and functions.
On deeply embedded projects I always try to avoid using any standard library functions. Even simple functions like "strtol()" blow up the binary size. If possible just simply avoid those calls.
In most deeply embedded projects you don't need a versatile "printf()" or dynamic memory allocation (many controllers have 32kb or less RAM).
Instead of just using "printf()" I use a very simple custom "printf()", this function can only print numbers in hexadecimal or decimal format not more. Most data structures are preallocated at compile time.
Andrew EdgeCombe has a great list, but if you really want to scrape every last byte, sstrip is a good tool that is missing from the list and and can shave off a few more kB.
For example, when run on strip itself, it can shave off ~2kB.
From an old README (see the comments at the top of this indirect source file):
sstrip is a small utility that removes the contents at the end of an
ELF file that are not part of the program's memory image.
Most ELF executables are built with both a program header table and a
section header table. However, only the former is required in order
for the OS to load, link and execute a program. sstrip attempts to
extract the ELF header, the program header table, and its contents,
leaving everything else in the bit bucket. It can only remove parts of
the file that occur at the end, after the parts to be saved. However,
this almost always includes the section header table, and occasionally
a few random sections that are not used when running a program.
Note that due to some of the information that it removes, a sstrip'd executable is rumoured to have issues with some tools. This is discussed more in the comments of the source.
Also... for an entertaining/crazy read on how to make the smallest possible executable, this article is worth a read.
Just to double-check and document for future reference, but do you use Thumb instructions? They're 16 bit versions of the normal instructions. Sometimes you might need 2 16 bit instructions, so it won't save 50% in code space.
A decent linker should take just the functions needed. However, you might need compiler & linke settings to package functions for individual linking.
Ok so in the end I just reduced the project to it's simplest form, then slowly added files one by one until the function that I wanted to remove appeared in the 'readelf' file. Then when I had the file I commented everything out and slowly add things back in until the function popped up again. So in the end I found out what called it and removed all those calls...Now it works as desired...sweet!
Must be a better way to do it though.
To answer this specific need:
•I want to omit those functions (if possible) but I can't find what's
calling them!! Could be called from any number of library functions I
guess.
If you want to analyze your code base to see who calls what, by whom a given function is being called and things like that, there is a great tool out there called "Understand C" provided by SciTools.
https://scitools.com/
I have used it very often in the past to perform static code analysis. It can really help to determine library dependency tree. It allows to easily browse up and down the calling tree among other things.
They provide a limited time evaluation, then you must purchase a license.
You could look at something like executable compression.