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I was benchmarking some code that I'm working on recently, and happened to notice that my code was noticeably faster with SDL checks (/sdl) enabled. From my testing, it is consistently about 20% faster with SDL checks, all other basic optimizations enabled. This surprised me, as I did not expect SDL checks to make any performance difference, or if anything I would have expected it to be slower. The only reason I even noticed is because I noticed that the code was slower when built with CMake compared to Visual Studio, which I eventually narrowed down to the /sdl flag. With inlining or optimizations disabled, the results are reversed with /sdl being slightly slower. Profiling shows no major differences in the functions that are using most of the compute time, they all just seem to be a little bit slower.
I assume this is not a general pattern, and probably has something to do with the specific nature of my code. It's a library for parsing a file into a bunch of structs, so lots of shuffling bytes around, not too many allocations or pointers (mostly just a few vectors). The code is here if anyone wishes to peruse it. I don't even know if this would be observed on other computers, but I have no easy way to test that.
My best guess is that /sdl is either intentionally or unintentionally triggering some other optimization, but I cannot figure out or guess what that might be. Normally I wouldn't particularly care about this, but since I had already been benchmarking this code and considering the surprisingly large difference, I can't stop thinking about it.
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I have two functions performing same process but with different techniques and I need to know on a large scale which technique is faster than the other of maybe in the future will be more techniques available. So my question is, how can I do that in c++ specially? Is there a specific method and header to be used to perform this task?
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For example the isLargest() uses three parameters and it has two versions, one uses a nested if technique and the other uses initializers and less if statements. So if I need to know which one is faster, how can I do that?
Try your code in the real world and measure
There is a tool called a profiler that is meant to solve this problem. Broadly speaking, there are two kinds (note: some are a mix between the two):
Sampling profilers.
Instrumenting profilers.
It's worth learning about what each does and their pros/cons, but if you don't know what to use go with a sampling profiler.
There are many sampling profilers, but support depends on your platform. If you're on Windows, Visual Studio comes with a really nice sampling profiler and I recommend you start there!
If you go down this route, it's important to make sure you use your functions as you would "for real" when you're profiling them, as there are many subtle factors that can affect the result.
An alternative
If you don't want to try your code running in a real program, perhaps if you're just trying to understand general characteristics of the function, there are libraries to help you do this such as Google Benchmark.
Benchmarking code can be surprisingly difficult to get right, so I would strongly recommend using existing benchmarking tools where like Google Benchmark wherever possible.
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After a lot of research i could not find any solution to my question (if i did i woudln't be here ...)
I'm looking for solutions that permits me to reduce the flash memory used by my program.
I'm programming an embedded c++ programm and when i Flash my electronic card in release mode everything is fine cause it doesn't overflow the space of the flash memory, but that is not the case when i do it in Debug mode... I want to know if it is possible to find functions (my goal is to do it without reducing the code) that could reduce Flash memory.I already thought about defragmentation but I don't find how to do it in embedded even though i don't even know if i can ... I also tried the -Os cmd from gcc but without any big success
So I'm taking any advices or support and i'll be there at any question about my issue ;)
Thanks !
Look at your map file. Is there something there you don't
expect? Functions you aren't expecting (like floating point, or
exception handling, etc.) or something unreasonably large?
Turn on optimization except for the file you're interested in.
Make sure you've actually got optimizations turned on (look at the build log and ensure that you've got -Os being passed to each compile step)
Consider link time optimizations, but don't expect miracles
Welcome to embedded programming. 90% of the job is figuring out how to stuff the never ending requirements into the memory available. Rinse and repeat.
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I'm currently doing research about memory checking tools for Windows 7, MS Visual Studio C++. For tools which are free, or have demo available, I would like to run special test: run a simple application which would contain as many different memory errors as possible so I could check how many of them the tool can detect.
So I would like to ask whether there is available source code in C++ for such application.
Notes:
I Googled this thing, but all I could find are questions and sites about memory checking tools.
Of course I could write such app (and I will do it if there is no such code available), but I know I will probably forget some kinds of errors. For example I even didn't think about checking whether memcpy's parameters are overlapping (which is an error - use memmove instead, or revise the code) until valgrind found it in my personal project (in this case, I copied an array into multiple places and I forgot (in a loop) to omit the first (source) array). I believe there is much more error types I never thought about.
I cannot use valgrind as it MUST be Windows MS Visual C++ application, I cannot change this decision, and valgrind under Wine looks like too complicated so I probably won't be able to convince my colleagues to use this solution. And it's not even sure that Wine will run our app...
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I work on embedded systems with limited memory and throughput. Making a system more robust requires more memory, and time on the processor. I like the idea of the Chaos Monkey to figure out if your system is fault tolerant, however with limited resources I'm not sure how feasible it is to just keep adding code to it. Are there certain design considerations whether in the architecture or otherwise that would improve the fault handling capabilities, without necessarily adding "more code" to a system?
One way I have seen to help in preventing writing an if then statement in c (or c++) that assigns rather then compares a static value, recommends writing that on the left hand side of the comparison, this way if you try to assign your variable to say the number 5, things will complain and you're likely to find any issues right away.
Are there Architectural or Design decisions that can be made early on that prevent possible redundancy/reliability issues in a similar way?
Yes many other techniques can be used. You'd do well to purchase and read "Code Complete".
Code Complete on Amazon
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I have a program in which I've lost the C++ source code. Are there any good C++ decompilers out there?
I've already ran across Boomerang.
You can use IDA Pro by Hex-Rays. You will usually not get good C++ out of a binary unless you compiled in debugging information. Prepare to spend a lot of manual labor reversing the code.
If you didn't strip the binaries there is some hope as IDA Pro can produce C-alike code for you to work with. Usually it is very rough though, at least when I used it a couple of years ago.
information is discarded in the compiling process. Even if a decompiler could produce the logical equivalent code with classes and everything (it probably can't), the self-documenting part is gone in optimized release code. No variable names, no routine names, no class names - just addresses.
Yes, but none of them will manage to produce readable enough code to worth the effort. You will spend more time trying to read the decompiled source with assembler blocks inside, than rewriting your old app from scratch.
I haven't seen any decompilers that generate C++ code. I've seen a few experimental ones that make a reasonable attempt at generating C code, but they tended to be dependent on matching the code-generation patterns of a particular compiler (that may have changed, it's been awhile since I last looked into this). Of course any symbolic information will be gone. Google for "decompiler".
Depending on how large and how well-written the original code was, it might be worth starting again in your favourite language (which might still be C++) and learning from any mistakes made in the last version. Didn't someone once say about writing one to throw away?
n.b. Clearly if this is a huge product, then it may not be worth the time.