I wonder if currently there is support for clang static analysis with various flows checking. For example - there is a checker to check for zero division, but can clang find a flow which a divider can evaluate to zero ?
simple example: in the bellow example there is a flow (i=0) that b will evaluate to 0. do I get warning here ?
for(int i = 10; i>=0; i--){
int a = div(i);
...
}
int div(int b){
return 100000 / b;
}
if not, is there a plan to support this ?
Related
I'd like to extend my instrumental Profiler in order to avoid it affect too much performances.
Im my current implementation, I'm using a ProfilerHelper taking one string, which is put whereever you want in the profiling f().
The ctor is starting the measurement and the dector is closing it, logging the Delta in an unordered_map entry, which is key is the string.
Now, I'd like to turn all of that into a faster stuff.
First of all, I'd like to create a string LUT (Look Up Table) contaning the f()s names at compile time, and turn the unordered_map to a plain vector which is paired by the string function LUT.
Now the question is: I've managed to create a LUT but std::string_view, but I cannot find a way to extend it at compile time.
A first rought trial sounds like this:
template<unsigned N>
constexpr auto LUT() {
std::array<std::string_view, N> Strs{};
for (unsigned n = 0; n < N; n++) {
Strs[n] = "";
}
return Strs;
};
constexpr std::array<std::string_view, 0> StringsLUT { LUT<0>() };
constexpr auto AddString(std::string_view const& Str)
{
constexpr auto Size = StringsLUT.size();
std::array<std::string_view, Size + 1> Copy{};
for (auto i = 0; i < Size; ++i)
Copy[i] = StringsLUT[i];
Copy[Size] = Str;
return Copy;
};
int main()
{
constexpr auto Strs = AddString(__builtin_FUNCTION());
//for (auto const Str : Strs)
std::cout << Strs[0] << std::endl;
}
So my idea should be to recall the AddString whenever needed in my f()s to be profiled, extending this list at compile time.
But of course I should take the returned Copy and replace the StringsLUT everytime, to land to a final StringsLUT with all the f() names inside it.
Is there a way to do that at compile time?
Sorry, but I'm just entering the magic "new" world of constexpr applied to LUT right in these days.
Tx for your support in advance.
This is my code:
int foo(int x) {
return x + 1; // I have more complex code here
}
int main() {
int s = 0;
for (int i = 0; i < 1000000; ++i) {
s += foo(42);
}
}
Without -O3 this code works for a few minutes. With -O3 it returns the same result in no time. Clang++, I believe, caches the value of foo(42) (it's a pure function) and doesn't call it a million times. How can I instruct it NOT to apply this particular optimization for this particular function call?
Out of curiosity, can you share why you would want to disable that optimization?
Anyway, about your question:
In your example code, s is never read after the loop, so the compiler would throw the whole loop away. So let's assume that s is used after the loop.
I'm not aware of any pragmas or compiler options to disable a particular optimization in a particular section of code.
Is changing the code an option?
To prevent that optimization in a portable manner, you can look for a creative way to compute the function call argument in a way such that the compiler is no longer able to treat the argument as constant. Of course the challenge here is to actually use a trick that does not rely on undefined behavior and that cannot be "outsmarted" by a newer compiler version.
See the commented example below.
pro: you use a trick that uses only the language that you can apply selectively
con: you get an additional memory access in every loop iteration; however, the access will be satisfied by your CPU cache most of the time
I verified the generated assembly for your particular example with clang++ -O3 -S. The compiler now generates your loop and no longer caches the result. However, the function gets inlined. If you want to prevent that as well, you can declare foo with __attribute__((noinline)), for example.
int foo(int x) {
return x + 1; // I have more complex code here
}
volatile int dummy = 0; // initialized to 0 and never changed
int main() {
int s = 0;
for (int i = 0; i < 1000000; ++i) {
// Because of the volatile variable, the compiler is forced to assume
// that the function call argument is different for each loop
// iteration and it is no longer able to use a cached result.
s += foo(42 + dummy);
}
}
I store various formulas in Postgres and I want to use those formulas in my code. It would look something like this:
var amount = 100;
var formula = '5/105'; // normally something I would fetch from Postgres
var total = amount * formula; // should return 4.76
Is there a way to evaluate the string in this manner?
As far as I'm aware, there isn't a formula solver package developed for Dart yet. (If one exists or gets created after this post, we can edit it into the answer.)
EDIT: Mattia in the comments points out the math_expressions package, which looks pretty robust and easy to use.
There is a way to execute arbitrary Dart code as a string, but it has several problems. A] It's very roundabout and convoluted; B] it becomes a massive security issue; and C] it only works if the Dart is compiled in JIT mode (so in Flutter this means it will only work in debug builds, not release builds).
So the answer is that unfortunately, you will have to implement it yourself. The good news is that, for simple 4-function arithmetic, this is pretty straight-forward, and you can follow a tutorial on writing a calculator app like this one to see how it's done.
Of course, if all your formulas only contain two terms with an operator between them like in your example snippet, it becomes even easier. You can do the whole thing in just a few lines of code:
void main() {
final amount = 100;
final formula = '5/105';
final pattern = RegExp(r'(\d+)([\/+*-])(\d+)');
final match = pattern.firstMatch(formula);
final value = process(num.parse(match[1]), match[2], num.parse(match[3]));
final total = amount * value;
print(total); // Prints: 4.761904761904762
}
num process(num a, String operator, num b) {
switch (operator) {
case '+': return a + b;
case '-': return a - b;
case '*': return a * b;
case '/': return a / b;
}
throw ArgumentError(operator);
}
There are a few packages that can be used to accomplish this:
pub.dev/packages/function_tree
pub.dev/packages/math_expressions
pub.dev/packages/expressions
I used function_tree as follows:
double amount = 100.55;
String formula = '5/105*.5'; // From Postgres
final tax = amount * formula.interpret();
I haven't tried it, but using math_expressions it should look like this:
double amount = 100.55;
String formula = '5/105*.5'; // From Postgres
Parser p = Parser();
// Context is used to evaluate variables, can be empty in this case.
ContextModel cm = ContextModel();
Expression exp = p.parse(formula) * p.parse(amount.toString());
// or..
//Expression exp = p.parse(formula) * Number(amount);
double result = exp.evaluate(EvaluationType.REAL, cm);
// Result: 2.394047619047619
print('Result: ${result}');
Thanks to fkleon for the math_expressions help.
I'm currently building a prime number finder, and am having a memory problem:
This may be due to a corruption of the heap, which indicates a bug in PrimeNumbers.exe or any of the DLLs it has loaded.
PS. Please don't say to me if this isn't the way to find prime numbers, I want to figure it out myself!
Code:
// PrimeNumbers.cpp : main project file.
#include "stdafx.h"
#include <vector>
using namespace System;
using namespace std;
int main(array<System::String ^> ^args)
{
Console::WriteLine(L"Until what number do you want to stop?");
signed const int numtstop = Convert::ToInt16(Console::ReadLine());
bool * isvalid = new bool[numtstop];
int allattempts = numtstop*numtstop; // Find all the possible combinations of numbers
for (int currentnumb = 0; currentnumb <= allattempts; currentnumb++) // For each number try to find a combination
{
for (int i = 0; i <= numtstop; i++)
{
for (int tnumb = 0; tnumb <= numtstop; tnumb++)
{
if (i*tnumb == currentnumb)
{
isvalid[currentnumb] = false;
Console::WriteLine("Error");
}
}
}
}
Console::WriteLine(L"\nAll prime number in the range of:" + Convert::ToString(numtstop));
for (int pnts = 0; pnts <= numtstop; pnts++)
{
if (isvalid[pnts] != false)
{
Console::WriteLine(pnts);
}
}
return 0;
}
I don't see the memory problem.
Please help.
You are allocating numtstop booleans, but you index that array using a variable that ranges from zero to numtstop*numtstop. This will be severely out of bounds for all numstop values greater than 1.
You should either allocate more booleans (numtstop*numtstop) or use a different variable to index into isvalid (for example, i, which ranges from 0 to numstop). I am sorry, I cannot be more precise than that because of your request not to comment on your algorithm of finding primes.
P.S. If you would like to read something on the topic of finding small primes, here is a link to a great book by Dijkstra. He teaches you how to construct a program for the first 1000 primes on pages 35..49.
Problem is that you use native C++ in managed C++/CLI code. And use new without delete of course.
`currentnumb` :
is bigger than the size of the array, which is just numtstop. You are probably going out of bound, this might be your issue.
You never delete[] your isvalid local, this is a memory leak.
In my previous question I was looking for a way of evaulating complex mathematical expressions in C, most of the suggestions required implementing some type of parser.
However one answer, suggested using Lua for evaluating the expression. I am interested in this approach but I don't know anything about Lua.
Can some one with experience in Lua shed some light?
Specifically what I'd like to know is
Which API if any does Lua provide that can evaluate mathematical expressions passed in as a string? If there is no API to do such a thing, may be some one can shed some light on the linked answer as it seemed like a good approach :)
Thanks
The type of expression I'd like to evaluate is given some user input such as
y = x^2 + 1/x - cos(x)
evaluate y for a range of values of x
It is straightforward to set up a Lua interpreter instance, and pass it expressions to be evaluated, getting back a function to call that evaluates the expression. You can even let the user have variables...
Here's the sample code I cooked up and edited into my other answer. It is probably better placed on a question tagged Lua in any case, so I'm adding it here as well. I compiled this and tried it for a few cases, but it certainly should not be trusted in production code without some attention to error handling and so forth. All the usual caveats apply here.
I compiled and tested this on Windows using Lua 5.1.4 from Lua for Windows. On other platforms, you'll have to find Lua from your usual source, or from www.lua.org.
Update: This sample uses simple and direct techniques to hide the full power and complexity of the Lua API behind as simple as possible an interface. It is probably useful as-is, but could be improved in a number of ways.
I would encourage readers to look into the much more production-ready ae library by lhf for code that takes advantage of the API to avoid some of the quick and dirty string manipulation I've used. His library also promotes the math library into the global name space so that the user can say sin(x) or 2 * pi without having to say math.sin and so forth.
Public interface to LE
Here is the file le.h:
/* Public API for the LE library.
*/
int le_init();
int le_loadexpr(char *expr, char **pmsg);
double le_eval(int cookie, char **pmsg);
void le_unref(int cookie);
void le_setvar(char *name, double value);
double le_getvar(char *name);
Sample code using LE
Here is the file t-le.c, demonstrating a simple use of this library. It takes its single command-line argument, loads it as an expression, and evaluates it with the global variable x changing from 0.0 to 1.0 in 11 steps:
#include <stdio.h>
#include "le.h"
int main(int argc, char **argv)
{
int cookie;
int i;
char *msg = NULL;
if (!le_init()) {
printf("can't init LE\n");
return 1;
}
if (argc<2) {
printf("Usage: t-le \"expression\"\n");
return 1;
}
cookie = le_loadexpr(argv[1], &msg);
if (msg) {
printf("can't load: %s\n", msg);
free(msg);
return 1;
}
printf(" x %s\n"
"------ --------\n", argv[1]);
for (i=0; i<11; ++i) {
double x = i/10.;
double y;
le_setvar("x",x);
y = le_eval(cookie, &msg);
if (msg) {
printf("can't eval: %s\n", msg);
free(msg);
return 1;
}
printf("%6.2f %.3f\n", x,y);
}
}
Here is some output from t-le:
E:...>t-le "math.sin(math.pi * x)"
x math.sin(math.pi * x)
------ --------
0.00 0.000
0.10 0.309
0.20 0.588
0.30 0.809
0.40 0.951
0.50 1.000
0.60 0.951
0.70 0.809
0.80 0.588
0.90 0.309
1.00 0.000
E:...>
Implementation of LE
Here is le.c, implementing the Lua Expression evaluator:
#include <lua.h>
#include <lauxlib.h>
#include <stdlib.h>
#include <string.h>
static lua_State *L = NULL;
/* Initialize the LE library by creating a Lua state.
*
* The new Lua interpreter state has the "usual" standard libraries
* open.
*/
int le_init()
{
L = luaL_newstate();
if (L)
luaL_openlibs(L);
return !!L;
}
/* Load an expression, returning a cookie that can be used later to
* select this expression for evaluation by le_eval(). Note that
* le_unref() must eventually be called to free the expression.
*
* The cookie is a lua_ref() reference to a function that evaluates the
* expression when called. Any variables in the expression are assumed
* to refer to the global environment, which is _G in the interpreter.
* A refinement might be to isolate the function envioronment from the
* globals.
*
* The implementation rewrites the expr as "return "..expr so that the
* anonymous function actually produced by lua_load() looks like:
*
* function() return expr end
*
*
* If there is an error and the pmsg parameter is non-NULL, the char *
* it points to is filled with an error message. The message is
* allocated by strdup() so the caller is responsible for freeing the
* storage.
*
* Returns a valid cookie or the constant LUA_NOREF (-2).
*/
int le_loadexpr(char *expr, char **pmsg)
{
int err;
char *buf;
if (!L) {
if (pmsg)
*pmsg = strdup("LE library not initialized");
return LUA_NOREF;
}
buf = malloc(strlen(expr)+8);
if (!buf) {
if (pmsg)
*pmsg = strdup("Insufficient memory");
return LUA_NOREF;
}
strcpy(buf, "return ");
strcat(buf, expr);
err = luaL_loadstring(L,buf);
free(buf);
if (err) {
if (pmsg)
*pmsg = strdup(lua_tostring(L,-1));
lua_pop(L,1);
return LUA_NOREF;
}
if (pmsg)
*pmsg = NULL;
return luaL_ref(L, LUA_REGISTRYINDEX);
}
/* Evaluate the loaded expression.
*
* If there is an error and the pmsg parameter is non-NULL, the char *
* it points to is filled with an error message. The message is
* allocated by strdup() so the caller is responsible for freeing the
* storage.
*
* Returns the result or 0 on error.
*/
double le_eval(int cookie, char **pmsg)
{
int err;
double ret;
if (!L) {
if (pmsg)
*pmsg = strdup("LE library not initialized");
return 0;
}
lua_rawgeti(L, LUA_REGISTRYINDEX, cookie);
err = lua_pcall(L,0,1,0);
if (err) {
if (pmsg)
*pmsg = strdup(lua_tostring(L,-1));
lua_pop(L,1);
return 0;
}
if (pmsg)
*pmsg = NULL;
ret = (double)lua_tonumber(L,-1);
lua_pop(L,1);
return ret;
}
/* Free the loaded expression.
*/
void le_unref(int cookie)
{
if (!L)
return;
luaL_unref(L, LUA_REGISTRYINDEX, cookie);
}
/* Set a variable for use in an expression.
*/
void le_setvar(char *name, double value)
{
if (!L)
return;
lua_pushnumber(L,value);
lua_setglobal(L,name);
}
/* Retrieve the current value of a variable.
*/
double le_getvar(char *name)
{
double ret;
if (!L)
return 0;
lua_getglobal(L,name);
ret = (double)lua_tonumber(L,-1);
lua_pop(L,1);
return ret;
}
Remarks
The above sample consists of 189 lines of code total, including a spattering of comments, blank lines, and the demonstration. Not bad for a quick function evaluator that knows how to evaluate reasonably arbitrary expressions of one variable, and has rich library of standard math functions at its beck and call.
You have a Turing-complete language underneath it all, and it would be an easy extension to allow the user to define complete functions as well as to evaluate simple expressions.
Since you're lazy, like most programmers, here's a link to a simple example that you can use to parse some arbitrary code using Lua. From there, it should be simple to create your expression parser.
This is for Lua users that are looking for a Lua equivalent of "eval".
The magic word used to be loadstring but it is now, since Lua 5.2, an upgraded version of load.
i=0
f = load("i = i + 1") -- f is a function
f() ; print(i) -- will produce 1
f() ; print(i) -- will produce 2
Another example, that delivers a value :
f=load('return 2+3')
print(f()) -- print 5
As a quick-and-dirty way to do, you can consider the following equivalent of eval(s), where s is a string to evaluate :
load(s)()
As always, eval mechanisms should be avoided when possible since they are expensive and produce a code difficult to read.
I personally use this mechanism with LuaTex/LuaLatex to make math operations in Latex.
The Lua documentation contains a section titled The Application Programming Interface which describes how to call Lua from your C program. The documentation for Lua is very good and you may even be able to find an example of what you want to do in there.
It's a big world in there, so whether you choose your own parsing solution or an embeddable interpreter like Lua, you're going to have some work to do!
function calc(operation)
return load("return " .. operation)()
end