When using a pointer to a dynamically allocated array (directly using new[] and delete[]), the compiler optimized away the calls to operator new and operator delete even though they have observable side effects. This optimization is allowed by the C++ standard section 5.3.4 paragraph 10: An implementation is allowed to omit a call to a … Read more
A complete example Here’s a little example that shows the situation: module A where big :: () -> [Int] big _ = [1..10^7] Looks like a function, right? But what does GHC do? It floats the enum to the top level! A.big1 :: [Int] [ Unf=Unf{Src=<vanilla>, TopLvl=True, Arity=0, Value=False, ConLike=False, Cheap=False, Expandable=False, Guidance=IF_ARGS [] 7 … Read more
It’s a bug in your code, nothing wrong with the compiler or the optimization levels. The cinch is with expression templates (like the ones used by Boost Proto, and hence by Boost Spirit). They are only valid to the end of their enclosing full expression [1] The canonical workaound is: BOOST_SPIRIT_AUTO(ana, *~qi::char_(‘*’) > +qi::char_(‘*’)); You … Read more
The compiler is not allowed to perform “optimizations”, which would result in a different value computed, than the one computed according to abstract machine semantics. 5.1.2.3 Program execution [#1] The semantic descriptions in this International Standard describe the behavior of an abstract machine in which issues of optimization are irrelevant. [#3] In the abstract machine, … Read more
tl;dr doNotOptimizeAway creates an artificial “use”s. A little bit of terminology here: a “def” (“definition”) is a statement, which assigns a value to a variable; a “use” is a statement, which uses the value of a variable to perform some operation. If from the point immediately after a def, all the paths to the program … Read more
I guess the question that needs to be answered why well-intentioned people would write the checks in the first place. The most common case is probably if you have a class that is part of a naturally occurring recursive call. If you had: struct Node { Node* left; Node* right; }; in C, you might … Read more
This is a longstanding bug in the Visual C++ compiler. When a class derives from multiple empty base classes, only the initial empty base class will be optimized using the empty base optimization (EBO). This issue was reported on Microsoft Connect in 2006: Empty Base Optimization Not Working Properly. At the moment, old bugs are … Read more
There are multiple reasons why the C compiler cannot automatically reorder the fields: The C compiler doesn’t know whether the struct represents the memory structure of objects beyond the current compilation unit (for example: a foreign library, a file on disc, network data, CPU page tables, …). In such a case the binary structure of … Read more
That’s a good problem. In order to solve that problem you will also have to disable ASLR otherwise the address of g() will be unpredictable. Disable ASLR: sudo bash -c ‘echo 0 > /proc/sys/kernel/randomize_va_space’ Disable canaries: gcc overflow.c -o overflow -fno-stack-protector After canaries and ASLR are disabled it should be a straight forward attack like … Read more