Assuming alignment is a uintptr_t power of 2, looking for the next properly aligned address can be done using this expression: (address + alignment - 1u) & ~(alignment - 1u) This is used in cu...

Recently we switched to a more recent GCC and it optimized away a whole function and replaced it with "null pointer access" trap code instead when optimizing for size. Looking at godbolt,...

Now we all sometimes have to work with binary data. In C++ we work with sequences of bytes, and since the beginning char was the our building block. Defined to have sizeof of 1, it is the byte. And...

One of the examples of undefined behavior from the C standard reads (J.2): — An array subscript is out of range, even if an object is apparently accessible with the given subscript (as in the l...

I read that char *- and their signed and unsigned counterparts - can alias any type without violating the strict aliasing rule. However, having a char * point to an int variable and casting that ch...

This question refers to the current C++20 draft. The quoted passages have been slightly modified from previous standard iterations, but not in relevant ways as far as I know. I am looking for clar...

Consider the following valarray-like class: #include <stdlib.h> struct va { void add1(const va& other); void add2(const va& other); size_t* data; size_t size; }; void va::add1(...

std::byte is defined in C++17 as: enum class byte : unsigned char {}; I'm currently stuck at using C++14, and I wonder if I add the same definition in C++14 (in some non-std namespace, along with ...

After P0593R6 ('Implicit creation of objects for low-level object manipulation') was accepted in C++20, C++23 will get std::start_lifetime_as() which 'completes the functionality proposed in [P0593...

I have my custom little OOP-esque inheritance functionality, something like this: // base class struct BaseTag; typedef struct { int (*DoAwesomeStuff)(struct BaseTag* pInstance); } S_BaseVtable; ...

There are a few questions and answers on the site already concerning pointer-interconvertibility of structs and their first member variable, as well as structs and their first public base. This que...

There seems to be some agreement that you can't willy nilly point (an int*) into a char array because of the C++ aliasing rules. From this other question -- Generic char[] based storage and avoidi...

This example is copied from cppreference. struct Y { int z; }; alignas(Y) std::byte s[sizeof(Y)]; Y* q = new(&s) Y{2}; const int f = reinterpret_cast<Y*>(&s)->z; // Class member ac...

Can I use a std::array<int, N> to alias parts of a int[] without invoking UB? https://en.cppreference.com/w/cpp/container/array "This container is an aggregate type with the same semanti...

So I was re-reading C17 6.5/6 - 6.5/7 regarding effective type and strict aliasing, but couldn't figure out how to treat qualifiers. Some things confuse me: I always assumed that qualifiers aren't...

Given type definitions struct a { int a; }; struct b { int b; struct a ba;}; and a function taking struct a* a and struct b* b, the type information expresses possible overlap between a->a and ...

Reading https://en.cppreference.com/w/cpp/language/reinterpret_cast I wonder what are use-cases of reinterpret_cast that are not UB and are used in practice? The above description contains many cas...

Consider the following two snippets: #define ALIGN_BYTES 32 #define ASSUME_ALIGNED(x) x = __builtin_assume_aligned(x, ALIGN_BYTES) void fn0(const float *restrict a0, const float *restrict a1, fl...

While looking at the code for Dear Imgui, I found the following code (edited for relevance): struct ImVec2 { float x, y; float& operator[] (size_t idx) { return (&x)[idx]; } }; It's pret...

In C, what exactly are the performance benefits that come with observing strict aliasing?

The accepted answer to What is the strict aliasing rule? mentions that you can use char * to alias another type but not the other way. It doesn't make sense to me — if we have two pointers, one of...

Since strict aliasing may help compiler optimize better, C99 introduced the restrict keyword which can be used as a qualifier of a variable if programmers guarantee that it won't be accessed throug...

I believe 6.5p7 in the C standard defines the so-called strict aliasing rule as follows. An object shall have its stored value accessed only by an lvalue expression that has one of the following t...

I'm very confused by the LLVM AliasAnalysis implementation. Say I have this program: int* key = malloc(4); *key = 10; *key = 11; It gets transformed to IR code like this: %3 = call noalias i8* @m...

I'm trying to fix two warnings when compiling a specific program using GCC. The warnings are: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing] and t...