gistfile1.c

/* To my understanding, the following is well-defined C:
 *
 *  given: */
int* p;

// then:
char* c = (char*)p;
char firstByteAtP = *c;
// firstByteAtP is the first byte at the pointer p.

// Put another way, aliasing through char* is OK; otherwise, memcpy could not be written.

// Now, my question: is the following code well-defined?

char* c;
intptr_t i;
memcpy(&i, &p, sizeof(p));
memcpy(&c, &i, sizeof(c));
char firstByteAtP = *c;

/*
 * That is, can you alias c to p through a memcpy?
 * If so, C precludes an implementation where a pointer to a wider type has a different implementation,
 * such as indexing assuming aligned access.
 *
 * Emscripten could realize significant code size reductions if it could represent 32-bit loads and
 * stores as HEAP32[p] instead of HEAP32[p >> 2], but that would require shifting the pointers whenever
 * statically casting between char* and int* (such as from the result of malloc).
 */

It is well formed because there are no intervening possibly aliasing accesses to i.