EDIT: I see- a different reading of the post than my original take would suggest that the author is saying, you as a C/C++ programmer are "not allowed" to compile code that would cause under/overflow. The compiler assumes that you are following this rule, enabling these optimizations. The author's wording is unclear as to who or what is not being allowed to under/overflow, hence my confusion.

---

Right off the bat-

> Signed integers are not allowed to wrap in C and C++

What? This is...not true.

  $ cat > test.c
  #include <stdio.h>
  #include <limits.h>
  
  int main()
  {
      int i = INT_MAX;
      printf("%i\n%i\n", i, i+1);
  }
  
  $ cc test.c 
  $ ./a.out 
  2147483647
  -2147483648

Im not an expert on these matters, but I suspect signed integers are not defined as wrapping and in fact not defined as doing anything particular in under/overflow situations, though in one common convention, two's complement integer arithmetic, they often do. The optimizations rely on the fact that the programmer can't depend on any specific overflow or underflow behavior.