How to guide GCC optimizations based on assertions without runtime cost?

Is there a way to express that the condition in the contract has no side effect, so that it is always optimized out?

Not likely.

It’s known that you cannot take a big collection of assertions, turn them into assumptions (via __builtin_unreachable) and expect good results (e.g. Assertions Are Pessimistic, Assumptions Are Optimistic by John Regehr).

Some clues:

• CLANG, while already having the __builtin_unreachable intrinsic, introduced __builtin_assume exactly for this purpose.

• N4425 – Generalized Dynamic Assumptions^(*) notes that:

  GCC does not explicitly provide a general assumption facility, but general assumptions can be encoded using a combination of control flow and the __builtin_unreachable intrinsic

  …

  The existing implementations that provide generic assumptions use some keyword in the implementation­reserved identifier space (__assume, __builtin_assume, etc.). Because the expression argument is not evaluated (side effects are discarded), specifying this in terms of a special library function (e.g. std::assume) seems difficult.

• The Guidelines Support Library (GSL, hosted by Microsoft, but in no way Microsoft specific) has “merely” this code:

  #ifdef _MSC_VER
  #define GSL_ASSUME(cond) __assume(cond)
  #elif defined(__clang__)
  #define GSL_ASSUME(cond) __builtin_assume(cond)
  #elif defined(__GNUC__)
  #define GSL_ASSUME(cond) ((cond) ? static_cast<void>(0) : __builtin_unreachable())
  #else
  #define GSL_ASSUME(cond) static_cast<void>(!!(cond))
  #endif
  

  and notes that:

  // GSL_ASSUME(cond)
  //
  // Tell the optimizer that the predicate cond must hold. It is unspecified
  // whether or not cond is actually evaluated.
  

_{*) Paper rejected: EWG’s guidance was to provide the functionality within the proposed contract facilities.}