C++ attribute: assume (since C++23)

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Tells the compiler to assume that an expression will always evaluate to true at a given point, and asks the compiler to perform optimizations relying on that fact.

IMPORTANT: DANGEROUS OPERATION, USE WITH CARE. This is a dangerous operation for advanced experts only. It is used to inject a data fact directly into the compiler that the compiler cannot verify, and it must be impossible for the expression to ever be false under any circumstances. If the expression can ever evaluate to false, it will inject undefined behavior into your whole program (not just where the assumption appears). It is not the same as an assertion or precondition, and should never be used in place of an assertion or precondition.


[edit] Syntax

[[assume( expression )]]
expression - expression that must evaluate to true

[edit] Explanation

Can only be applied to a null statement, as in [[assume(x > 0)]];. This statement is called an assumption. If the expression (contextually converted to bool) would not evaluate to true at the place the assumption occurs, the behavior is undefined. Otherwise, the statement does nothing. In particular, the expression is not evaluated (but it is still potentially evaluated).

The purpose of an assumption is to allow compiler optimizations based on the information given.

The expression may not be a comma operator expression, but enclosing the expression in parentheses will allow the comma operator to be used.

[edit] Notes

If the expression would have undefined behavior, or if it would cause an exception to be thrown, then it does not evaluate to true.

Since assumptions cause undefined behavior if they do not hold, they should be used sparingly. They are not intended as a mechanism to document the preconditions of a function or to diagnose violations of preconditions. Also, it should not be presumed, without checking, that the compiler actually makes use of any particular assumption.

[edit] Example

void f(int& x, int y)
    void g(int);
    void h();
    [[assume(x > 0)]]; // Compiler may assume x is positive
    g(x / 2); // More efficient code possibly generated
    x = 3;
    int z = x;
    [[assume((h(), x == z))]]; // Compiler may assume x would have the same value after
                               // calling h
                               // The assumption does not cause a call to h
    g(x); // Compiler may replace this with g(3);
    g(x); // Compiler may NOT replace this with g(3);
          // An assumption applies only at the point where it appears
    z = std::abs(y);
    [[assume((g(z), true))]]; // Compiler may assume g(z) will return
    g(z); // Due to above and below assumptions, compiler may replace this with g(10);
    [[assume(y == -10)]]; // Undefined behavior if y != -10 at this point
    [[assume((x - 1) * 3 == 12)]];
    g(x); // Compiler may replace this with g(5);

[edit] References

  • C++23 standard (ISO/IEC 14882:2024):
  • 9.12.3 Assumption attribute [dcl.attr.assume]

[edit] See also

marks unreachable point of execution
(function) [edit]

[edit] External links

1.  Clang language extensions doc: __builtin_assume.
2.  Clang attribute reference doc: assume.
3.  MSVC doc: __assume built-in.
4.  GCC doc: __attribute__((assume(...))).