9 Declarations [dcl.dcl]

9.12 Attributes [dcl.attr]

9.12.3 Assumption attribute [dcl.attr.assume]

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The attribute-token assume may be applied to a null statement; such a statement is an assumption.
An attribute-argument-clause shall be present and shall have the form:
( conditional-expression )
The expression is contextually converted to bool ([conv.general]).
The expression is not evaluated.
If the converted expression would evaluate to true at the point where the assumption appears, the assumption has no effect.
Otherwise, evaluation of the assumption has runtime-undefined behavior.
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[Note 1: 
The expression is potentially evaluated ([basic.def.odr]).
The use of assumptions is intended to allow implementations to analyze the form of the expression and deduce information used to optimize the program.
Implementations are not required to deduce any information from any particular assumption.
It is expected that the value of a has-attribute-expression for the assume attribute is 0 if an implementation does not attempt to deduce any such information from assumptions.
— end note]
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[Example 1: int divide_by_32(int x) { [[assume(x >= 0)]]; return x/32; // The instructions produced for the division // may omit handling of negative values. } int f(int y) { [[assume(++y == 43)]]; // y is not incremented return y; // statement may be replaced with return 42; } — end example]