Хотелось-бы пруфлинк, если можно. Сколько копал на тему volatile - нигде не видел подобного разделения на то где требуется, а где не требуется volatile.

5.1.2.3 Program execution
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8
EXAMPLE 1
An implementation might define a one-to-one correspondence between abstract and actual
semantics: at every sequence point, the values of the actual objects would agree with those specified by the
abstract semantics. The keyword volatile would then be redundant.
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Alternatively, an implementation might perform various optimizations within each translation unit, such
that the actual semantics would agree with the abstract semantics only when making function calls across
translation unit boundaries. In such an implementation, at the time of each function entry and function
return where the calling function and the called function are in different translation units, the values of all
externally linked objects and of all objects accessible via pointers therein would agree with the abstract
semantics. Furthermore, at the time of each such function entry the values of the parameters of the called
function and of all objects accessible via pointers therein would agree with the abstract semantics. In this
type of implementation, objects referred to by interrupt service routines activated by the signal function
would require explicit specification of volatile storage, as well as other implementation-defined
restrictions.

а мэй нот дэфайн...