Question

How portable is code that uses #pragma optimize? Do most compilers support it and how complete is the support for this #pragma?

Answer 1

#pragma is the sanctioned and portable way for compilers to add non-sanctioned and non-portable language extensions ^*.

Basically, you never know for sure, and at least one major C++ compiler (g++) does not support this pragma as is.

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^*:

From the C++ standard (N3242):

16.6 Pragma directive [cpp.pragma]

A preprocessing directive of the form

# pragma pp-tokens_opt new-line

causes the implementation to behave in an implementation-defined manner. The behavior might cause translation to fail or cause the translator or the resulting program to behave in a non-conforming manner. Any pragma that is not recognized by the implementation is ignored.

From the C standard (Committee Draft — April 12, 2011):

6.10.6 Pragma directive

Semantics

A preprocessing directive of the form

# pragma pp-tokens_opt new-line

where the preprocessing token STDC does not immediately follow pragma in the
directive (prior to any macro replacement)¹⁷⁴⁾ causes the implementation to behave in an
implementation-defined manner. The behavior might cause translation to fail or cause the
translator or the resulting program to behave in a non-conforming manner. Any such
pragma that is not recognized by the implementation is ignored.

And here’s an example:

int main () {
    #pragma omp parallel for
    for (int i=0; i<16; ++i) {}
}

A big part of the C and C++ OpenMP API is implemented as #pragmas.