HP C/aC++ for PA-RISC Software manual Iostream Performance Improvement Pragma, #pragma Optimize on

IOSTREAM Performance Improvement Pragma

The -AA-D_HP_NONSTD_FAST_IOSTREAMPerformance Improvement macro can be used to improve the -AA iostream performance.

Syntax:

#define _HP_NONSTD_FAST_IOSTREAM 1 (or)

aCC options -D_HP_NONSTD_FAST_IOSTREAM

This macro enables the following non-standard features:

•Sets std::ios_base::sync_with_stdio(false), which disables the default synchronization with stdio

•Sets std::cin.tie(0). which unties the cin from other streams.

•Replaces all occurrences of “std::endl” with “\n”.

Enabling this macro can provide noticeable performance improvement if the application uses iostreams often.

NOTE: Note: Do not enable the HP_NONSTD_FAST_IOSTREAM macro in any of the following cases:.

•If the application assumes a C++ stream to be in sync with a C stream.

•If the application depends on stream flushing behavior with endl.

•If the user uses "std::cout.unsetf(ios::unitbuf)" to unit buffer the output stream.

Pragma Directive (#pragma) and _Pragma Operator

A#pragma directive is an instruction to the compiler. Use a pragma to control the actions of the compiler in a particular portion of a program without affecting the program as a whole.

Syntax

pragma-directive ::=

#pragma [token-list]

Description

The #pragma directive is ignored by the preprocessor, and instead is passed on to the HP aC++ compiler. It provides implementation-dependent information to HP aC++. Any pragma that is not recognized by HP aC++ will generate a warning from the compiler.

The _Pragma operator, supported in non-strict C++98/C++03 mode and in all C++0x modes, has the effect of expanding the pragma specified in the string (in double-quotes) in just the way a #pragma directive would. For example:

_Pragma ("pack 1"); struct Packed { char c;

int i; };

int main () {

int iPackedSize = sizeof(Packed);

}

See Chapter 3 (page 96) for more information on pragmas.

Example

#pragma OPTIMIZE ON

Overview of the Preprocessor 129