XL Fortran for AIX 8.1

User's Guide

Maximizing Floating-Point Performance

If performance is your primary concern and you want your program to be relatively safe but do not mind if results are slightly different (generally more precise) from what they would be otherwise, optimize the program with the -O option, and specify -qfloat=rsqrt:hssngl:fltint.

The following section describes the functions of these suboptions:

In cases where speed is so important that you can make an informed decision to sacrifice correctness at boundary conditions, you can replace hssngl and fltint with the hsflt suboption; it does the same thing as fltint and suppresses rounding operations.

In suppressing rounding operations, hsflt works like hssngl, but it also suppresses rounding operations when double-precision values are assigned to single-precision memory locations. Single-precision overflow is not detected in such assignments, and the assigned value is not correctly rounded according to the current rounding mode.

Attention: When you use the hsflt suboption, observe these restrictions, or your program may produce incorrect results without warning:

Therefore, we recommend that you use this suboption only with extreme caution. It is for use by knowledgeable programmers in specific applications, such as graphics programs, where the computational characteristics are known. If you are at all unsure whether a program is suitable or if the program produces unexpected results when you use this suboption, use hssngl instead.

Related Information:
Technical Details of the -qfloat=hsflt Option provides additional technical information about this suboption.

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