HP-UX Floating-Point Guide

184 Chapter 7

Performance Tuning

Mixed-Precision Expressions

Expressions that contain a mixture of single-precision and

double-precision data generally execute more slowly than expressions of

just one precision type because they require extra conversions. (Whether

the compiler converts a mixed expression to single-precision or

double-precision depends on the rules of the high-level language.) The

reason is that HP 9000 instructions require their operands to be of the

same precision. For example, for an add or multiply or any other

two-operand instruction, both operands must be either single-precision

or double-precision; they cannot be of mixed precision.

Aside from performance considerations, it is generally a good idea to

avoid mixed-precision expressions for numerical analysis reasons. If you

must mix precisions in a single expression, choose the precisions

carefully so that the compiler does not need to perform repetitive

conversions.