The -mp option restricts some optimizations to maintain declared precision and to ensure that floating-point arithmetic conforms more closely to the ANSI and IEEE standards.
For most programs, specifying this option adversely affects performance. If you are not sure whether your application needs this option, try compiling and running your program both with and without it to evaluate the effects on performance versus precision.
Specifying this option has the following effects on program compilation:
User variables declared as floating-point types are not assigned to registers.
Floating-point arithmetic comparisons conform to IEEE 754 except for NaN behavior.
The exact operations specified in the code are performed. For example, division is never changed to multiplication by the reciprocal.
The compiler performs floating-point operations in the order specified without reassociation.
The compiler does not perform the constant folding on floating-point values. Constant folding also eliminates any multiplication by 1, division by 1, and addition or subtraction of 0. For example, code that adds 0.0 to a number is executed exactly as written. Compile-time floating-point arithmetic is not performed to ensure that floating-point exceptions are also maintained.
For IA-32 systems, whenever an expression is spilled, it is spilled as 80 bits (EXTENDED PRECISION), not 64 bits (DOUBLE PRECISION). Floating-point operations conform to IEEE 754. When assignments to type REAL and DOUBLE PRECISION are made, the precision is rounded from 80 bits (EXTENDED) down to 32 bits (REAL) or 64 bits (DOUBLE PRECISION). When you do not specify -O0, the extra bits of precision are not always rounded away before the variable is reused.
Even if vectorization is enabled by the -xK option, the compiler does not vectorize reduction loops (loops computing the dot product) and loops with mixed precision types.