Intel® Fortran Compiler 17.0 Developer Guide and Reference

Using the -fp-model (/fp) Option

The -fp-model (Linux* and OS X*) or /fp (Windows*) option allows you to control the optimizations on floating-point data. You can use this option to tune the performance, level of accuracy, or result consistency for floating-point applications across platforms and optimization levels.

For applications that do not require support for denormalized numbers, the -fp-model or /fp option can be combined with the [Q]ftz option to flush denormalized results to zero in order to obtain improved runtime performance on processors based on all Intel® architectures.

You can use keywords to specify the semantics to be used. The keywords specified for this option may influence the choice of math routines that are invoked. Many routines in the libirc, libm, and libsvml libraries are more highly optimized for Intel microprocessors than for non-Intel microprocessors. Possible values of the keywords are as follows:

Keyword

Description

precise

Enables value-safe optimizations on floating-point data and rounds intermediate results to source-defined precision.

fast[=1|2]

Enables more aggressive optimizations on floating-point data.

consistent

Enables consistent, reproducible results for different optimization levels or between different processors of the same architecture. This setting is equivalent to the use of the following options:

Windows*: /fp:precise /Qfma- /Qimf-arch-consistency:true

Linux* and OS X*: -fp-model precise -no-fma -fimf-arch-consistency=true

strict

Enables precise and except, disables contractions, and enables the property that allows modification of the floating-point environment.

source

Enables value-safe optimizations on floating-point data and rounds intermediate results to source-defined precision (same as precise keyword).

[no-]except (Linux* and OS X*) or
except[-] (Windows*)

Determines whether strict floating-point exception semantics are used.

The default value of the option is -fp-model fast=1 or /fp:fast=1, which means that the compiler uses more aggressive optimizations on floating-point calculations.

Note

Using the default option keyword -fp-model fast or /fp:fast, you may get significant differences in your result depending on whether the compiler uses x87 or SSE/AVX instructions to implement floating-point operations. Results are more consistent when the other option keywords are used.

Several examples are provided to illustrate the usage of the keywords. These examples show:

-fp-model fast or /fp:fast

Example source code:

Example

REAL T0, T1, T2; 
... 
T0 = 4.0E + 0.1E + T1 + T2;

When this option is specified, the compiler applies the following semantics:

Using these semantics, some possible ways the compiler may interpret the original code are given below:

Example

REAL T0, T1, T2; 
  ... 
T0 = (T1 + T2) + 4.1E;
REAL T0, T1, T2; 
  ... 
T0 = (T1 + 4.1E) + T2;

-fp-model source or /fp:source

This setting is equivalent to -fp-model precise or /fp:precise on systems based on the Intel® 64 architecture.

Source code example

REAL T0, T1, T2; 
... 
T0 = 4.0E + 0.1E + T1 + T2;

When this option is specified, the compiler applies the following semantics:

Using these semantics, a possible way the compiler may interpret the original code is shown below:

Example

REAL T0, T1, T2; 
  ... 
T0 = ((4.1E + T1) + T2);

-fp-model strict or /fp:strict

Source code example

REAL T0, T1, T2; 
  ... 
T0 = 4.0E + 0.1E + T1 + T2;

When this option is specified, the compiler applies the following semantics:

Using these semantics, a possible way the compiler may interpret the original code is shown below:

Example

REAL T0, T1, T2; 
  ... 
T0 = REAL ((((REAL)4.0E + (REAL)0.1E) + (REAL)T1) + (REAL)T2);

See Also