# Use the -fp-model, /fp Option

The -fp-model (Linux and macOS) 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 subnormal numbers, the -fp-model or /fp option can be combined with the [Q]ftz option to flush subnormal results to zero. This flush can improve 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 macOS: `-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 macOS) 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 Intel® Streaming SIMD Extensions (Intel® SSE)/Intel® Advanced Vector Extensions (Intel® 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:

- A small example of source code.

  ### Note

  The same source code is considered in all the included examples.

- The semantics that are used to interpret floating-point calculations in the source code.

- One or more possible ways the compiler may interpret the source code.

  ### Note

  There are several ways that the compiler may interpret the code; we show just some of these possibilities.

## -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:

- Additions may be performed in any order.

- Intermediate expressions may use single, double, or extended double precision.

- The constant addition may be pre-computed, assuming the default rounding mode.

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;

Parent topic: Floating-Point Operations

## See Also

fp-model, fp compiler option