Intel® C++ Compiler 16.0 User and Reference Guide
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 svml library 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. |
|
fast[=1|2] |
Enables more aggressive optimizations on floating-point data. |
|
strict |
Enables precise and except, disables contractions, and enables pragma stdc fenv_access. |
|
source |
Rounds intermediate results to source-defined precision and enables value-safe optimizations. |
|
double |
Rounds intermediate results to 53-bit (double) precision and enables value-safe optimizations. |
|
extended |
Rounds intermediate results to 64-bit (extended) precision and enables value-safe optimizations. |
|
[no-]except (Linux* and OS X*) or |
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.
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:
A small example of source code.
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.
There are several ways the compiler may interpret the code; we show just some of these possibilities.
Example source code:
|
Example |
|---|
float t0, t1, t2; ... t0 = 4.0f + 0.1f + 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 doubleprecision.
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 |
|---|
float t0, t1, t2; ... t0 = (float)((double)t1 + (double)t2) + 4.1f; float t0, t1, t2; ... t0 = (t1 + t2) + 4.1f; float t0, t1, t2; ... t0 = (t1 + 4.1f) + t2; |
This setting is equivalent to -fp-model precise on Linux* operating systems based on the IA-32 architecture.
Example source code:
float t0, t1, t2; ... t0 = 4.0f + 0.1f + t1 + t2;
When this option is specified, the compiler applies the following semantics:
Additions are performed in program order
Intermediate expressions use extended double precision
The constant addition may be pre-computed, assuming the default rounding mode
Using these semantics, a possible way the compiler may interpret the original code is shown below:
float t0, t1, t2; ... t0 = (float)(((long double)4.1 + (long double)t1) + (long double)t2);
This setting is equivalent to -fp-model precise or /fp:precise on systems based on the Intel® 64 architecture.
|
Source code example |
|---|
float t0, t1, t2; ... t0 = 4.0f + 0.1f + t1 + t2; |
When this option is specified, the compiler applies the following semantics:
Additions are performed in program order.
Intermediate expressions use the precision specified in the source code, that is, single-precision.
The constant addition may be pre-computed, assuming the default rounding mode.
Using these semantics, a possible way the compiler may interpret the original code is shown below:
|
Example |
|---|
float t0, t1, t2; ... t0 = ((4.1f + t1) + t2); |
This setting is equivalent to -fp-model precise or /fp:precise on Windows systems based on the IA-32 architecture.
Example source code:
float t0, t1, t2; ... t0 = 4.0f + 0.1f + t1 + t2;
When this option is specified, the compiler applies the following semantics:
Additions are performed in program order
Intermediate expressions use double precision
The constant addition may be pre-computed, assuming the default rounding mode
Using these semantics, a possible way the compiler may interpret the original code is shown below:
float t0, t1, t2; ... t0 = (float)(((double)4.1 + (double)t1) + (double)t
|
Source code example |
|---|
float t0, t1, t2; ... t0 = 4.0f + 0.1f + t1 + t2; |
When this option is specified, the compiler applies the following semantics:
Additions are performed in program order
Expression evaluation matches expression evaluation under keyword precise.
The constant addition is not pre-computed because there is no way to tell what rounding mode will be active when the program runs.
Using these semantics, a possible way the compiler may interpret the original code is shown below:
|
Example |
|---|
float t0, t1, t2; ... t0 = (float)((((long double)4.0f + (long double)0.1f) + (long double)t1) + (long double)t2); |