Intel® C++ Compiler 16.0 User and Reference Guide
Controls the semantics of floating-point calculations.
Linux and OS X: | -fp-model keyword |
Windows: | /fp:keyword |
keyword |
Specifies the semantics to be used. Possible values are:
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-fp-model fast=1
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The compiler uses more aggressive optimizations on floating-point calculations. |
This option controls the semantics of floating-point calculations.
The keywords can be considered in groups:
Group A: precise, fast, strict
Group B: source, double, extended
Group C: except (or the negative form)
You can use more than one keyword. However, the following rules apply:
You cannot specify fast and except together in the same compilation. You can specify any other combination of group A, group B, and group C.
Since fast is the default, you must not specify except without a group A or group B keyword.
You should specify only one keyword from group A and only one keyword from group B. If you try to specify more than one keyword from either group A or group B, the last (rightmost) one takes effect.
If you specify except more than once, the last (rightmost) one takes effect.
The floating-point (FP) environment is a collection of registers that control the behavior of FP machine instructions and indicate the current FP status. The floating-point environment may include rounding-mode controls, exception masks, flush-to-zero controls, exception status flags, and other floating-point related features.
Option |
Description |
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-fp-model precise or /fp:precise |
Tells the compiler to strictly adhere to value-safe optimizations when implementing floating-point calculations. It disables optimizations that can change the result of floating-point calculations, which is required for strict ANSI conformance. These semantics ensure the reproducibility of floating-point computations for serial code, including code vectorized or auto-parallelized by the compiler, but they may slow performance. They do not ensure value safety or run-to-run reproducibility of other parallel code. Run-to-run reproducibility for floating-point reductions in OpenMP* code may be obtained for a fixed number of threads through the KMP_DETERMINISTIC_REDUCTION environment variable. For more information about this environment variable, see topic "Supported Environment Variables". The compiler assumes the default floating-point environment; you are not allowed to modify it. Intermediate results are computed with the precision shown in the following table, unless it is overridden by a keyword from Group B:
Floating-point exception semantics are disabled by default. To enable these semantics, you must also specify -fp-model except or /fp:except. |
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-fp-model fast[=1|2] or /fp:fast[=1|2] |
Tells the compiler to use more aggressive optimizations when implementing floating-point calculations. These optimizations increase speed, but may affect the accuracy or reproducibility of floating-point computations. Specifying fast is the same as specifying fast=1. fast=2 may produce faster and less accurate results. Floating-point exception semantics are disabled by default and they cannot be enabled because you cannot specify fast and except together in the same compilation. To enable exception semantics, you must explicitly specify another keyword (see other keyword descriptions for details). |
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-fp-model strict or /fp:strict |
Tells the compiler to strictly adhere to value-safe optimizations when implementing floating-point calculations and enables floating-point exception semantics. This is the strictest floating-point model. The compiler does not assume the default floating-point environment; you are allowed to modify it. Floating-point exception semantics can be disabled by explicitly specifying -fp-model no-except or /fp:except-. |
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-fp-model source or /fp:source |
This option causes intermediate results to be rounded to the precision defined in the source code. It also implies keyword precise unless it is overridden by a keyword from Group A. Intermediate expressions use the precision of the operand with higher precision, if any.
The compiler assumes the default floating-point environment; you are not allowed to modify it. |
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-fp-model double or /fp:double |
This option causes intermediate results to be rounded as follows:
This option also implies keyword precise unless it is overridden by a keyword from Group A. The compiler assumes the default floating-point environment; you are not allowed to modify it. |
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-fp-model extended or /fp:extended |
This option causes intermediate results to be rounded as follows:
This option also implies keyword precise unless it is overridden by a keyword from Group A. The compiler assumes the default floating-point environment; you are not allowed to modify it. |
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-fp-model except or /fp:except |
Tells the compiler to follow strict floating-point exception semantics. |
The -fp-model and /fp options determine the setting for the maximum allowable relative error for math library function results (max-error) if none of the following options are specified:
-fimf-accuracy-bits (Linux* OS and OS X*) or /Qimf-accuracy-bits (Windows* OS)
-fimf-max-error (Linux OS and OS X) or /Qimf-max-error (Windows OS)
-fimf-precision (Linux OS and OS X) or /Qimf-precision (Windows OS)
[Q]fast-transcendentals
Option -fp-model fast (and /fp:fast) sets option -fimf-precision=medium (/Qimf-precision:medium) and option -fp-model precise (and /fp:precise) implies -fimf-precision=high (and /Qimf-precision:high). Option -fp-model fast=2 (and /fp:fast2) sets option -fimf-precision=medium (and /Qimf-precision:medium) and option -fimf-domain-exclusion=15 (and /Qimf-domain-exclusion=15).
On Windows and Linux operating systems on IA-32 architecture, the compiler, by default, implements floating-point (FP) arithmetic using SSE2 and SSE instructions. This can cause differences in floating-point results when compared to previous x87 implementations.
In Visual Studio 2010, when you create a Microsoft* Visual C++ project, option /fp:precise is set by default. It sets the floating-point model to improve consistency for floating-point operations by disabling certain optimizations that may reduce performance. To set the option back to the general default /fp:fast, change the IDE project property for Floating Point Model to Fast.
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Intel's compilers may or may not optimize to the same degree for non-Intel microprocessors for optimizations that are not unique to Intel microprocessors. These optimizations include SSE2, SSE3, and SSSE3 instruction sets and other optimizations. Intel does not guarantee the availability, functionality, or effectiveness of any optimization on microprocessors not manufactured by Intel. Microprocessor-dependent optimizations in this product are intended for use with Intel microprocessors. Certain optimizations not specific to Intel microarchitecture are reserved for Intel microprocessors. Please refer to the applicable product User and Reference Guides for more information regarding the specific instruction sets covered by this notice. Notice revision #20110804 |
Visual Studio: Code Generation>Floating Point Model
Code Generation>Enable Floating Point Exceptions
Code Generation> Floating Point Expression Evaluation
Eclipse: Floating Point > Floating Point Model
Xcode: Floating Point > Floating Point Model
Floating Point > Reliable Floating Point Exceptions Model
None
The article titled: Consistency of Floating-Point Results using the Intel® Compiler, which is located in http://software.intel.com/en-us/articles/consistency-of-floating-point-results-using-the-intel-compiler/