Developer Guide and Reference

Contents

ftz, Qftz

Flushes
denormal
results to zero.

Syntax

Linux and macOS:
-ftz
-no-ftz
Windows:
/Qftz
/Qftz-
Arguments
None
Default
-ftz
or
/Qftz
Denormal
results are flushed to zero.
Every optimization option
O
level, except
O0
, sets
[Q]ftz
.
Description
This option flushes
denormal
results to zero when the application is in the gradual underflow mode. It may improve performance if the
denormal
values are not critical to your application's behavior.
The
[Q]ftz
option has no effect during compile-time optimization.
The
[Q]ftz
option sets or resets the FTZ and the DAZ hardware flags. If FTZ is ON,
denormal
results from floating-point calculations will be set to the value zero. If FTZ is OFF,
denormal
results remain as is. If DAZ is ON,
denormal
values used as input to floating-point instructions will be treated as zero. If DAZ is OFF,
denormal
instruction inputs remain as is. Systems using Intel® 64 architecture have both FTZ and DAZ. FTZ and DAZ are not supported on all IA-32 architectures.
When the
[Q]ftz
option is used in combination with an SSE-enabling option on systems using IA-32 architecture (for example, the
[Q]xSSE2
option), the compiler will insert code in the main routine to set FTZ and DAZ. When
[Q]ftz
is used without such an option, the compiler will insert code to conditionally set FTZ/DAZ based on a run-time processor check.
If you specify option
-no-ftz
(Linux and
macOS*
) or option
/Qftz-
(Windows), it prevents the compiler from inserting any code that might set FTZ or DAZ.
Option
[Q]ftz
only has an effect when the main program is being compiled. It sets the FTZ/DAZ mode for the process. The initial thread and any threads subsequently created by that process will operate in FTZ/DAZ mode.
If this option produces undesirable results of the numerical behavior of your program, you can turn the FTZ/DAZ mode off by specifying
-no-ftz
or
/Qftz-
in the command line while still benefiting from the
O3
optimizations.
Option
[Q]ftz
is a performance option. Setting this option does not guarantee that all
denormals
in a program are flushed to zero. The option only causes
denormals
generated at run time to be flushed to zero.
Alternate Options
None

Product and Performance Information

1

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