Intel® oneAPI DPC++/C++ Compiler Developer Guide and Reference

ID 767253
Date 3/22/2024
Public

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Set the FTZ and DAZ Flags

For Intel® x86-basedprocessors, the flush-to-zero (FTZ) and denormals-are-zero (DAZ) flags in the MXCSR register are used to control floating-point calculations. Intel® Streaming SIMD Extensions (Intel® SSE) and Intel® Advanced Vector Extensions (Intel® AVX) instructions, including scalar and vector instructions, benefit from enabling the FTZ and DAZ flags. Floating-point computations using the Intel® SSE and Intel® AVX instructions are accelerated when the FTZ and DAZ flags are enabled. This improves the application's performance.

Use the [Q]ftz option to flush denormal results to zero when the application is in the gradual underflow mode. This option may improve performance if the denormal values are not critical to the application's behavior. The [Q]ftz option, when applied to the main program, sets the FTZ and the DAZ hardware flags. The negative forms of the [Q]ftz option (-no-ftz for Linux* and /Qftz- for Windows*) leave the flags as they are.

The following table describes how the compiler processes denormal values based on the status of the FTZ and DAZ flags:

Flag

When set to ON, the compiler...

When set to OFF, the compiler...

FTZ

...sets denormal results from floating-point calculations to zero.

...does not change the denormal results.

DAZ

...treats denormal values used as input to floating-point instructions as zero.

...does not change the denormal instruction inputs.

  • FTZ only applies to Intel® SSE and Intel® AVX instructions. If the application generates denormals using x86 instructions, FTZ does not apply.
  • DAZ and FTZ flags are not compatible with the IEEE 754 standard, and should only be enabled when compliance to the IEEE standard is not required.

Options for [Q]ftz are performance options. Setting these options does not guarantee that all denormals in a program are flushed to zero. They only cause denormals generated at runtime to be flushed to zero.

By default the compiler inserts code into the main routine to set the FTZ and DAZ flags. Using the negative form of [Q]ftz prevents the compiler from inserting any code that sets FTZ or DAZ flags.

The [Q]ftz option 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 subsequently created threads, operate in the FTZ/DAZ mode.

With the default floating-point model (fast), every optimization option O level, except O0, sets [Q]ftz.

If this option produces undesirable results of the numerical behavior of the program, turn the FTZ/DAZ mode off by using the negative form of [Q]ftz in the command line.

Manually set the FTZ flags with the following macros:

_MM_SET_FLUSH_ZERO_MODE(_MM_FLUSH_ZERO_ON)

Manually set the DAZ flags with the following macros:

_MM_SET_DENORMALS_ZERO_MODE(_MM_DENORMALS_ZERO_ON)

The prototypes for these macros are in xmmintrin.h (FTZ) and pmmintrin.h (DAZ).

See Also