Processor-specific Runtime Checks, IA-32 Systems

The Intel® C++  Compiler optimizations take effect at run time. For IA-32 systems, the compiler enhances processor-specific optimizations by inserting a code segment in the program that performs the run-time checks described below.

Check for Supported Processor with -xN, -xB, or -xP

To prevent execution errors, the compiler inserts code in the program to check for proper processor usage.  Programs compiled with options -xN, -xB, or -xP will check at run time whether they are being executed on the Intel® Pentium® 4 processor, Intel Pentium M processor, or the Intel processors code-named "Prescott", respectively, or a compatible Intel processor. If the program is not executed on one of these processors, the program terminates with an error.

Example

To optimize the program prog.cpp for the Intel processors code-named "Prescott", issue the following command:

prompt>icpc -xP prog.cpp

The resulting executable aborts if it is executed on a processor that does not support the Intel processors code-named "Prescott", such as the Intel Pentium III or Intel Pentium 4 processor.

If you intend to run your programs on multiple IA-32 processors, do not use the -x{} options that optimize for processor-specific features; consider using -ax{} to attain processor specific performance and portability among different processors.

Setting FTZ and DAZ Flags

Previously, the values of the flags flush-to-zero (FTZ) and denormals-as-zero (DAZ) for IA-32 processors were off by default.  However, even at the cost of losing IEEE compliance, turning these flags on significantly increases the performance of programs with denormal floating-point values in the gradual underflow mode run on the most recent IA-32 processors.  Hence, for the Intel Pentium III, Pentium 4, Pentium M, Intel processors code-named "Prescott", and compatible IA-32 processors, the compiler's default behavior is to turn these flags on. The compiler inserts code in the program to perform a run-time check for the processor on which the program runs to verify it is one of the afore-listed Intel processors.

Examples

• Executing a program on a Pentium III processor enables FTZ, but not DAZ.
• Executing a program on an Intel Pentium M processor or Intel processors code-named "Prescott" enables both FTZ and DAZ.

These flags are only turned on by Intel processors that have been validated to support them.

For non-Intel processors, you can set the flags manually with the following macros:

Enable FTZ: _MM_SET_FLUSH_ZERO_MODE(_MM_FLUSH_ZERO_ON)

Enable DAZ: _MM_SET_DENORMALS_ZERO_MODE(_MM_DENORMALS_ZERO_ON)

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