The compiler provides a CPU-dispatching feature that enables users to provide different implementations of their functionality. The CPU-dispatching mechanism checks the target architecture and then selects the best implementation at runtime. However, the mechanism has two related potential limitations:
- Users cannot control the selected code path when they have a particular reason to do so; this can be decided only by the CPU-dispatching mechanism at runtime according to the target platform.
- Users cannot test their different implementations on the same machine.
The CPU-Spoofing feature addresses these limitations.
Usage
The CPU-Spoofing feature does not add any command-line options. Instead, the user must set a new environment variable,
INTEL_ISA_DISABLE, before running their application.
The user can set the environment variable as follows (Linux* example):
$ export INTEL_ISA_DISABLE=features
where
features is a comma-separated list of features such as
sse2,clwb.
Note
The feature names are those used with the
-m option.
Setting the environment variable causes the named features not to be visible on the host even if the
CPUID reports that it has them onboard. This has the following implications:
- If the user disables a CPU feature (for example,
_FEATURE_SSE2) using
export INTEL_ISA_DISABLE=sse2, then
_may_i_use_cpu_feature(_FEATURE_SSE2) will return false; however, there will be no impact on other features for
_may_i_use_cpu_feature.
- The CPU-dispatching mechanism will be affected; that is, dispatching will not take paths that require features disabled via
INTEL_ISA_DISABLE.
- Libraries that use
libirc for their CPU dispatching (such as
mkl and
libimf/libsvml) will be affected by
INTEL_ISA_DISABLE in the same way.
Additional Information
- CPU-Spoofing has no architecture restrictions. Users can set the environment variable effectively on all our current architectures.
- CPU-Spoofing has no default setting (such as OFF or ON). The feature is triggered by the
INTEL_ISA_DISABLE environment variable, so if users do not set that variable before running their application, everything works normally (with no CPU spoofing). Also, if users specify invalid feature names within the environment variable's value, those names will be ignored.
- There is no IDE equivalent for the CPU-Spoofing feature.
Most important points to remember
- The value of environment variable
INTEL_ISA_DISABLE is a feature list string comprising feature names separated by commas. The feature names are those used with the
-m option.
- Users must set
INTEL_ISA_DISABLE before running their application.
- Users
must not disable any feature that is requested by the
-x
target option. For example, if you compile with
-xcore-avx2 and then disable
fma (which is required by avx2) via the
INTEL_ISA_DISABLE environment variable, a runtime error will occur indicating that the CPU is not supported.
Example
hide_avx.c:
#include "immintrin.h"
#define CHECK(feature) \
printf("%3s: %s\n", _may_i_use_cpu_feature(feature) ? "yes" : "no", #feature);
int main() {
CHECK(_FEATURE_GENERIC_IA32);
CHECK(_FEATURE_SSE4_2);
CHECK(_FEATURE_AVX);
CHECK(_FEATURE_AVX2);
return 0;
}
Build
hide_avx.c using
icc:
icc hide_avx.c –o hide_avx.exe
Run
hide_avx.exe on a machine with avx2, producing the following output:
yes: _FEATURE_GENERIC_IA32
yes: _FEATURE_SSE4_2
yes: _FEATURE_AVX
yes: _FEATURE_AVX2
Then set the environment variable on the command line:
export INTEL_ISA_DISABLE=avx2,avx
And then run
hide_avx.exe again, producing the following output:
yes: _FEATURE_GENERIC_IA32
yes: _FEATURE_SSE4_2
no: _FEATURE_AVX
no: _FEATURE_AVX2