man OPENSSL_ia32cap () - finding the IA-32 processor capabilities


OPENSSL_ia32cap - finding the IA-32 processor capabilities


 unsigned long *OPENSSL_ia32cap_loc(void);
 #define OPENSSL_ia32cap (*(OPENSSL_ia32cap_loc()))


Value returned by OPENSSL_ia32cap_loc() is address of a variable containing IA-32 processor capabilities bit vector as it appears in EDX register after executing CPUID instruction with EAX=1 input value (see Intel Application Note #241618). Naturally it's meaningful on IA-32[E] platforms only. The variable is normally set up automatically upon toolkit initialization, but can be manipulated afterwards to modify crypto library behaviour. For the moment of this writing three bits are significant, namely bit #28 denoting Hyperthreading, which is used to distinguish Intel P4 core, bit #26 denoting SSE2 support, and bit #4 denoting presence of Time-Stamp Counter. Clearing bit #26 at run-time for example disables high-performance SSE2 code present in the crypto library. You might have to do this if target OpenSSL application is executed on SSE2 capable CPU, but under control of OS which does not support SSE2 extentions. Even though you can manipulate the value programmatically, you most likely will find it more appropriate to set up an environment variable with the same name prior starting target application, e.g. 'env OPENSSL_ia32cap=0x10 apps/openssl', to achieve same effect without modifying the application source code. Alternatively you can reconfigure the toolkit with no-sse2 option and recompile.