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 six bits are significant, namely:
1. bit #28 denoting Hyperthreading, which is used to distiguish cores with shared cache; 2. bit #26 denoting SSE2 support; 3. bit #25 denoting SSE support; 4. bit #23 denoting MMX support; 5. bit #20, reserved by Intel, is used to choose between RC4 code pathes; 6. bit #4 denoting presence of Time-Stamp Counter.
For example, clearing bit #26 at run-time 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. on Intel P4 processor 'env OPENSSL_ia32cap=0x12900010 apps/openssl', to achieve same effect without modifying the application source code. Alternatively you can reconfigure the toolkit with no-sse2 option and recompile.