openssl/doc/crypto/OPENSSL_ia32cap.pod

=pod

=head1 NAME

OPENSSL_ia32cap - finding the IA-32 processor capabilities

=head1 SYNOPSIS

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

=head1 DESCRIPTION

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.

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