=pod

=head1 NAME

OPENSSL_ia32cap, OPENSSL_ia32cap_loc - the IA-32 processor capabilities vector

=head1 SYNOPSIS

 unsigned int *OPENSSL_ia32cap_loc(void);
 #define OPENSSL_ia32cap ((OPENSSL_ia32cap_loc())[0])

=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:ECX register pair after executing CPUID instruction with EAX=1
input value (see Intel Application Note #241618). Naturally it's
meaningful on x86 and x86_64 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 following bits are significant:

=over

=item bit #4 denoting presence of Time-Stamp Counter.

=item bit #19 denoting availability of CLFLUSH instruction;

=item bit #20, reserved by Intel, is used to choose among RC4 code paths;

=item bit #23 denoting MMX support;

=item bit #24, FXSR bit, denoting availability of XMM registers;

=item bit #25 denoting SSE support;

=item bit #26 denoting SSE2 support;

=item bit #28 denoting Hyperthreading, which is used to distinguish
cores with shared cache;

=item bit #30, reserved by Intel, denotes specifically Intel CPUs;

=item bit #33 denoting availability of PCLMULQDQ instruction;

=item bit #41 denoting SSSE3, Supplemental SSE3, support;

=item bit #43 denoting AMD XOP support (forced to zero on non-AMD CPUs);

=item bit #54 denoting availability of MOVBE instruction;

=item bit #57 denoting AES-NI instruction set extension;

=item bit #58, XSAVE bit, lack of which in combination with MOVBE is used
to identify Atom Silvermont core;

=item bit #59, OSXSAVE bit, denoting availability of YMM registers;

=item bit #60 denoting AVX extension;

=item bit #62 denoting availability of RDRAND instruction;

=back

For example, in 32-bit application context clearing bit #26 at run-time
disables high-performance SSE2 code present in the crypto library, while
clearing bit #24 disables SSE2 code operating on 128-bit XMM register
bank. You might have to do the latter if target OpenSSL application is
executed on SSE2 capable CPU, but under control of OS that does not
enable XMM registers. 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=0x16980010
apps/openssl', or better yet 'env OPENSSL_ia32cap=~0x1000000
apps/openssl' to achieve same effect without modifying the application
source code. Alternatively you can reconfigure the toolkit with no-sse2
option and recompile.

Less intuitive is clearing bit #28. The truth is that it's not copied
from CPUID output verbatim, but is adjusted to reflect whether or not
the data cache is actually shared between logical cores. This in turn
affects the decision on whether or not expensive countermeasures
against cache-timing attacks are applied, most notably in AES assembler
module.

The capability vector is further extended with EBX value returned by
CPUID with EAX=7 and ECX=0 as input. Following bits are significant:

=over

=item bit #64+3 denoting availability of BMI1 instructions, e.g. ANDN;

=item bit #64+5 denoting availability of AVX2 instructions;

=item bit #64+8 denoting availability of BMI2 instructions, e.g. MULX
and RORX;

=item bit #64+16 denoting availability of AVX512F extension;

=item bit #64+18 denoting availability of RDSEED instruction;

=item bit #64+19 denoting availability of ADCX and ADOX instructions;

=item bit #64+29 denoting availability of SHA extension;

=item bit #64+30 denoting availability of AVX512BW extension;

=item bit #64+31 denoting availability of AVX512VL extension;

=back

To control this extended capability word use ':' as delimiter when
setting up OPENSSL_ia32cap environment variable. For example assigning
':~0x20' would disable AVX2 code paths, and ':0' - all post-AVX
extensions.

It should be noted that whether or not some of the most "fancy"
extension code paths are actually assembled depends on current assembler
version. Base minimum of AES-NI/PCLMULQDQ, SSSE3 and SHA extension code
paths are always assembled. Besides that, minimum assembler version
requirements are summarized in below table:

   Extension   | GNU as | nasm   | llvm
   ------------+--------+--------+--------
   AVX         | 2.19   | 2.09   | 3.0
   AVX2        | 2.22   | 2.10   | 3.1
   AVX512      | 2.25   | 2.11.8 | 3.6

=head1 COPYRIGHT

Copyright 2004-2016 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the OpenSSL license (the "License").  You may not use
this file except in compliance with the License.  You can obtain a copy
in the file LICENSE in the source distribution or at
L<https://www.openssl.org/source/license.html>.

=cut