NAME

       OPENSSL_ia32cap - the x86[_64] processor capabilities vector

SYNOPSIS

        env OPENSSL_ia32cap=... <application>

DESCRIPTION

       OpenSSL supports a range of x86[_64] instruction set extensions. These extensions are
       denoted by individual bits in capability vector returned by processor in EDX:ECX register
       pair after executing CPUID instruction with EAX=1 input value (see Intel Application Note
       #241618). This vector is copied to memory upon toolkit initialization and used to choose
       between different code paths to provide optimal performance across wide range of
       processors. For the moment of this writing following bits are significant:

       bit #4 denoting presence of Time-Stamp Counter.
       bit #19 denoting availability of CLFLUSH instruction;
       bit #20, reserved by Intel, is used to choose among RC4 code paths;
       bit #23 denoting MMX support;
       bit #24, FXSR bit, denoting availability of XMM registers;
       bit #25 denoting SSE support;
       bit #26 denoting SSE2 support;
       bit #28 denoting Hyperthreading, which is used to distinguish cores with shared cache;
       bit #30, reserved by Intel, denotes specifically Intel CPUs;
       bit #33 denoting availability of PCLMULQDQ instruction;
       bit #41 denoting SSSE3, Supplemental SSE3, support;
       bit #43 denoting AMD XOP support (forced to zero on non-AMD CPUs);
       bit #54 denoting availability of MOVBE instruction;
       bit #57 denoting AES-NI instruction set extension;
       bit #58, XSAVE bit, lack of which in combination with MOVBE is used to identify Atom
       Silvermont core;
       bit #59, OSXSAVE bit, denoting availability of YMM registers;
       bit #60 denoting AVX extension;
       bit #62 denoting availability of RDRAND instruction;

       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. Historically address of the capability vector copy was exposed to
       application through OPENSSL_ia32cap_loc(), but not anymore. Now the only way to affect the
       capability detection is to set OPENSSL_ia32cap environment variable prior target
       application start. To give a specific example, on Intel P4 processor 'env
       OPENSSL_ia32cap=0x16980010 apps/openssl', or better yet 'env OPENSSL_ia32cap=~0x1000000
       apps/openssl' would achieve the desired effect. Alternatively you can reconfigure the
       toolkit with no-sse2 option and recompile.

       Less intuitive is clearing bit #28, or ~0x10000000 in the "environment variable" terms.
       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:

       bit #64+3 denoting availability of BMI1 instructions, e.g. ANDN;
       bit #64+5 denoting availability of AVX2 instructions;
       bit #64+8 denoting availability of BMI2 instructions, e.g. MULX and RORX;
       bit #64+16 denoting availability of AVX512F extension;
       bit #64+18 denoting availability of RDSEED instruction;
       bit #64+19 denoting availability of ADCX and ADOX instructions;
       bit #64+21 denoting availability of VPMADD52[LH]UQ instructions, a.k.a. AVX512IFMA
       extension;
       bit #64+29 denoting availability of SHA extension;
       bit #64+30 denoting availability of AVX512BW extension;
       bit #64+31 denoting availability of AVX512VL extension;
       bit #64+41 denoting availability of VAES extension;
       bit #64+42 denoting availability of VPCLMULQDQ extension;

       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. Apart from 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
          ADCX/ADOX   | 2.23   | 2.10   | 3.3
          AVX512      | 2.25   | 2.11.8 | see NOTES
          AVX512IFMA  | 2.26   | 2.11.8 | see NOTES
          VAES        | 2.30   | 2.13.3 |

NOTES

       Even though AVX512 support was implemented in llvm 3.6, compilation of assembly modules
       apparently requires explicit -march flag. But then compiler generates processor-specific
       code, which in turn contradicts the mere idea of run-time switch execution facilitated by
       the variable in question. Till the limitation is lifted, it's possible to work around the
       problem by making build procedure use following script:

          #!/bin/sh
          exec clang -no-integrated-as "$@"

       instead of real clang. In which case it doesn't matter which clang version is used, as it
       is GNU assembler version that will be checked.

RETURN VALUES

       Not available.

COPYRIGHT

       Copyright 2004-2018 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 <https://www.openssl.org/source/license.html>.