What OpenSSL does is not necessarily obvious.  The INSTALL document talks about 
the no-asm configuration option.  Details about what the assembler code does in 
terms of optimization are only available by reading the source code comments in 
the various Perl files that generate the assembler, mostly.

On x86, the assembly code uses the CPUID instruction (see the 
OPENSSL_ia32cap.pod manpage) to determine if various instructions (AES, SSE, 
MMX, etc) are available and will use them if so.  For other processors, similar 
tests are performed if at all possible.

I have added this to the FAQ

Senior Architect, Akamai Technologies
Member, OpenSSL Dev Team
IM: richs...@jabber.at Twitter: RichSalz

From: - JinsongJi [mailto:jjsb...@hotmail.com]
Sent: Wednesday, August 09, 2017 9:09 AM
To: openssl-users@openssl.org
Subject: [openssl-users] Does openssl pick low level interface or high level 
interface to do encrypt?


For one simple operation: openssl enc -aes-256-cbc -salt -in foo.txt -out 
Does openssl pick classic implementation or AES-NI implementation to do this 

Does any user/application always pick classic implementation for AES operation 
regardless of AES-NI improves speed much?

Is there any document about this interface selection?

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