bug#42998: 27.1; gnutls-symmetric-encrypt slow when providing cipher as symbol

[Vasilij Schneidermann]

I've recently debugged bad performance for a piece of code showing
abysmal performance in its encryption loop.  Compare the following
benchmark results:

(benchmark 10000
  '(gnutls-symmetric-encrypt 'AES-128-CBC (make-string 16 0)
                                          (make-string 16 0)
                                          (make-string 16 0)))
;; => "Elapsed time: 1.808647s (1.708427s in 124 GCs)"
(benchmark 10000
  '(gnutls-symmetric-encrypt 4 (make-string 16 0)
                               (make-string 16 0)
                               (make-string 16 0)))
;; => "Elapsed time: 0.103690s (0.089148s in 6 GCs)"

This seems to be the fault of how the cipher value is mapped to the
cipher, if it's a symbol, the C code performs the equivalent of `(assoc
cipher (gnutls-ciphers))`.  Generating the cipher list over and over
again seems like a waste of processing power.  I'd consider either to
document this behavior or making the mapping more efficient, for example
by caching the cipher list instead of recomputing it over and over
again.  The latter would match the behavior as seen in other
cryptographic library wrappers, for example the OpenSSL one in Ruby.


In GNU Emacs 27.1 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.22)
 of 2020-08-13 built on juergen
Windowing system distributor 'The X.Org Foundation', version 11.0.12008000
System Description: Arch Linux

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 'configure --prefix=/usr --sysconfdir=/etc --libexecdir=/usr/lib
 --localstatedir=/var --with-x-toolkit=gtk3 --with-xft --with-wide-int
 --with-modules 'CFLAGS=-march=x86-64 -mtune=generic -O2 -pipe -fno-plt'
 CPPFLAGS=-D_FORTIFY_SOURCE=2
 LDFLAGS=-Wl,-O1,--sort-common,--as-needed,-z,relro,-z,now'

Configured features:
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ACL GNUTLS LIBXML2 FREETYPE HARFBUZZ M17N_FLT LIBOTF XFT ZLIB
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Memory information:
((conses 16 45005 6273)
 (symbols 48 6004 1)
 (strings 32 15483 2032)
 (string-bytes 1 504714)
 (vectors 16 10090)
 (vector-slots 8 130067 10258)
 (floats 8 19 27)
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