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Re: [PATCH] Add a mechanism for passing unibyte strings from lisp to mod

From: Brennan Vincent
Subject: Re: [PATCH] Add a mechanism for passing unibyte strings from lisp to modules.
Date: 2024年6月26日 23:36:16 -0400 
Eli Zaretskii <eliz@gnu.org> writes:
>> Date: 2024年6月26日 15:33:09 +0200
>> From: tomas@tuxteam.de
>> Cc: brennan@umanwizard.com, acorallo@gnu.org, stefankangas@gmail.com,
>> emacs-devel@gnu.org
>> 
>> > > > How will it be different from the Lisp vectors we already have?
>> > > 
>> > > The box around every byte.
>> > 
>> > What box? Please tell more, as I don't think I follow.
>> 
>> Maybe I'm all wrong, but AFAIU, a vector can contain arbitrary Lisp
>> values. That makes 64bits/8bits plus boxing/unboxing (which is, I
>> assume, quick, but nonzero).
>> 
>> Having a specialized "array of bytes" (as there is one for bools)
>> might be beneficial for big arrays, and perhaps avoid big data moving
>> operations over the C/LISP fence.
>
> If you are saying that using 64-bit values there incurs a run-time
> performance penalty, then accessing bytes does that as well. Someone
> should profile this and present evidence wrt the relative performance
> of these, then we can discuss whether the penalty is real and whether
> it is worth adding yet another data type to Emacs.
Sure, I wrote a quick benchmark that passes a 10MB buffer to a module
which just sums the bytes and returns and integer. It is about 200x
faster using a unibyte string (with my original patch) than a vector.
C code:
// Compile with gcc -O3 -fPIC -shared -o test-module.so test.c
#include <emacs-module.h>
#include <stdlib.h>
int plugin_is_GPL_compatible;
static emacs_value
Fcall_test(emacs_env *env, ptrdiff_t nargs, emacs_value args[], void *) 
EMACS_NOEXCEPT
{
 unsigned char sum = 0;
 emacs_value vec = args[0];
 size_t sz = env->vec_size(env, vec);
 for (int i = 0; i < sz; ++i)
 sum += env->extract_integer(env, env->vec_get(env, vec, i));
 return env->make_integer(env, sum);
}
static emacs_value
Fcall_test2(emacs_env *env, ptrdiff_t nargs, emacs_value args[], void *) 
EMACS_NOEXCEPT
{
 unsigned char sum = 0;
 emacs_value arr = args[0];
 char *buf;
 ptrdiff_t sz = 0;
 env->copy_unibyte_string_contents(env, arr, NULL, &sz);
 buf = malloc(sz);
 env->copy_unibyte_string_contents(env, arr, buf, &sz);
 for (int i = 0; i < sz - 1; ++i)
 sum += buf[i];
 return env->make_integer(env, sum);
}
/* bind c_func (native) to e_func (elisp) */
static void
bind(emacs_env *env, emacs_value (*c_func) (emacs_env *env,
 ptrdiff_t nargs,
 emacs_value args[],
 void *) EMACS_NOEXCEPT,
 const char *e_func,
 ptrdiff_t min_arity,
 ptrdiff_t max_arity,
 const char *doc,
 void *data)
{
 emacs_value fset_args[2];
 
 fset_args[0] = env->intern(env, e_func);
 fset_args[1] = env->make_function(env, min_arity, max_arity, c_func, doc, 
data);
 env->funcall(env, env->intern(env, "fset"), 2, fset_args);
}
int
emacs_module_init(struct emacs_runtime *ert)
{
 emacs_env *env = ert->get_environment(ert); 
 
 bind(env,
 Fcall_test, "btv--test", 1, 1,
 "test using vector",
 NULL);
 bind(env,
 Fcall_test2, "btv--test2", 1, 1,
 "test using byte array",
 NULL);
 emacs_value provide_arg = env->intern(env, "test-module");
 env->funcall(env, env->intern(env, "provide"), 1, &provide_arg);
 return 0;
}
Elisp code:
(require 'test-module)
(require 'benchmark)
(setq v (make-vector 10000001 37))
(setq v2 (make-string 10000001 37))
`(,(benchmark-elapse (btv--test v))
 ,(benchmark-elapse (btv--test2 v2)))
Result of evaluating elisp code:
(0.17861138 0.000805208)
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