From: Alejandro Colomar <alx.manpages@gmail.com>
To: gcc@gcc.gnu.org
Subject: Missing optimization: mempcpy(3) vs memcpy(3)
Date: Fri, 9 Dec 2022 18:11:17 +0100 [thread overview]
Message-ID: <fdc766b8-7cfd-806b-3602-53e1ba9b277e@gmail.com> (raw)
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Hi!
I expect mempcpy(3) to be at least as fast as memcpy(3), since it performs the
same operations, with the exception that mempcpy(3) returns something useful (as
opposed to memcpy(3), which could perfectly return void), and in fact something
more likely to be in cache, if the copy is performed upwards.
The following two files are alternative implementations of a function, each one
written in terms of one of memcpy(3) and mempcpy(3):
$ cat usts2stp1.c
#include <string.h>
struct ustr_s {
size_t len;
char *ustr;
};
char *
usts2stp(char *restrict dst, const struct ustr_s *restrict src)
{
memcpy(dst, src->ustr, src->len);
dst[src->len] = '\0';
return dst + src->len;
}
$ cat usts2stp3.c
#define _GNU_SOURCE
#include <string.h>
struct ustr_s {
size_t len;
char *ustr;
};
char *
usts2stp(char *restrict dst, const struct ustr_s *restrict src)
{
char *end;
end = mempcpy(dst, src->ustr, src->len);
*end = '\0';
return end;
}
I expect the compiler to be knowledgeable enough to call whatever is fastest,
whatever it is, but be consistent in both cases. However, here are the results:
$ cc -Wall -Wextra -O3 -S usts2stp*.c
$ diff -u usts2stp[13].s
--- usts2stp1.s 2022-12-09 18:06:11.708367061 +0100
+++ usts2stp3.s 2022-12-09 18:06:11.740366451 +0100
@@ -1,4 +1,4 @@
- .file "usts2stp1.c"
+ .file "usts2stp3.c"
.text
.p2align 4
.globl usts2stp
@@ -6,16 +6,13 @@
usts2stp:
.LFB0:
.cfi_startproc
- pushq %rbx
+ subq $8, %rsp
.cfi_def_cfa_offset 16
- .cfi_offset 3, -16
- movq (%rsi), %rbx
+ movq (%rsi), %rdx
movq 8(%rsi), %rsi
- movq %rbx, %rdx
- call memcpy@PLT
- leaq (%rax,%rbx), %rax
+ call mempcpy@PLT
movb $0, (%rax)
- popq %rbx
+ addq $8, %rsp
.cfi_def_cfa_offset 8
ret
.cfi_endproc
The code with memcpy(3) seems to be worse (assuming both calls to be
equivalent). Shouldn't GCC produce the same code for both implementations?
Cheers,
Alex
--
<http://www.alejandro-colomar.es/>
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next reply other threads:[~2022-12-09 17:11 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-12-09 17:11 Alejandro Colomar [this message]
2022-12-12 13:37 ` Martin Liška
2022-12-12 13:44 ` Alejandro Colomar
2022-12-12 13:56 ` Jakub Jelinek
2022-12-12 14:05 ` Alejandro Colomar
2022-12-12 14:48 ` Jonathan Wakely
2022-12-12 14:53 ` Jakub Jelinek
2022-12-12 15:56 ` Alejandro Colomar
2022-12-12 16:09 ` Jakub Jelinek
2022-12-12 17:15 ` Alejandro Colomar
2022-12-12 17:42 ` Jonathan Wakely
2022-12-12 14:34 Wilco Dijkstra
2022-12-12 14:57 ` Cristian Rodríguez
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