Missing optimization: mempcpy(3) vs memcpy(3)

[Alejandro Colomar]

Hi Martin,

On 12/12/22 14:37, Martin Liška wrote:
> On 12/9/22 18:11, Alejandro Colomar via Gcc wrote:
>> 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:
> 
> Hi.
> 
> Note the glibc implementation of mempcpy typically uses (calls) memcpy, thus

Thanks for the info.  I CCed glibc now, and copied my original email below for 
completeness.

> I don't see any problem with the code snippets you provided.

Well, then the optimization may be the other way around (although I question why 
it is implemented that way, and not the other way around, but I'm not a hardware 
or libc guy, so there may be reasons).

If calling memcpy(3) is better, then the code calling mempcpy(3) could be 
expanded inline to call it (but I doubt it).

If calling mempcpy(3) is better, then the hand-made pattern resembling 
mempcpy(3) should probably be merged as a call to mempcpy(3).

But acting different on equivalent calls to both of them seems inconsistent to 
me, unless you trust the programmer to know better how to optimize, that is...

Cheers,

Alex


-------- Forwarded Message --------
Subject: Missing optimization: mempcpy(3) vs memcpy(3)
Date: Fri, 9 Dec 2022 18:11:17 +0100
From: Alejandro Colomar <alx.manpages@gmail.com>
To: gcc@gcc.gnu.org

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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