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Re: Optimisation questions

From Dann Corbit <dcorbit@connx.com>
Newsgroups comp.lang.c
Subject Re: Optimisation questions
Date 2012-02-06 09:40 -0800
Organization Aioe.org NNTP Server
Message-ID <MPG.2999b0533378d8cf989693@aioe.org> (permalink)
References <jgkbiu$432$1@speranza.aioe.org> <jgkfso$1bh$2@dont-email.me> <jgkgra$886$1@dont-email.me> <jgkou6$g96$1@dont-email.me> <20120205034114.797@kylheku.com>

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In article <20120205034114.797@kylheku.com>, kaz@kylheku.com says...
> int div8_0(int x)
> {
>   return x >> 3;
> }
> 
> int div8_1(int x)
> {
>   return x / 8;
> }
> 
> 

Using gcc 4.7 with -O3 on this program:

#include <stdio.h>
#include <stdlib.h>

int div8_0(int x)
{
    return x >> 3;
}

int div8_1(int x)
{
    return x / 8;
}

int main(void)
{
    int a = rand();
    int result0 = div8_0(a);
    int result1 = div8_1(a);
    printf("%d / 8 = %d, %d\n", a, result0, result1);
    return 0;
}

gave this output, which adds additional instructions for the division as 
opposed to the shift:

C:\tmp>type test.s
        .file   "test.c"
        .text
        .p2align 4,,15
        .globl  div8_0
        .def    div8_0; .scl    2;      .type   32;     .endef
        .seh_proc       div8_0
div8_0:
        .seh_endprologue
        movl    %ecx, %eax
        sarl    $3, %eax
        ret
        .seh_endproc
        .p2align 4,,15
        .globl  div8_1
        .def    div8_1; .scl    2;      .type   32;     .endef
        .seh_proc       div8_1
div8_1:
        .seh_endprologue
        leal    7(%rcx), %eax
        testl   %ecx, %ecx
        cmovns  %ecx, %eax
        sarl    $3, %eax
        ret
        .seh_endproc
        .def    __main; .scl    2;      .type   32;     .endef
        .section .rdata,"dr"
.LC0:
        .ascii "%d / 8 = %d, %d\12\0"
        .section        .text.startup,"x"
        .p2align 4,,15
        .globl  main
        .def    main;   .scl    2;      .type   32;     .endef
        .seh_proc       main
main:
        subq    $40, %rsp
        .seh_stackalloc 40
        .seh_endprologue
        call    __main
        call    rand
        leal    7(%rax), %r9d
        testl   %eax, %eax
        movl    %eax, %r8d
        leaq    .LC0(%rip), %rcx
        movl    %eax, %edx
        cmovns  %eax, %r9d
        sarl    $3, %r8d
        sarl    $3, %r9d
        call    printf
        xorl    %eax, %eax
        addq    $40, %rsp
        ret
        .seh_endproc
        .def    rand;   .scl    2;      .type   32;     .endef
        .def    printf; .scl    2;      .type   32;     .endef

With similar output for the MS compiler:
; Listing generated by Microsoft (R) Optimizing Compiler Version 
16.00.40219.01 

include listing.inc

INCLUDELIB LIBCMT
INCLUDELIB OLDNAMES

_DATA	SEGMENT
$SG4006	DB	'%d / 8 = %d, %d', 0aH, 00H
_DATA	ENDS
PUBLIC	div8_0
; Function compile flags: /Ogtpy
_TEXT	SEGMENT
x$ = 8
div8_0	PROC
; File c:\tmp\test.c
; Line 6
	sar	ecx, 3
	mov	eax, ecx
; Line 7
	ret	0
div8_0	ENDP
_TEXT	ENDS
PUBLIC	div8_1
; Function compile flags: /Ogtpy
_TEXT	SEGMENT
x$ = 8
div8_1	PROC
; Line 11
	mov	eax, ecx
	cdq
	and	edx, 7
	add	eax, edx
	sar	eax, 3
; Line 12
	ret	0
div8_1	ENDP
_TEXT	ENDS
PUBLIC	main
EXTRN	printf:PROC
EXTRN	rand:PROC
pdata	SEGMENT
$pdata$main DD	imagerel $LN7
	DD	imagerel $LN7+53
	DD	imagerel $unwind$main
pdata	ENDS
xdata	SEGMENT
$unwind$main DD	010401H
	DD	04204H
; Function compile flags: /Ogtpy
xdata	ENDS
_TEXT	SEGMENT
main	PROC
; Line 15
$LN7:
	sub	rsp, 40					; 00000028H
; Line 16
	call	rand
; Line 19
	lea	rcx, OFFSET FLAT:$SG4006
	mov	r11d, eax
	cdq
	and	edx, 7
	mov	r8d, r11d
	lea	r9d, DWORD PTR [rdx+rax]
	sar	r8d, 3
	mov	edx, r11d
	sar	r9d, 3
	call	printf
; Line 20
	xor	eax, eax
; Line 21
	add	rsp, 40					; 00000028H
	ret	0
main	ENDP
_TEXT	ENDS
END

I didn't bother to time it, but it seems that state of the art compilers 
create faster code for the shift than for the division.

I guess that the bottom line is that if you need the last little 
smackerel of performance then you have better try all the alternatives 
and benchmark them.

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Thread

Optimisation questions Raj Pashwar <raj121190@hotmail.NOSPAM.com> - 2012-02-04 22:30 +0000
  Re: Optimisation questions Ian Collins <ian-news@hotmail.com> - 2012-02-05 11:44 +1300
    Re: Optimisation questions Keith Thompson <kst-u@mib.org> - 2012-02-04 14:52 -0800
      Re: Optimisation questions Ian Collins <ian-news@hotmail.com> - 2012-02-05 11:55 +1300
      Re: Optimisation questions Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2012-02-21 18:41 +0200
  Re: Optimisation questions "BartC" <bc@freeuk.com> - 2012-02-04 22:44 +0000
    Re: Optimisation questions "BartC" <bc@freeuk.com> - 2012-02-04 23:41 +0000
  Re: Optimisation questions Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-02-04 17:51 -0500
  Re: Optimisation questions Keith Thompson <kst-u@mib.org> - 2012-02-04 14:52 -0800
  Re: Optimisation questions James Kuyper <jameskuyper@verizon.net> - 2012-02-04 18:44 -0500
    Re: Optimisation questions Stephen Sprunk <stephen@sprunk.org> - 2012-02-04 18:00 -0600
      Re: Optimisation questions James Kuyper <jameskuyper@verizon.net> - 2012-02-04 21:18 -0500
        Re: Optimisation questions Kaz Kylheku <kaz@kylheku.com> - 2012-02-05 03:04 +0000
          Re: Optimisation questions Dann Corbit <dcorbit@connx.com> - 2012-02-06 09:40 -0800
            Re: Optimisation questions Willem <willem@toad.stack.nl> - 2012-02-06 18:51 +0000
            Re: Optimisation questions Markus Wichmann <nullplan@gmx.net> - 2012-02-07 14:19 +0100
              Re: Optimisation questions Dann Corbit <dcorbit@connx.com> - 2012-02-07 14:25 -0800
          Re: Optimisation questions Stephen Sprunk <stephen@sprunk.org> - 2012-02-06 17:29 -0600
            Re: Optimisation questions Kaz Kylheku <kaz@kylheku.com> - 2012-02-06 23:52 +0000
            Re: Optimisation questions pete <pfiland@mindspring.com> - 2012-02-06 19:10 -0500
    Re: Optimisation questions Raj Pashwar <raj121190@hotmail.NOSPAM.com> - 2012-02-05 10:51 +0000
      Re: Optimisation questions Ian Collins <ian-news@hotmail.com> - 2012-02-06 00:08 +1300
        Re: Optimisation questions Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-02-05 08:13 -0500
      Re: Optimisation questions "BartC" <bc@freeuk.com> - 2012-02-05 11:33 +0000
      Re: Optimisation questions Zoltan Kocsi <zoltan@bendor.com.au> - 2012-02-05 23:24 +1100
        Re: Optimisation questions Raj Pashwar <raj121190@hotmail.NOSPAM.com> - 2012-02-05 22:28 +0000
          Re: Optimisation questions Kaz Kylheku <kaz@kylheku.com> - 2012-02-05 22:37 +0000
          Re: Optimisation questions Eric Sosman <esosman@ieee-dot-org.invalid> - 2012-02-05 17:48 -0500
          Re: Optimisation questions Keith Thompson <kst-u@mib.org> - 2012-02-05 15:57 -0800
      Re: Optimisation questions "BartC" <bc@freeuk.com> - 2012-02-05 12:53 +0000
  Re: Optimisation questions Geoff <geoff@invalid.invalid> - 2012-02-04 16:07 -0800
  Re: Optimisation questions "christian.bau" <christian.bau@cbau.wanadoo.co.uk> - 2012-02-04 15:37 -0800
    Re: Optimisation questions Nick Keighley <nick_keighley_nospam@hotmail.com> - 2012-02-05 04:26 -0800
      Re: Optimisation questions lawrence.jones@siemens.com - 2012-02-07 10:56 -0500
  Re: Optimisation questions Nick Keighley <nick_keighley_nospam@hotmail.com> - 2012-02-05 04:29 -0800
  Re: Optimisation questions Michael Angelo Ravera <maravera@prodigy.net> - 2012-02-05 23:24 -0800

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