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Date: Thu, 1 Nov 2007 00:55:26 +0300
From: Andrey Chernov <ache@nagual.pp.ru>
To: Juli Mallett <juli@clockworksquid.com>
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Cc: cvs-src@FreeBSD.ORG, src-committers@FreeBSD.ORG, cvs-all@FreeBSD.ORG
Subject: Re: cvs commit: src/include _ctype.h
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On Tue, Oct 30, 2007 at 10:03:31AM -1000, Juli Mallett wrote:
> * "Andrey A. Chernov" <ache@FreeBSD.org> [ 2007-10-27 ]
> [ cvs commit: src/include _ctype.h ]
> > ache 2007-10-27 22:32:28 UTC
> >
> > FreeBSD src repository
> >
> > Modified files:
> > include _ctype.h
> > Log:
> > Micro-optimization of prev. commit, change
> > (_c < 0 || _c >= 128) to (_c & ~0x7F)
>
> Isn't that a non-optimization in code and a minor pessimization of readability?
> Maybe I'm getting rusty, but those seem to result in nearly identical code on
> i386 with a relatively modern GCC. Did you look at the compiler output for this
> optimization? Is there a specific expensive instruction you're trying to avoid?
> For such thoroughyl bit-aligned range checks, you shouldn't even get a branch
> for the former case. Is there a platform other than i386 I should look at where
> the previous expression is more clearly pessimized? Or a different compiler
> than GCC?
For ones who doubts there two tests compiled with -O2. As you may see the
result is almost identical (andl vs cmpl):
-------------------- a.c --------------------
main () {
int c;
return (c & ~0x7f) ? 0 : c * 2;
}
-------------------- a.s --------------------
.file "a.c"
.text
.p2align 4,,15
.globl main
.type main, @function
main:
leal 4(%esp), %ecx
andl $-16, %esp
pushl -4(%ecx)
movl %eax, %edx
andl $-128, %edx
addl %eax, %eax
cmpl $1, %edx
sbbl %edx, %edx
pushl %ebp
andl %edx, %eax
movl %esp, %ebp
pushl %ecx
popl %ecx
popl %ebp
leal -4(%ecx), %esp
ret
.size main, .-main
.ident "GCC: (GNU) 4.2.1 20070719 [FreeBSD]"
-------------------- a1.c --------------------
main () {
int c;
return (c < 0 || c >= 128) ? 0 : c * 2;
}
-------------------- a1.s --------------------
.file "a1.c"
.text
.p2align 4,,15
.globl main
.type main, @function
main:
leal 4(%esp), %ecx
andl $-16, %esp
pushl -4(%ecx)
addl %eax, %eax
cmpl $128, %eax
sbbl %edx, %edx
andl %edx, %eax
pushl %ebp
movl %esp, %ebp
pushl %ecx
popl %ecx
popl %ebp
leal -4(%ecx), %esp
ret
.size main, .-main
.ident "GCC: (GNU) 4.2.1 20070719 [FreeBSD]"
--
http://ache.pp.ru/