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From: Brian Inglis <Brian DOT Inglis AT SystematicSw DOT ab DOT ca>
Newsgroups: comp.os.msdos.djgpp,comp.lang.c
Subject: Re: gcc 2.953 output - is this a bug?
Date: Thu, 07 Feb 2002 21:07:40 -0700
Organization: Systematic Software
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On Thu, 07 Feb 2002 02:31:00 GMT, CBFalconer
<cbfalconer AT yahoo DOT com> wrote:

>Please look at the following disassembly:
>
>00000578 <_hshstrehash>:
>
>/* 1------------------1 */
>
>/* ReHash a string quantity */
>unsigned long hshstrehash(const char * string)
>{
> 578:   55                      push   %ebp
> 579:   89 e5                   mov    %esp,%ebp
> 57b:   8b 4d 08                mov    0x8(%ebp),%ecx
>   unsigned long h;
>
>   h = 0;
> 57e:   31 d2                   xor    %edx,%edx
>   while (*string) {
> 580:   80 39 00                cmpb   $0x0,(%ecx)
> 583:   74 12                   je     597 <_hshstrehash+0x1f>
>      h = h * 37UL + (unsigned char) *string++;
> 585:   8d 04 d2                lea    (%edx,%edx,8),%eax <<***
> 588:   8d 04 82                lea    (%edx,%eax,4),%eax <<***
> 58b:   31 d2                   xor    %edx,%edx
> 58d:   8a 11                   mov    (%ecx),%dl
> 58f:   41                      inc    %ecx
> 590:   01 c2                   add    %eax,%edx
>   }
> 592:   80 39 00                cmpb   $0x0,(%ecx)
> 595:   75 ee                   jne    585 <_hshstrehash+0xd>
>   return h;
> 597:   89 d0                   mov    %edx,%eax
> 599:   89 ec                   mov    %ebp,%esp
> 59b:   5d                      pop    %ebp
> 59c:   c3                      ret
>} /* hshstrehash */
>
>look at the lines marked "<<***".  I fail to see how this
>implements h * 37UL.  This is compiled with -O2 and disassembled
>by objdump -dS.
>
>c.l.c justification - purely standard source code :-)  I AM
>impressed by gcc's code generation.

As am I -- I've got to download and read that source -- really
nice optimization -- anyone point me to the module? 

-- 

Thanks. Take care, Brian Inglis 	Calgary, Alberta, Canada

Brian DOT Inglis AT CSi DOT com 	(Brian dot Inglis at SystematicSw dot ab dot ca)
    fake address		use address above to reply

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