To: linux-kernel@vger.kernel.org
From: hpa@zytor.com (H. Peter Anvin)
Subject: Re: [PATCH 1/12] random pt4: Create new rol32/ror32 bitops
Date: Mon, 24 Jan 2005 22:16:35 +0000 (UTC)
Organization: Mostly alphabetical, except Q, which We do not fancy
Message-ID: <ct3s43$ree$1@terminus.zytor.com>
References: <200501230247_MC3-1-93FA-7A4E@compuserve.com>
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Followup to:  <200501230247_MC3-1-93FA-7A4E@compuserve.com>
By author:    Chuck Ebbert <76306.1226@compuserve.com>
In newsgroup: linux.dev.kernel
>
> On Sat, 22 Jan 2005 at 20:13:24 -0800 Matt Mackall wrote:
> 
> > So I think tweaks for x86 at least are unnecessary. 
> 
>  So the compiler looks for that specific sequence of instructions:
> 
>         (a << b) | (a >> (sizeof(a) * 8 - b)
> 
> and recognizes that it means rotation?  Wow.
> 

You know, there is a LOT of instructions like that.  x86 has a single
instruction to do:

	a = b + (c << 3) + 3600;

(Assuming a, b and c are in eax, ebx, and ecx, respecively, the
instruction is "leal 3600(%ebx,%ecx,3),%eax".)

The C compiler really needs to recognize these kinds of idioms, not
just in the source, but that occur natually as a result of code
generation and optimizations.  The compiler lingo for this is
"peephole optimization."

	-hpa
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