Followup to: <[EMAIL PROTECTED]>
By author: Chuck Ebbert <[EMAIL PROTECTED]>
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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