From: sommerfeld@orchard.medford.ma.us (Bill Sommerfeld)
Date: Thu, 7 Jul 94 08:33:17 PDT
To: ifarqhar@laurel.ocs.mq.edu.au
Subject: Re: Counting bits
In-Reply-To: <199407070647.AA12059@laurel.ocs.mq.edu.au>
Message-ID: <199407071518.LAA00484@orchard.medford.ma.us>
MIME-Version: 1.0
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Since people are playing "my processor is better than your processor"...

This case (counting number of bits set in n-bit word) takes 2n+1
instructions on the HP PA-RISC processor.  (HP's compiler generates
2n+2 instructions, GCC takes 2n+1).  No branch instructions are
generated in either case.

HP's compiler uses the conditional skip feature of the PA
architecture, while GCC converts

	if (x&(1<<n)) y++;

into the equivalent branchless form:

	y += ((x>>n)&1);

( (x>>n)&1 being a single-instruction bitfield extract on the PA).

						- Bill