Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!ames!pasteur!ucbvax!amdcad!crackle!tim
From: tim@crackle.amd.com (Tim Olson)
Newsgroups: comp.arch
Subject: Re: Understanding variations in Dhrystone performance
Message-ID: <25679@amdcad.AMD.COM>
Date: 18 May 89 19:13:30 GMT
References: <474@estevax.UUCP> <1989May15.173631.3029@utzoo.uucp> <1989May16.172354.1417@utzoo.uucp> <839@ultb.UUCP>
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In article <839@ultb.UUCP> jed4885@ultb.UUCP (J.E. Dyer (713ICS)) writes:
| 	This does seem like alot of work to test for a null byte...  Has
| 	anyone considered putting in a test-word-for-byte (twfb?)
| 	instruction on their favorite processor?

From the Am29000 User's Manual:

CPBYTE								CPBYTE
			    Compare Bytes

Operation:	if (srca.byte0 = srcb.byte0) or
		   (srca.byte1 = srcb.byte1) or
		   (srca.byte2 = srcb.byte2) or
		   (srca.byte3 = srcb.byte3)
		then
			dest <- TRUE
		else
			dest <- FALSE

Description:
	Each byte of the srca operand is compared to the corresponding
	byte of the srcb operand.  If any corresponding bytes are equal,
	a Boolean TRUE is placed into the DEST location; otherwise, a
	Boolean FALSE is placed in the DEST location.

Assembler Syntax:

	cpbyte	rc, ra, rb
	cpbyte	rc, ra, const8


|	It seems to me that
| 	adding this kind of a function to an alu would be pretty
| 	trivial, and it would make a significant improvement in some
| 	kinds of string operations.

Yes, it is trivial.  We added it because we felt that it was an easy way
to speed up 'C' string operations (str[n]cmp, str[n]cpy, strlen) which
must constantly search for a terminating byte.  With the cpbyte
instruction, string operations can be performed a word at a time.

|	Of course, your strings would have
| 	to be aligned on word boundaries, but that shouldn't be to
| 	difficult to add to a compiler.

Not necessarily.  All you have to do is take care of the boundary
conditions correctly.

	-- Tim Olson
	Advanced Micro Devices
	(tim@amd.com)