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From: simpson@xenna.UUCP (Rich Simpson,Look for the plant on the pole.,x2609,5082636474)
Newsgroups: comp.sys.encore
Subject: Re: word in registers
Message-ID: <10854@xenna.Encore.COM>
Date: 17 Jul 89 13:47:41 GMT
References: <15266@ut-emx.UUCP>
Sender: news@Encore.COM
Lines: 41

From article <15266@ut-emx.UUCP>, by reeder@ut-emx.UUCP (William P. Reeder):
-- 
-- Here is why I ask:  I have a FORTRAN program in which 1000 is greater
-- than 3.  I look at the assembly instructions generated by the Encore
-- f77 compiler and don't understand why things are busted.

-- Here is the relevant assembler code (produced with f77 -S):
-- 	movd	8(fp),r1	# R1 contains address of ptr
-- 	movqw	$2,r2		# R2 contains word value 2 (lkey)
-- 	movqw	$1,0(r1)	# store 1 in word addressed in R1 (ptr=1)
-- 	movxwd	0(r1),r0	# bring ptr into R0 as double
-- 	addqd	$1,r0		# add (as doubles) 1 to contents of R0 (ptr+=1)
-- 	movw	r0,0(r1)	# store low word back in memory
-- 	movxwd	0(r1),r0	# bring it back
-- 	addd	r2,r0		# add R2 to R0 as doubles (ptr+lkey)
The problem is here. The value in r2 is a word and the value in r0 is a
double. Either the compiler should have not done the movxwd into r0,
done an addw and then widened the result, or it should have widened
the value in r2 to double before the addd. The net effect is that
the addd picks up noise from the top half of r2 which throws the
cmpd off.
-- 	addqd	$-1,r0		# subtract 1 from R0 as doubles (ptr+lkey-1)
-- 	cmpd	r0,$1000	# compare double in R0 with 1000 (clear N?)
-- 	ble	.L15		# branch if N flag clear

-- Can anyone explain what is going on with the this assembler?  And
-- can anyone from Encore tell me why f77 is generating this apparently
-- incorrect code?  I will be submitting a bug report to Encore about
-- this soon.  For those who care, I have an Encore Multimax 320 with
-- APC's, running Umax 4.2 Release 3.2.1, and with f77 release 2.0.r051.
-- -- 
-- William "Wills" Reeder, The University of Texas at Austin, Computation Center
-- reeder@emx.utexas.edu, uunet!cs.utexas.edu!ut-emx!reeder, (512)471-3241

This problem is fixed in the 2.1 version of f77 which is currently in final
release testing and should be available within a month. There were a variety
of problems with handling INTEGER*2 expressions which have been fixed in 
this release. 

Rich Simpson
Encore Computer