From mboxrd@z Thu Jan  1 00:00:00 1970
X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me
X-Spam-Level: 
X-Spam-Status: No, score=0.2 required=5.0 tests=BAYES_00,FREEMAIL_FROM,
	INVALID_MSGID,REPLYTO_WITHOUT_TO_CC autolearn=no autolearn_force=no
	version=3.4.4
X-Google-Language: ENGLISH,ASCII-7-bit
X-Google-Thread: 103376,270d47706116d3c,start
X-Google-Attributes: gid103376,public
From: bwburnsed@aol.com (BWBurnsed)
Subject: Can compilers do this?
Date: 1996/02/22
Message-ID: <4gjd6g$mfq@newsbf02.news.aol.com>#1/1
X-Deja-AN: 140717456
sender: root@newsbf02.news.aol.com
organization: America Online, Inc. (1-800-827-6364)
reply-to: bwburnsed@aol.com (BWBurnsed)
newsgroups: comp.lang.ada
Date: 1996-02-22T00:00:00+00:00
List-Id: <comp.lang.ada>

I came across some very strange looking code that someone else wrote,
long, long ago, and (apparently) in a universe far, far away. But before I

make too many critical comments, I want to be sure I'm not missing
something.

Repeatedly in this code (in many files), there are places where a floating
point variable is tested to see if it is negative. However, the way it is
done is:

     if  X * abs(X)  <  0.0  then  ...

Is there (or was there ever) some pathological anomaly about floating
point 
implementations that would make a conversion (abs) and floating point
multiply 
preferable to testing a sign bit? Can, and will, optimizing compilers
recognize the
real test desired in such constructs, i.e. reduce it to a sign bit test?

Also, suppose Y and X are floating point variables, and M and B are 
CONSTANT floating point variables (not named numbers) initialized to
1.0  and  0.0  respectively. If one writes

     Y := M * X  +  B ;

can (and will) any compiler reduce this to

     Y := X ;

If not, could it do so if M and B were named numbers?

Thanks,
BwB