Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!purdue!mentor.cc.purdue.edu!l.cc.purdue.edu!cik
From: cik@l.cc.purdue.edu (Herman Rubin)
Newsgroups: comp.lang.c
Subject: Re: Type punning in C
Summary: There are problems with this
Message-ID: <1647@l.cc.purdue.edu>
Date: 10 Oct 89 14:25:15 GMT
References: <475@idacrd.UUCP> <2165@dataio.Data-IO.COM>
Followup-To: comp.lang.c
Organization: Purdue University Statistics Department
Lines: 31

In article <2165@dataio.Data-IO.COM>, bright@Data-IO.COM (Walter Bright) writes:
> In article <475@idacrd.UUCP> desj@idacrd.UUCP (David desJardins) writes:
| <   Does anyone have any ideas on how one should go about converting an
| <integer to the floating-point number with the same bit pattern?
> 
> Try a union:
> 	union blah { int i; float f; } x;
> 	x.f = f;
> 	/* now manipulate x.i */

			.......................

| <      int i;
| <      float x;
| <      * (int *) & x = i;
| <to produce a single register-to-register transfer (or not even that).
> 
> Not at all. It's a pretty much standard and trivial optimization to
> convert *(int*)&x=i into MOV X,I. Feel free to use it, with the caveat
> that float format is not the same from machine to machine. I've done
> it many times to do special manipulations on doubles.

This may or may not work.  It is likely that the "&" by itself will force
x to memory.  But event if a #define is used to eliminate that, al least on
may compilers the MOV X,I or equivalent will not work.  Reluctantly, I would
have to say that it might be necessary to put in a kludge and then edit the
.s files, which is what I find myself forced to do in these situations.
-- 
Herman Rubin, Dept. of Statistics, Purdue Univ., West Lafayette IN47907
Phone: (317)494-6054
hrubin@l.cc.purdue.edu (Internet, bitnet, UUCP)