Path: utzoo!mnetor!uunet!husc6!bloom-beacon!gatech!purdue!umd5!brl-adm!adm!bzs%bu-cs.bu.edu@bu-it.BU.EDU
From: bzs%bu-cs.bu.edu@bu-it.BU.EDU (Barry Shein)
Newsgroups: comp.unix.wizards
Subject: casting to float without converting to float?
Message-ID: <11174@brl-adm.ARPA>
Date: 7 Jan 88 23:25:49 GMT
Sender: news@brl-adm.ARPA
Lines: 46


The attached program seems to work. Whether or not it ends up in a
register seems to be compiler dependant but I had no complaints about
the syntax and it always produced the expected results.

On the Sun it ends up in a register when compiled with a -O flag but
not if not. On an Encore/Multimax (Greenhill's C) and Vax (4.3bsd) it
never ends up in a register. I couldn't really tell on the Celerity
but I think it ends up in a register both optimized and unoptimized (I
had trouble reading the machine code, not their fault, mostly I was
too lazy to try to follow it carefully.) The optimizer for PCC on the
3090 (IBM370) is not available but it ended up in a register
unoptimized.

I'll leave it to the language lawyers as to whether or not it *should*
have worked.

	-Barry Shein, Boston University

#include <stdio.h>
/*
 * Bit patterns for 23.7 on various machines
 */
#define SUN 1
#if SUN | UMAX | CELERITY
#define TWENTYTHREEPTSEVEN 0x41bd999a
#endif
#if IBM370
#define TWENTYTHREEPTSEVEN 0x4217b333
#endif
#if VAX
#define TWENTYTHREEPTSEVEN 0x999a42bd
#endif

main()
{
  register union foo {
    int a;
    float b;
  } x;
  float z;

  x.a = TWENTYTHREEPTSEVEN;	/* bit pattern for 23.7 float */
  z = x.b; 
  printf("%f\n",z);
}