From mboxrd@z Thu Jan  1 00:00:00 1970
Message-ID: <d68515d6a85237b56c150190c4ac45d0@plan9.bell-labs.com>
From: David Presotto <presotto@closedmind.org>
To: 9fans@cse.psu.edu
Subject: Re: [9fans] remember when C compiler used to produce working code? i don't!
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Date: Fri, 30 Jan 2004 17:29:21 -0500
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It does work if you replace:

	        elasticity *= ratio;

with

	        elasticity = elasticity * ratio;

Clearly we're analyzing wrong when there's only an assignment.
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To: 9fans@cse.psu.edu
From: andrey mirtchovski <mirtchov@cpsc.ucalgary.ca>
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Subject: [9fans] remember when C compiler used to produce working code? i don't!
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I got bitten by this one pretty hard today, took me the better part of
the day to find out...  The 'elasticity *= ratio' code comes from the
XScreensaver distribution and was written sometime in 1997, so
presumably this has been working since then (and braking when compiled
with 8c?).

In Plan 9 the following code produces 800/0:

	plan9-2% cat t.c
	#include <u.h>
	#include <libc.h>

	main()
	{
		long elasticity = 8000L;
		double ratio = 0.1;

		/* this is fine */
		elasticity = (long)(((double)elasticity)*ratio);
		print("%ld\n", elasticity);

		/* this breaks with 8c */
		elasticity *= ratio;
		print("%ld\n", elasticity);

	}
	plan9-2% 8c t.c; 8l t.8
	plan9-2% 8.out
	800
	0
	plan9-2%


Its (almost) identical lunix counterpart, when compiled with gcc 3.2,
produces 800/80:

	mirtchov@fbsd$ cat t.c
	main()
	{
	        long elasticity = 8000L;
	        double ratio = 0.1;

		/* this is fine */
	        elasticity = (long)(((float)elasticity)*ratio);
	        printf("%ld\n", elasticity);

		/* this if fine with gcc 3.2 */
	        elasticity *= ratio;
	        printf("%ld\n", elasticity);

	}
	mirtchov@fbsd$ gcc t.c
	mirtchov@fbsd$ ./a.out
	800
	80
	mirtchov@fbsd$


Not looking to assign any blame, just letting you know...

andrey

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