From mboxrd@z Thu Jan  1 00:00:00 1970
Message-ID: <d45fb131231fd5a3515e0f7518a6568a@plan9.ucalgary.ca>
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!
Date: Fri, 30 Jan 2004 15:05:43 -0700
Topicbox-Message-UUID: c83a9aa6-eacc-11e9-9e20-41e7f4b1d025

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