Date: Thu, 27 Aug 1998 20:54:37 +1000 From: Bruce Evans <bde@zeta.org.au> To: bde@zeta.org.au, cracauer@cons.org, current@FreeBSD.ORG, luoqi@watermarkgroup.com, shocking@prth.pgs.com Subject: Re: Floating Point Exceptions, signal handlers & subsequent ops Message-ID: <199808271054.UAA18772@godzilla.zeta.org.au>
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>> 2) if the SIGFPE was for an FP operation, then the FP operatation will be
>> restarted. The kernel has cleared the trap-pending flag before delivering
>> the SIGFPE to the application, and on i386's traps are delivered to the
>> kernel on the the first non-control FP operation after the one that
>> caused the exception, so it is certain that the restarted FP operation
>> won't trap; it may cause an exception which will be delivered to the
>> kernel on the next non-control FP operation.
>
>I'm not able to reproduce this behaviour. I wrote a short test program
>that runs like this:
It's easiest to reproduce it using assembler code - fill up the stack
with near-garbage using 8 fldz's and then attempt to clean up the stack
using 8 fdivp's. The fdivp's will have no effect except to generate
FP exceptions, but if a compiler had generated them, it would expect
the stack to be clean at the end.
I wrote the assembler code. Run it under gdb and look at the FP
state using `info float'. Homework: explain why this generates
only 6 SIGFPE's although it divides by 0.0 by 0.0 8 times.
Bruce
#include <floatingpoint.h>
#include <signal.h>
int
main(void)
{
/* I'm too lazy to set up signal handler. Use gdb to watch signals. */
signal(SIGFPE, SIG_IGN);
/* Ensure a SIGFPE for 0.0/0.0. */
fpsetmask(FP_X_INV);
asm("fldz");
asm("fldz");
asm("fldz");
asm("fldz");
asm("fldz");
asm("fldz");
asm("fldz");
asm("fldz");
asm("fdivp %st(0)");
asm("fdivp %st(0)");
asm("fdivp %st(0)");
asm("fdivp %st(0)");
asm("fdivp %st(0)");
asm("fdivp %st(0)");
asm("fdivp %st(0)");
asm("fdivp %st(0)");
return (0);
}
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