Date: Thu, 20 Feb 2014 14:06:55 +1100 (EST) From: Bruce Evans <brde@optusnet.com.au> To: David Hines <freebsd@dph.fluff.org> Cc: freebsd-gnats-submit@freebsd.org, freebsd-i386@freebsd.org Subject: Re: i386/186848: CLANG/LLVM code generation bug with optimisation on i386 Message-ID: <20140220135253.K889@besplex.bde.org> In-Reply-To: <201402171745.s1HHjCsG010826@cgiserv.freebsd.org> References: <201402171745.s1HHjCsG010826@cgiserv.freebsd.org>
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On Mon, 17 Feb 2014, David Hines wrote:
>> Description:
> "cc -O -o clang_bug-O clang_bug.c" generates incorrect code on i386, with the attached sample code. Without the "-O", or on an amd64 install the problem does not occur.
This is a bug in clang_bug.c. It's behaviour is undefined.
> Xunion
> X{
> X int i;
> X} u;
> X
> X
> Xint
> Xmain(int argc, char *argv[])
> X{
> X int j = 1;
> X
> X for (u.i = 1; u.i += u.i; ++j)
> X ;
> X printf("An int has %d bits\n", j);
> X
> X return 0;
> X}
Undefined behaviour occurs when the addition overflows. clang somehow
notices this (u.i starts as 1, and repeated doublings of it cannot
make it 0 unless overflow occurs). The addition does in fact overflow.
The result can be anything.
Some people (not me) don't like clang not giving the "normal" behaviour
on overflow.
Bruce
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