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Date: Thu, 20 Feb 2014 14:06:55 +1100 (EST)
From: Bruce Evans <brde@optusnet.com.au>
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To: David Hines <freebsd@dph.fluff.org>
Subject: Re: i386/186848: CLANG/LLVM code generation bug with optimisation
 on i386
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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