Re: [PATCH] futex: avoid undefined behaviour when shift exponent is negative

[Thomas Gleixner]

On Thu, 29 Jun 2017, zhong jiang wrote:
> On 2017/6/29 6:13, Thomas Gleixner wrote:
> > That's simply wrong. If oparg is negative and the SHIFT bit is set then the
> > result is undefined today and there is no way that this can be used at
> > all.
> >
> > On x86:
> >
> >    1 << -1	= 0x80000000
> >    1 << -2048	= 0x00000001
> >    1 << -2047	= 0x00000002
>   but I test the cases in x86_64 all is zero.   I wonder whether it is related to gcc or not
> 
>   zj.c:15:8: warning: left shift count is negative [-Wshift-count-negative]
>   j = 1 << -2048;
>         ^
> [root@localhost zhongjiang]# ./zj
> j = 0

Which is not a surprise because the compiler can detect it as the shift is
a constant. oparg is not so constant ...

Thanks,

	tglx