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Re: O2 Agressive Optimisation by GCC
- From: Allan Sandfeld Jensen <linux at carewolf dot com>
- To: gcc at gcc dot gnu dot org
- Cc: Jonathan Wakely <jwakely dot gcc at gmail dot com>, Umesh Kalappa <umesh dot kalappa0 at gmail dot com>
- Date: Sat, 21 Jul 2018 00:50:31 +0200
- Subject: Re: O2 Agressive Optimisation by GCC
- References: <CAGfacvTvkR6d+dqG0Et04gxqcz9JcH3JV1vi8S=WbfJbOnv3JA@mail.gmail.com> <1700194.Gr8vF7hmjn@twilight> <CAH6eHdRsNQkfjTXeOxXT3Dt0+oVt=bzjnyMCSGJQyXTQTohPFQ@mail.gmail.com>
On Samstag, 21. Juli 2018 00:21:48 CEST Jonathan Wakely wrote:
> On Fri, 20 Jul 2018 at 23:06, Allan Sandfeld Jensen wrote:
> > On Freitag, 20. Juli 2018 14:19:12 CEST Umesh Kalappa wrote:
> > > Hi All ,
> > >
> > > We are looking at the C sample i.e
> > >
> > > extern int i,j;
> > >
> > > int test()
> > > {
> > > while(1)
> > > { i++;
> > >
> > > j=20;
> > >
> > > }
> > > return 0;
> > > }
> > >
> > > command used :(gcc 8.1.0)
> > > gcc -S test.c -O2
> > >
> > > the generated asm for x86
> > >
> > > .L2:
> > > jmp .L2
> > >
> > > we understand that,the infinite loop is not deterministic ,compiler
> > > is free to treat as that as UB and do aggressive optimization ,but we
> > > need keep the side effects like j=20 untouched by optimization .
> > >
> > > Please note that using the volatile qualifier for i and j or empty
> > > asm("") in the while loop,will stop the optimizer ,but we don't want
> > > do that.
> >
> > But you need to do that! If you want changes to a variable to be
> > observable in another thread, you need to use either volatile,
>
> No, volatile doesn't work for that.
>
It does, but you shouldn't use for that due to many other reasons (though the
linux kernel still does) But if the guy wants to code primitive without using
system calls or atomics, he might as well go traditional
'Allan