On Wed, Sep 05, 2012 at 05:30:31PM +0900, Takuya Yoshikawa wrote:
> On Thu, 30 Aug 2012 19:49:23 +0300
> "Michael S. Tsirkin" <[email protected]> wrote:
>
> > On Fri, Aug 31, 2012 at 01:09:56AM +0900, Takuya Yoshikawa wrote:
> > > On Thu, 30 Aug 2012 16:21:31 +0300
> > > "Michael S. Tsirkin" <[email protected]> wrote:
> > >
> > > > > +static u32 apic_read_reg(int reg_off, void *bitmap)
> > > > > +{
> > > > > + return *((u32 *)(bitmap + reg_off));
> > > > > +}
> > > > > +
> > > >
> > > > Contrast with apic_set_reg which gets apic,
> > > > add fact that all callers invoke REG_POS and you will
> > > > see this is a bad API.
> > > >
> > > > I played with some APIs but in the end it's
> > > > probably better to just open-code this.
> > >
> > > I don't mind open-coding this.
> > >
> > > > As a bonus, open-coding will avoid the need
> > > > for cast above, which is good: casts make code more
> > > > fragile.
> > >
> > > But I still don't understand why we can eliminate casting:
> > >
> > > u32 reg_val;
> > >
> > > reg_val = *((u32 *)(bitmap + REG_POS(vec)));
> > > if (reg_val)
> > > return __fls(reg_val) + vec;
> > >
> > > (I'm not sure compilers are allowed to push out the value and
> > > do multiple references for this code as explained in
> > > https://lwn.net/Articles/508991/
> >
> > So you *were* talking about concurrency?
>
> Yes and no, please see below.
>
> > And you expect to solve it somehow without barriers
> > explicit or implicit?
>
> What I want to make clear is that the value we pass to
> __fls() is not zero, not any more, to avoid undefined
> behaviour.
>
> So as you showed below, if the value passed to __fls() is
> exactly from the register, which we did non-zero check,
> that's fine. Barriers are not related here.
>
> But as can be seen in the last part of the article above,
> that's may theoretically not be guranteed?
It's not guaranteed if another thread can modify the bitmap.
Is this the case here? If yes we need at least ACCESS_ONCE.
> Anyway, I'm now thinking that we do not care about such
> things here, and can just follow your advice, yes?
Unless you see an issue with it ...
> >
> > > )
> > >
> > >
> > > If you mean
> > >
> > > u32 *reg;
> > >
> > > reg = bitmap + REG_POS(vec);
> > > if (*reg)
> > > return __fls(*reg) + vec;
> >
> > yes
> >
> > > I'm still not confident if this is a good style.
> > > I rarely see code doing
> > >
> > > if (*p)
> > > __fls(*p);
> > >
> > > This looks like explicite multiple references: I'm not saying
> > > this will actually be compiled to do multiple references.
> > >
> > > Thanks,
> > > Takuya
> >
> > It's just weird. Both versions are exactly equivalent in C.
> > Adding a temporary changes *nothing* so the best readability
> > wins. And IMHO, a version that does not cast wins hands down.
> > I did a small test just to give you an example:
>
> Thank you for the example.
>
> What you showed is what I wanted to mean by
> "I'm not saying this will actually be compiled to ..."
>
> Thanks,
> Takuya
>
> >
> > [mst@robin ~]$ cat a.c
> >
> > int foo(void *bitmap)
> > {
> > unsigned *reg;
> >
> > reg = bitmap + 4;
> > if (*reg)
> > return *reg + 1;
> >
> > return -1;
> > }
> > [mst@robin ~]$ cat b.c
> >
> > int foo(void *bitmap)
> > {
> > unsigned reg;
> >
> > reg = *((unsigned *)(bitmap + 4));
> > if (reg)
> > return reg + 1;
> >
> > return -1;
> > }
> >
> > [mst@robin ~]$ gcc -O2 -c a.c
> > [mst@robin ~]$ gcc -O2 -c b.c
> >
> >
> > [mst@robin ~]$ objdump -ld a.o
> >
> > a.o: file format elf32-i386
> >
> >
> > Disassembly of section .text:
> >
> > 00000000 <foo>:
> > foo():
> > 0: 8b 44 24 04 mov 0x4(%esp),%eax
> > 4: 8b 50 04 mov 0x4(%eax),%edx
> > 7: b8 ff ff ff ff mov $0xffffffff,%eax
> > c: 8d 4a 01 lea 0x1(%edx),%ecx
> > f: 85 d2 test %edx,%edx
> > 11: 0f 45 c1 cmovne %ecx,%eax
> > 14: c3 ret
> > [mst@robin ~]$ objdump -ld b.o
> >
> > b.o: file format elf32-i386
> >
> >
> > Disassembly of section .text:
> >
> > 00000000 <foo>:
> > foo():
> > 0: 8b 44 24 04 mov 0x4(%esp),%eax
> > 4: 8b 50 04 mov 0x4(%eax),%edx
> > 7: b8 ff ff ff ff mov $0xffffffff,%eax
> > c: 8d 4a 01 lea 0x1(%edx),%ecx
> > f: 85 d2 test %edx,%edx
> > 11: 0f 45 c1 cmovne %ecx,%eax
> > 14: c3 ret
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