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" <mst@xxxxxxxxxx> 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?
And you expect to solve it somehow without barriers
explicit or implicit?
> )
>
>
> 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:
[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
--
MST
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