On 04/16/2012 05:14 PM, Takuya Yoshikawa wrote:
> On Sun, 15 Apr 2012 14:25:30 +0300
> Avi Kivity <avi@xxxxxxxxxx> wrote:
>
> > > > @@ -1689,7 +1690,7 @@ static void mmu_sync_children(struct kvm_vcpu *vcpu,
> > > >
> > > > kvm_mmu_pages_init(parent, &parents, &pages);
> > > > while (mmu_unsync_walk(parent, &pages)) {
> > > > - int protected = 0;
> > > > + bool protected = false;
> > > >
> > > > for_each_sp(pages, sp, parents, i)
> > > > protected |= rmap_write_protect(vcpu->kvm, sp->gfn);
> > >
> > > Isn't this the reason we prefer int to bool?
> > >
> > > Not sure people like to use |= with boolean.
> > >
> >
> > Why not?
> >
>
> The code "bitwise OR assignment" is assuming the internal representations
> of true and false: true=1, false=0.
No, it doesn't. |= converts the result back to bool.
In fact it's better than
int x;
...
x |= some_value() & MASK;
since MASK might be of type longer than int, and the result can be
truncated. With bool |=, it cannot.
Disassembly of section .text:
0000000000000000 <f>:
static bool x;
void f(long y)
{
x |= y;
0: 0f b6 05 00 00 00 00 movzbl 0x0(%rip),%eax # 7 <f+0x7>
3: R_X86_64_PC32 .bss-0x4
7: 48 09 c7 or %rax,%rdi
a: 0f 95 05 00 00 00 00 setne 0x0(%rip) # 11 <f+0x11>
d: R_X86_64_PC32 .bss-0x4
}
11: c3 retq
The corresponding code with 'int x' would just store the truncated
result back into x.
> I might have seen some point if it had been
> protected = protected || rmap_...
>
>
> But the real question is whether there is any point in re-writing completely
> correct C code: there are tons of int like this in the kernel code.
>
> __rmap_write_protect() was introduced recently, so if this conversion is
> really worthwhile, I should have been told to use bool at that time, no?
It's up to developer and maintainer preference. I like bool since it
documents the usage and is safer, but sometimes I miss it on review.
--
error compiling committee.c: too many arguments to function
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