Re: [PATCH] locking/rwsem: Remove arch specific rwsem files

Peter Zijlstra
2019-02-11T17:05:26Z

On Mon, Feb 11, 2019 at 11:35:24AM -0500, Waiman Long wrote:
> On 02/11/2019 06:58 AM, Peter Zijlstra wrote:
> > Which is clearly worse. Now we can write that as:
> >
> > int __down_read_trylock2(unsigned long *l)
> > {
> > long tmp = READ_ONCE(*l);
> >
> > while (tmp >= 0) {
> > if (try_cmpxchg(l, &tmp, tmp + 1))
> > return 1;
> > }
> >
> > return 0;
> > }
> >
> > which generates:
> >
> > 0000000000000030 <__down_read_trylock2>:
> > 30: 48 8b 07 mov (%rdi),%rax
> > 33: 48 85 c0 test %rax,%rax
> > 36: 78 18 js 50 <__down_read_trylock2+0x20>
> > 38: 48 8d 50 01 lea 0x1(%rax),%rdx
> > 3c: f0 48 0f b1 17 lock cmpxchg %rdx,(%rdi)
> > 41: 75 f0 jne 33 <__down_read_trylock2+0x3>
> > 43: b8 01 00 00 00 mov $0x1,%eax
> > 48: c3 retq
> > 49: 0f 1f 80 00 00 00 00 nopl 0x0(%rax)
> > 50: 31 c0 xor %eax,%eax
> > 52: c3 retq
> >
> > Which is a lot better; but not quite there yet.
> >
> >
> > I've tried quite a bit, but I can't seem to get GCC to generate the:
> >
> > add $1,%rdx
> > jle
> >
> > required; stuff like:
> >
> > new = old + 1;
> > if (new <= 0)
> >
> > generates:
> >
> > lea 0x1(%rax),%rdx
> > test %rdx, %rdx
> > jle
>
> Thanks for the suggested code snippet. So you want to replace "lea
> 0x1(%rax), %rdx" by "add $1,%rdx"?
>
> I think the compiler is doing that so as to use the address generation
> unit for addition instead of using the ALU. That will leave the ALU
> available for doing other arithmetic operation in parallel. I don't
> think it is a good idea to override the compiler and force it to use
> ALU. So I am not going to try doing that. It is only 1 or 2 more of
> codes anyway.

Yeah, I was trying to see what I could make it do.. #2 really should be
good enough, but you know how it is once you're poking at it :-)