On Fri, Sep 15, 2023 at 07:01:14PM -0700, Linus Torvalds wrote: > On Fri, 15 Sept 2023 at 17:38, Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > > > > On Fri, Sep 15, 2023 at 05:27:17PM -0700, Linus Torvalds wrote: > > > On Fri, 15 Sept 2023 at 11:37, Matthew Wilcox (Oracle) > > > <willy@xxxxxxxxxxxxx> wrote: > > > > > > > > + "1: ldl_l %0,%4\n" > > > > + " xor %0,%3,%0\n" > > > > + " xor %0,%3,%2\n" > > > > + " stl_c %0,%1\n" > > > > > > What an odd thing to do. > > > > > > Why don't you just save the old value? That double xor looks all kinds > > > of strange, and is a data dependency for no good reason that I can > > > see. > > > > > > Why isn't this "ldl_l + mov %0,%2 + xor + stl_c" instead? > > > > > > Not that I think alpha matters, but since I was looking through the > > > series, this just made me go "Whaa?" > > > > Well, this is my first time writing Alpha assembler ;-) I stole this > > from ATOMIC_OP_RETURN: > > > > "1: ldl_l %0,%1\n" \ > > " " #asm_op " %0,%3,%2\n" \ > > " " #asm_op " %0,%3,%0\n" \ > > Note how that does "orig" assignment first (ie the '%2" destination is > the first instruction), unlike your version. Wow. I totally missed that I'd transposed those two lines. I read it back with the lines in the order that they should have been in. Every time I read it. I was wondering why you were talking about a data dependency, and I just couldn't see it. With the lines in the order that they're actually in, it's quite obvious and totally not what I meant. Of course, it doesn't matter which order they're in from the point of view of testing the waiters bit since we don't change the waiters bit. > Does any of this matter? Nope. It's alpha. There's probably a handful > of machines, and it's maybe one extra cycle. It's really the oddity > that threw me. I'll admit to spending far more time on the m68k version of this than the alpha version ;-)
