On Sun, Jun 06, 2021 at 03:26:16PM -0500, Segher Boessenkool wrote: > On Sun, Jun 06, 2021 at 01:11:53PM -0700, Linus Torvalds wrote: > > On Sun, Jun 6, 2021 at 12:56 PM Segher Boessenkool > > <segher@xxxxxxxxxxxxxxxxxxx> wrote: > > > > > > Yes, I know. But it is literally the *only* way to *always* get a > > > conditional branch: by writing one. > > > > The thing is, I don't actually believe you. > > Fortune favours the bold! > > > The barrier() thing can work - all we need to do is to simply make it > > impossible for gcc to validly create anything but a conditional > > branch. > > And the only foolproof way of doing that is by writing a branch. > > > If either side of the thing have an asm that cannot be combined, gcc > > simply doesn't have any choice in the matter. There's no other valid > > model than a conditional branch around it (of some sort - doing an > > indirect branch that has a data dependency isn't wrong either, it just > > wouldn't be something that a sane compiler would generate because it's > > obviously much slower and more complicated). > > Or push something to the stack and return. Or rewrite the whole thing > as an FSM. Or or or. > > (And yes, there are existing compilers that can do both of these things > on some code). > > > We are very used to just making the compiler generate the code we > > need. That is, fundamentally, what any use of inline asm is all about. > > We want the compiler to generate all the common cases and all the > > regular instructions. > > > > The conditional branch itself - and the instructions leading up to it > > - are exactly those "common regular instructions" that we'd want the > > compiler to generate. That is in fact more true here than for most > > inline asm, exactly because there are so many different possible > > combinations of conditional branches (equal, not equal, less than,..) > > and so many ways to generate the code that generates the condition. > > > > So we are much better off letting the compiler do all that for us - > > it's very much what the compiler is good at. > > Yes, exactly. > > I am saying that if you depend on that some C code you write will result > in some particular machine code, without actually *forcing* the compiler > to output that exact machine code, then you will be disappointed. Maybe > not today, and maybe it will take years, if you are lucky. > > (s/forcing/instructing/ of course, compilers have feelings too!) OK, I will bite... What would you suggest as a way of instructing the compiler to emit the conditional branch that we are looking for? Thanx, Paul
