On Thursday, December 17, 2015 at 2:17:01 PM UTC-8, Ben Smith wrote: > > > > On Thursday, December 17, 2015 at 11:26:48 AM UTC-8, Benedikt Meurer wrote: >> >> Can't you use the same instruction sequence, even from C++ code? >> > > Yeah, I could, just not using the compiler intrinsics. I was hoping to > avoid duplicating the sequence in multiple locations, but it might be the > best way. > > >> Or compile a small stub that is callable from C++ directly and thereby >> use the same instruction sequence? >> > > Right, this way makes the most sense to me. >
OK, I've investigated this some more and have some more questions: The Atomics functions are polymorphic over the different TypedArray types (similar to ArrayBuffer keyed property access). So currently the functions check the TypedArray base type and forward to the current atomic instruction for that type. But TurboFan for asm.js should have enough type information to determine the correct TypedArray type without this check, so it makes sense that the codestubs should not have this check. So assuming we have codestubs like AtomicLoad8, AtomicLoad16, etc. how can I call these from FCG? It looks like the current way code stubs are called in FCG is via intrinsics (e.g. Math.pow or String.fromCharCode). But if I am hooking to the JavaScript functions then I'll have to match Atomics.load first, then do the TypedArray check in the FCG compiler so I can forward to the correct codestub. Looks like it should work, but I'm wondering if it is the right way to do things. It seems like the current path uses a type feedback in an IC to handle this, though that seems like a lot more work, and AFAICT won't be optimized without CS anyway, is that correct? And speaking of CrankShaft, doesn't implementing the type check in FCG require me to do the same check in CS? Or is there a way to share this code between the two? Thinking through it more, perhaps the best solution is to use inline assembly in the runtime functions. Inline assembly doesn't seem to be used much in v8 (mostly just the atomics implementations borrowed for Chromium), but it would really simplify the code here: I could use the same runtime functions I'm using now for FCG and CS, then have TF generate the same instruction sequence. The biggest drawback is that keeping the instruction sequences the same would have to be maintained manually. What do you think? > > >> Also the code stub should work if that's being used by CS and FCG, while >> you can inline the code into TF. You just need to make sure to generate the >> exactly the same instructions I guess? >> > > Yeah, I suppose inlining the TF can be a later optimization if desired. > > >> IIRC Jaro thought about this already, maybe he can give some advice. >> >> On Thu, Dec 17, 2015 at 8:06 PM, Ben Smith <bi...@chromium.org> wrote: >> >>> >>> >>> On Wednesday, December 16, 2015 at 7:16:07 PM UTC-8, Benedikt Meurer >>> wrote: >>>> >>>> You probably need the same guarantees for CS. So how about putting them >>>> into a code stub? And calling the stub for the %_ cases in all three >>>> compilers? >>>> >>> >>> Yes, that's what I was thinking too, but I was wondering if there was >>> another way -- AIUI the code stub has the overhead of a function call for >>> all compilers, right? >>> >>> >>>> BTW wouldn't it work if FCG always uses the C++ version? >>>> >>> >>> Maybe I'm misunderstanding when the various compilers are used. But if >>> it's possible that some code that was compiled with FCG is being used at >>> the same time that compiled with TF is used and they are accessing the same >>> shared memory, then it's necessary that they have the same instruction >>> sequence. >>> >>> >>>> -- Benedikt >>>> >>>> Ben Smith <bi...@chromium.org> schrieb am Do., 17. Dez. 2015, 00:01: >>>> >>>>> Hi v8-dev, >>>>> >>>>> I added the Atomics API earlier this year (in >>>>> src/js/harmony-atomics.js and src/runtime/runtime-atomics.cc). It's >>>>> currently implemented as runtime functions using compiler intrinsics. I'd >>>>> like to add a TurboFan implementation, but I need to make sure that the >>>>> generated assembly is the same in both the full-codegen and for TF. >>>>> What's >>>>> the best way to do this? >>>>> >>>>> FYI, here's the associated bug: >>>>> https://bugs.chromium.org/p/v8/issues/detail?id=4614 >>>>> >>>>> Thanks, >>>>> -Ben >>>>> >>>>> -- >>>>> -- >>>>> v8-dev mailing list >>>>> v8-...@googlegroups.com >>>>> http://groups.google.com/group/v8-dev >>>>> --- >>>>> You received this message because you are subscribed to the Google >>>>> Groups "v8-dev" group. >>>>> To unsubscribe from this group and stop receiving emails from it, send >>>>> an email to v8-dev+un...@googlegroups.com. >>>>> For more options, visit https://groups.google.com/d/optout. >>>>> >>>> -- >>> -- >>> v8-dev mailing list >>> v8-...@googlegroups.com >>> http://groups.google.com/group/v8-dev >>> --- >>> You received this message because you are subscribed to the Google >>> Groups "v8-dev" group. >>> To unsubscribe from this group and stop receiving emails from it, send >>> an email to v8-dev+un...@googlegroups.com. >>> For more options, visit https://groups.google.com/d/optout. >>> >> >> -- -- v8-dev mailing list v8-dev@googlegroups.com http://groups.google.com/group/v8-dev --- You received this message because you are subscribed to the Google Groups "v8-dev" group. To unsubscribe from this group and stop receiving emails from it, send an email to v8-dev+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
