Re: [webkit-dev] SIMD support in JavaScript
Hi Nadav, - Original Message - Hi Dan! On Sep 28, 2014, at 6:44 AM, Dan Gohman sunf...@mozilla.com wrote: Hi Nadav, I agree with much of your assessment of the the proposed SIMD.js API. However, I don't believe it's unsuitability for some problems invalidates it for solving other very important problems, which it is well suited for. Performance portability is actually one of SIMD.js' biggest strengths: it's not the kind of performance portability that aims for a consistent percentage of peak on every machine (which, as you note, of course an explicit 128-bit SIMD API won't achieve), it's the kind of performance portability that achieves predictable performance and minimizes surprises across machines (though yes, there are some unavoidable ones, but overall the picture is quite good). There is a tradeoff between the performance portability of the SIMD.js ISA and its usefulness. A small number of instructions (that only targets 32bit data types, no masks, etc) is not useful for developing non-trivial vector programs. You need 16bit vector elements to support WebGL vertex indices, and lane-masking for implementing predicated control flow for programs like ray tracers. Introducing a large number of vector instructions will expose the performance portability problems. I don’t believe that there is a sweet spot in this tradeoff. I don’t think that we can find a small set of instructions that will be useful for writing non-trivial vector code that is performance portable. My belief in the existence of a sweet spot is based on looking at other systems, hardware and software, that have already gone there. For an interesting example, take a look at this page: https://software.intel.com/en-us/articles/interactive-ray-tracing Every SIMD operation used in that article is directly supported by a corresponding function in SIMD.js today. We do have an open question on whether we should do something different for the rsqrt instruction, since the hardware only provides an approximation. In this case the code requires some Newton-Raphson, which may give us some flexibility, but several things are possible there. And of course, sweet spot doesn't mean cure-all. Also, I am preparing to propose that SIMD.js handle 16-bit vector elements too (int16x8). It fits pretty naturally into the overall model. There are some challenges on some architectures, but there are challenges with alternative approaches too, and overall the story looks good. Other changes are also being discussed too. In general, the SIMD.js spec is still evolving; participation is welcome :-). This is an example of a weakness of depending on automatic vectorization alone. High-level language features create complications which can lead to surprising performance problems. Compiler transformations to target specialized hardware features often have widely varying applicability. Expensive analyses can sometimes enable more and better vectorization, but when a compiler has to do an expensive complex analysis in order to optimize, it's unlikely that a programmer can count on other compilers doing the exact same analysis and optimizing in all the same cases. This is a problem we already face in many areas of compilers, but it's more pronounced with vectorization than many other optimizations. I agree with this argument. Compiler optimizations are unpredictable. You never know when the register allocator will decide to spill a variable inside a hot loop. or a memory operation confuse the alias analysis. I also agree that loop vectorization is especially sensitive. However, it looks like the kind of vectorization that is needed to replace SIMD.js is a very simple SLP vectorization http://llvm.org/docs/Vectorizers.html#the-slp-vectorizer (BB vectorization). It is really easy for a compiler to combine a few scalar arithmetic operations into a vector. LLVM’s SLP-vectorizer support vectorization of computations across basic blocks and succeeds in surprising places, like vectorization of STDLIB code where the ‘begin' and ‘end' iterators fit into a 128-bit register! That's a surprising trick! I agree that SLP vectorization doesn't have the same level of performance cliff as loop vectorization. And, it may be a desirable thing for JS JITs to start doing. Even so, there is still value in an explicit SIMD API in the present. For the core features, instead of giving developers sets of expression patterns to follow to ensure SLP recognition, we are giving names to those patterns and letting developers identify which patterns they wish to use by their names. We can coordinate, compare, and standardize them by name across browsers, and in the future we may make a variety of interesting extensions to the API which developers will be able to feature-test for. And if, in the future, SLP vectorization proves itself reliable enough in JS, then we can drop our custom JIT
Re: [webkit-dev] SIMD support in JavaScript
Hi Maciej, - Original Message - Dan, you say that SIMD.js delivers performance portability, and Nadav says it doesn’t. Nadav’s argument seems to come down to (as I understand it): - The set of vector operations supported on different CPU architectures varies widely. This is true, but it's also true that there is a core set of features which is pretty consistent across popular SIMD architectures. This commonality exists because it's a very popular set. The proposed SIMD.js doesn't solve all problems, but it does solve a large number of important problems well, and it is following numerous precedents. We are also exploring the possibility of exposing additional instructions outside this core set. Several creative ideas are being discussed which could expand the API's reach while preserving a portability story. However, regardless of what we do there, I expect the core set will remain a prominent part of the API, due to its applicability. - Executing vector intrinsics on processors that don’t support them is slower than executing multiple scalar instructions because the compiler can’t always generate efficient with the same semantics.” This is also true, however the intent of SIMD.js *is* to be implementable on all popular architectures. The SIMD.js spec is originally derived from the Dart SIMD spec, which is already implemented and in use on at least x86 and ARM. We are also taking some ideas from OpenCL, which offers a very similar set of core functionality, and which is implemented on even more architectures. We have several reasons to expect that SIMD.js can cover enough functionality to be useful while still being sufficiently portable. - Even when vector intrinsics are supported by the CPU, whether it is profitable to use them may depend in non-obvious ways on exact characteristics of the target CPU and the surrounding code (the Port5 example). With SIMD.js, there are plain integer types, so developers directly bypass plain JS number semantics, so there are fewer corner cases for the compiler to insert extra code to check for. This means fewer branches, and among other things, should mean less port 5 contention overall on Sandy Bridge. Furthermore, automatic vectorization often requires the compiler make conservative assumptions about key information like pointer aliasing, trip counts, integer overflow, array indexing, load safety, scatter ordering, alignment, and more. In order to preserve observable semantics, these assumptions cause compilers to insert extra instructions, which are typically things like selects, shuffles, branches or other things, to handle all the possible corner cases. This is extra overhead that human programmers can often avoid, because they can more easily determine what corner cases are relevant in a given piece of code. And on Sandy Bridge in particular, these extra selects, shuffles, and branches hit port 5. For these reasons, Nadav says that it’s better to autovectorize, and that this is the norm even for languages with explicit vector data. In other words, he’s saying that SIMD.js will result in code that is not performance-portable between different CPUs. I question whether it is actually the norm. In C++, where auto-vectorization is available in every major compiler today, explicit SIMD APIs like xmmintrin.h are hugely popular. That particular header has become supported by Microsoft's C++ compiler, Intel's C++ compiler, GCC, and clang. I see many uses of xmmintrin.h in many contexts, including HPC, graphics, codecs, cryptography, and games. It seems many C++ developers are still willing to go through the pain of #ifdefs, preprocessor macros, and funny-looking syntax rather than rely on auto-vectorization, even with restrict and and other aids. Both auto-vectorization and SIMD.js have their strengths, and both have their weaknesses. I don't believe the fact that both solve some problems that the other doesn't rules out either of them. I don’t see a rebuttal to any of these points. Instead, you argue that, because SIMD.js does not require advanced compiler analysis, it is more likely to give similar results between different JITs (presumably when targeting the same CPU, or ones with the same supported vector operations and similar perf characteristics). That seems like a totally different sense of performance portability. Given these arguments, it’s possible that you and Nadav are both right[*]. That would mean that both these statements hold: (a) SIMD.js is not performance-portable between different CPU architectures and models. (b) SIMD.js is performance-portable between different JITs targeting the same CPU model. On net, I think that combination would be a strong argument *against* SIMD.js. The Web aims for portability between different hardware and not just different software. At Apple alone we support four major CPU instruction sets and a considerably greater number of
[webkit-dev] Adding support for gradient midpoint
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Re: [webkit-dev] Adding support for gradient midpoint
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[webkit-dev] New EWS status bubbles in Bugzilla
Hi, WebKit Bugzilla has new EWS status bubbles now, which will hopefully make it more clear what's going on with a patch. Mysterious yellow bubbles that could mean anything were eliminated, and most importantly, there is now detailed information presented on hover: Please try it out, and let me know if something breaks, or is not as good as it could be! - Alexey ___ webkit-dev mailing list webkit-dev@lists.webkit.org https://lists.webkit.org/mailman/listinfo/webkit-dev
