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Design philosophy

  • Performance is important but not the sole consideration. Anyone who goes to the trouble of using SIMD clearly cares about speed. However, portability, maintainability and readability also matter, otherwise we would write in assembly. We aim for performance within 10-20% of a hand-written assembly implementation on the development platform. There is no performance gap vs. intrinsics: Highway code can do anything they can. If necessary, you can use platform-specific instructions inside #if HWY_TARGET == HWY_NEON etc.

  • The guiding principles of C++ are "pay only for what you use" and "leave no room for a lower-level language below C++". We apply these by defining a SIMD API that ensures operation costs are visible, predictable and minimal.

  • Performance portability is important, i.e. the API should be efficient on all target platforms. Unfortunately, common idioms for one platform can be inefficient on others. For example: summing lanes horizontally versus shuffling. Documenting which operations are expensive does not prevent their use, as evidenced by widespread use of HADDPS. Performance acceptance tests may detect large regressions, but do not help choose the approach during initial development. Analysis tools can warn about some potential inefficiencies, but likely not all. We instead provide a carefully chosen set of vector types and operations that are efficient on all target platforms (Armv8, PPC8, x86).

  • Future SIMD hardware features are difficult to predict. For example, AVX2 came with surprising semantics (almost no interaction between 128-bit blocks) and AVX-512 added two kinds of predicates (writemask and zeromask). To ensure the API reflects hardware realities, we suggest a flexible approach that adds new operations as they become commonly available, with fallback implementations where necessary.

  • Masking/predication differs between platforms, and it is not clear how important the use cases are beyond the ternary operator IfThenElse. AVX-512/Arm SVE zeromasks are useful, but not supported by P0214R5. We provide IfThen[Zero]Else[Zero] variants.

  • "Width-agnostic" SIMD is more future-proof than user-specified fixed sizes. For example, valarray-like code can iterate over a 1D array with a library-specified vector width. This will result in better code when vector sizes increase, and matches the direction taken by Arm SVE and RiscV V as well as Agner Fog's ForwardCom instruction set proposal. However, some applications may require fixed sizes, so we also guarantee support for <= 128-bit vectors in each instruction set.

  • The API and its implementation should be usable and efficient with commonly used compilers, including MSVC. For example, we write ShiftLeft<3>(v) instead of v << 3 because MSVC 2017 (aarch64) does not propagate the literal (https://godbolt.org/g/rKx5Ga). Highway requires function-specific target attributes, supported by GCC 4.9 / Clang 3.9 / MSVC 2015.

  • Efficient and safe runtime dispatch is important. Modules such as image or video codecs are typically embedded into larger applications such as browsers, so they cannot require separate binaries for each CPU. Libraries also cannot predict whether the application already uses AVX2 (and pays the frequency throttling cost), so this decision must be left to the application. Using only the lowest-common denominator instructions sacrifices too much performance. Therefore, we provide code paths for multiple instruction sets and choose the most suitable at runtime. To reduce overhead, dispatch should be hoisted to higher layers instead of checking inside every low-level function. Highway supports inlining functions in the same file or in *-inl.h headers. We generate all code paths from the same source to reduce implementation- and debugging cost.

  • Not every CPU need be supported. To reduce code size and compile time, we group x86 targets into clusters. In particular, SSE3 instructions are only used/available if S-SSE3 is also available, and AVX only if AVX2 is also supported. Code generation for AVX3_DL also requires opting-in by defining HWY_WANT_AVX3_DL.

  • Access to platform-specific intrinsics is necessary for acceptance in performance-critical projects. We provide conversions to and from intrinsics to allow utilizing specialized platform-specific functionality, and simplify incremental porting of existing code.

  • The core API should be compact and easy to learn; we provide a concise reference.

Prior API designs

The author has been writing SIMD code since 2002: first via assembly language, then intrinsics, later Intel's F32vec4 wrapper, followed by three generations of custom vector classes. The first used macros to generate the classes, which reduces duplication but also readability. The second used templates instead. The third (used in highwayhash and PIK) added support for AVX2 and runtime dispatch. The current design (used in JPEG XL) enables code generation for multiple platforms and/or instruction sets from the same source, and improves runtime dispatch.

Overloaded function API

Most C++ vector APIs rely on class templates. However, the Arm SVE vector type is sizeless and cannot be wrapped in a class. We instead rely on overloaded functions. Overloading based on vector types is also undesirable because SVE vectors cannot be default-constructed. We instead use a dedicated tag type Simd for overloading, abbreviated to D for template arguments and d in lvalues.

Note that generic function templates are possible (see generic_ops-inl.h).

Masks

AVX-512 introduced a major change to the SIMD interface: special mask registers (one bit per lane) that serve as predicates. It would be expensive to force AVX-512 implementations to conform to the prior model of full vectors with lanes set to all one or all zero bits. We instead provide a Mask type that emulates a subset of this functionality on other platforms at zero cost.

Masks are returned by comparisons and TestBit; they serve as the input to IfThen*. We provide conversions between masks and vector lanes. On targets without dedicated mask registers, we use FF..FF as the definition of true. To also benefit from x86 instructions that only require the sign bit of floating-point inputs to be set, we provide a special ZeroIfNegative function.

Differences vs. P0214R5 / std::experimental::simd

  1. Allowing the use of built-in vector types by relying on non-member functions. By contrast, P0214R5 requires a wrapper class, which does not work for sizeless vector types currently used by Arm SVE and Risc-V.

  2. Supporting many more operations such as 128-bit compare/minmax, AES/CLMUL, AndNot, AverageRound, bit-shift by immediates, compress/expand, fixed-point mul, IfThenElse, interleaved load/store, lzcnt, mask find/set, masked load/store, popcount, reductions, saturated add/sub, scatter/gather.

  3. Designing the API to avoid or minimize overhead on AVX2/AVX-512 caused by crossing 128-bit 'block' boundaries.

  4. Avoiding the need for non-native vectors. By contrast, P0214R5's simd_cast returns fixed_size<> vectors which are more expensive to access because they reside on the stack. We can avoid this plus additional overhead on Arm/AVX2 by defining width-expanding operations as functions of a vector part, e.g. promoting half a vector of uint8_t lanes to one full vector of uint16_t, or demoting full vectors to half vectors with half-width lanes.

  5. Guaranteeing access to the underlying intrinsic vector type. This ensures all platform-specific capabilities can be used. P0214R5 instead only 'encourages' implementations to provide access.

  6. Enabling safe runtime dispatch and inlining in the same binary. P0214R5 is based on the Vc library, which does not provide assistance for linking multiple instruction sets into the same binary. The Vc documentation suggests compiling separate executables for each instruction set or using GCC's ifunc (indirect functions). The latter is compiler-specific and risks crashes due to ODR violations when compiling the same function with different compiler flags. We solve this problem via target-specific namespaces and attributes (see HOWTO section below). We also permit a mix of static target selection and runtime dispatch for hotspots that may benefit from newer instruction sets if available.

  7. Omitting inefficient or non-performance-portable operations such as hmax, operator[], and unsupported integer comparisons. Applications can often replace these operations at lower cost than emulating that exact behavior.

  8. Omitting long double types: these are not commonly available in hardware.

  9. Ensuring signed integer overflow has well-defined semantics (wraparound).

  10. Avoiding hidden performance costs. P0214R5 allows implicit conversions from integer to float, which costs 3-4 cycles on x86. We make these conversions explicit to ensure their cost is visible.

Other related work

  • Neat SIMD adopts a similar approach with interchangeable vector/scalar types and a compact interface. It allows access to the underlying intrinsics, but does not appear to be designed for other platforms than x86.

  • UME::SIMD (code, paper) also adopts an explicit vectorization model with vector classes. However, it exposes the union of all platform capabilities, which makes the API harder to learn (209-page spec) and implement (the estimated LOC count is 500K). The API is less performance-portable because it allows applications to use operations that are inefficient on other platforms.

  • Inastemp (code, paper) is a vector library for scientific computing with some innovative features: automatic FLOPS counting, and "if/else branches" using lambda functions. It supports IBM Power8, but only provides float and double types and does not support SVE without assuming the runtime vector size.