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reference runtime dispatch intro. Refs #2364 #2368

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reference runtime dispatch intro. Refs #2364 #2368

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23 changes: 14 additions & 9 deletions README.md
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Expand Up @@ -40,7 +40,8 @@ clouds or client devices, choosing the best available instruction set at
runtime. Alternatively, developers may choose to target a single instruction set
without any runtime overhead. In both cases, the application code is the same
except for swapping `HWY_STATIC_DISPATCH` with `HWY_DYNAMIC_DISPATCH` plus one
line of code.
line of code. See also @kfjahnke's
[introduction to dispatching](https://github.com/kfjahnke/zimt/blob/multi_isa/examples/multi_isa_example/multi_simd_isa.md).

**Suitable for a variety of domains**: Highway provides an extensive set of
operations, used for image processing (floating-point), compression, video
Expand Down Expand Up @@ -339,6 +340,10 @@ target-specific vector types.
You can prevent this by calling the following before any invocation of
`HWY_DYNAMIC_*`: `hwy::GetChosenTarget().Update(hwy::SupportedTargets());`.

See also a separate
[introduction to dynamic dispatch](https://github.com/kfjahnke/zimt/blob/multi_isa/examples/multi_isa_example/multi_simd_isa.md)
by @kfjahnke.

When using dynamic dispatch, `foreach_target.h` is included from translation
units (.cc files), not headers. Headers containing vector code shared between
several translation units require a special include guard, for example the
Expand Down Expand Up @@ -369,15 +374,15 @@ generally sufficient.
For MSVC, we recommend compiling with `/Gv` to allow non-inlined functions to
pass vector arguments in registers. If intending to use the AVX2 target together
with half-width vectors (e.g. for `PromoteTo`), it is also important to compile
with `/arch:AVX2`. This seems to be the only way to reliably generate VEX-encoded
SSE instructions on MSVC. Sometimes MSVC generates VEX-encoded SSE instructions,
if they are mixed with AVX, but not always, see
with `/arch:AVX2`. This seems to be the only way to reliably generate
VEX-encoded SSE instructions on MSVC. Sometimes MSVC generates VEX-encoded SSE
instructions, if they are mixed with AVX, but not always, see
[DevCom-10618264](https://developercommunity.visualstudio.com/t/10618264).
Otherwise, mixing VEX-encoded AVX2 instructions and non-VEX SSE may cause severe
performance degradation. Unfortunately, with `/arch:AVX2` option, the
resulting binary will then require AVX2. Note that no such flag is needed for
clang and GCC because they support target-specific attributes, which we use to
ensure proper VEX code generation for AVX2 targets.
Otherwise, mixing VEX-encoded AVX2 instructions and non-VEX SSE may cause severe
performance degradation. Unfortunately, with `/arch:AVX2` option, the resulting
binary will then require AVX2. Note that no such flag is needed for clang and
GCC because they support target-specific attributes, which we use to ensure
proper VEX code generation for AVX2 targets.

## Strip-mining loops

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