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SIMDe Dispatch Design

Source: vignettes/simde-dispatch.Rmd
simde-dispatch.Rmd

Rminibwa follows the RsimdDispatch/SIMDe downstream-package pattern for the SIMD-sensitive minibwa KSW kernels:

  • configure probes compiler support;
  • backend objects are staged under src/rmb-ksw/;
  • src/Makevars.in links those staged objects through PKG_LIBS;
  • ordinary minibwa sources are compiled once with baseline package flags;
  • runtime selection switches between compiled and CPU-supported backends.

This avoids putting ISA flags such as -mavx2 into global PKG_CPPFLAGS or PKG_CFLAGS.

Boundary

Dispatch starts at the KSW functions called by minibwa:

  • ksw_extz2_sse();
  • ksw_extd2_sse();
  • ksw_ll_qinit() and the ksw_ll_* helpers.

The rest of minibwa stays baseline-compiled once:

  • index loading;
  • BWT and seeding;
  • chaining;
  • pairing;
  • formatting;
  • R external-pointer glue.

Backends

Rminibwa currently exposes:

  • scalar: portable SIMDe fallback compiled with native intrinsics disabled and no ISA flags;
  • sse4: native SSE4.1/SIMDe-alias build of the KSW files;
  • avx2: native AVX2 build, including the widened dual-gap extd2 kernel.

The avx2 backend uses the wide ksw2_extd2_wide.c patch for the dominant dual-gap gap-filling path and uses AVX2-compiled KSW objects for the remaining KSW entry points.

R surface 

library(Rminibwa)
simd_info()
#> $dispatch_mode
#> [1] "rminibwa-ksw-staged"
#> 
#> $requested_backend
#> [1] "auto"
#> 
#> $selected_backend
#> [1] "avx2"
#> 
#> $compiled_backends
#> [1] "scalar" "sse4"   "avx2"  
#> 
#> $cpu_supported_backends
#> [1] "scalar" "sse4"   "avx2"  
#> 
#> $available_backends
#> [1] "scalar" "sse4"   "avx2"  
#> 
#> $target
#> [1] "x86_64"

Select a backend explicitly for diagnostics or benchmarks:

old <- simd_backend()
simd_set_backend("scalar")
simd_backend()
#> [1] "scalar"
simd_set_backend("auto")

simd_set_backend() errors if the requested backend was not compiled or is not supported by the current CPU/runtime.

Validation

For every available backend, Rminibwa runs the same native mapping fixture in tinytest and checks that the native alignment batch and ALTREP column accessors remain valid. The README make rdm path additionally benchmarks:

  1. internal Rminibwa-only scalar vs sse4 vs avx2; and
  2. Rminibwa AVX2 vs locally built Python/Rust bindings compiled with native AVX2 codegen.

Backend timing must use a workload that actually reaches KSW. Very short exact matches can be resolved by seeding/chaining and ungapped fast paths without a single ksw_ext* or ksw_ll* call, so apparent scalar/sse4/avx2 differences on those examples are just end-to-end mapper noise. The developer README benchmark uses an indel-mutated random read and records internal KSW call counters before timing count-only mapping.