Function Multi-Versioning for GNU Guix

This repository contains an experiment to implement automatic function multi-versioning (FMV) for GNU Guix packages, inspired by Clear Linux’ tutorial.

FMV is a technique whereby the compiler generates code for different instruction set architecture (ISA) extensions; at run time, the loader automatically chooses the implementation that performs best on the host CPU. Read Pre-built binaries vs. performance for background info.

Overview

To give it a spin, try running:

GUIX_FMV_PACKAGE=gsl guix build -f function-multi-versioning.scm

For the chosen package (GSL by default), it will perform the following steps:

1. Build the package with GCC’s -fopt-info-vec option and grab the source code location of all vectorization opportunities.
2. Compute a patch that adds =target_clones= attributes on the relevant functions.
3. Rebuild the package with that patch.

The resulting binary contains several versions of the functions:

$ objdump -t $(GUIX_FMV_PACKAGE=gsl guix build -f function-multi-versioning.scm)/lib/libgsl.so | grep avx | head
000000000009a480 l     F .text	00000000000003f5              bspline_pppack_bsplvd.avx2.0
000000000009a880 l     F .text	0000000000000400              bspline_pppack_bsplvd.arch_skylake_avx512.1
000000000009b080 l     F .text	0000000000000113              gsl_bspline_knots_uniform.avx2.0
000000000009b1a0 l     F .text	00000000000000d8              gsl_bspline_knots_uniform.arch_skylake_avx512.1
000000000009b3a0 l     F .text	000000000000012b              gsl_bspline_knots.avx2.0
000000000009b4d0 l     F .text	000000000000012b              gsl_bspline_knots.arch_skylake_avx512.1
000000000009be70 l     F .text	0000000000000125              gsl_bspline_eval.avx2.0
000000000009bfa0 l     F .text	0000000000000125              gsl_bspline_eval.arch_skylake_avx512.1
000000000009f120 l     F .text	0000000000000071              avl_t_copy.avx2.0
000000000009f1a0 l     F .text	0000000000000071              avl_t_copy.arch_skylake_avx512.1

Binary Size

The binary is larger. With FMV for avx2 and skylake-avx512, it is almost twice as big as the original binary:

$ size /gnu/store/xq64iaxx2gmlcgnipj31wjxlf1yd2g2p-gsl-2.6/lib/libgsl.so
   text	   data	    bss	    dec	    hex	filename
2899152	  84112	    864	2984128	 2d88c0	/gnu/store/xq64iaxx2gmlcgnipj31wjxlf1yd2g2p-gsl-2.6/lib/libgsl.so
$ size /gnu/store/s14ldmmlmig1pgwc49ci7lsp651zllgb-gsl-2.6/lib/libgsl.so
   text	   data	    bss	    dec	    hex	filename
5600712	  85256	    912	5686880	 56c660	/gnu/store/s14ldmmlmig1pgwc49ci7lsp651zllgb-gsl-2.6/lib/libgsl.so

Performance

WIP

Integration

If robust enough, this transformation could be made available as a package transformation option.