1. Make sure you have an x32-capable kernel (CONFIG_X86_X32=y); Debian amd64 kernels are configured with the conditional x32 patch and CONFIG_X86_X32_DISABLED=y
   (https://salsa.debian.org/kernel-team/linux/-/blob/a2f701044777053055ef9c0f145b24731f60f4ea/debian/patches/features/x86/x86-make-x32-syscall-support-conditional.patch),
   and will work out-of-box with just "syscall.x32=y" in the cmdline.
2. Untar nextpnr.tgz to somewhere (I used /mnt/zest/nextpnr for the build and tests,
    so the libraries would end up in /mnt/zest/nextpnr/nextpnr/ARCH/nextpnr-ecp5; you might or might not need to replicate that).
3. If your environment doesn't have python3.8, you can use the bare-bones (12MB!) chroot from nextpnr-chroot.tgz by untaring it,
   then nextpnr.tgz in nextpnr-chroot/mnt/zest/nextpnr, then chrooting to nextpnr-chroot and cding to mnt/zest/nextpnr/nexpnr
   (note that the chroot results might not be absolutely optimal since I haven't managed to compile the bytecode but I'm 99% sure the python isn't in the hot path, so).
4. From the newly-created directory run
   ./ld-linux-ARCH.so.2 --library-path ARCH ./ARCH/nextpnr-ecp5 --quiet --placer heap --um-45k --package CABGA381 --speed 8 --json JSON.json
   wherein blinky.json  is the ecp5_evn     example: https://github.com/SymbiFlow/prjtrellis/tree/1e03e4e09ae1e7bec0aea3cdfa5658932470a2e2/examples/ecp5_evn
   and     attosoc.json is the soc_ecp5_evn example: https://github.com/SymbiFlow/prjtrellis/tree/1e03e4e09ae1e7bec0aea3cdfa5658932470a2e2/examples/soc_ecp5_evn


./ld-linux-x86-64.so.2 --library-path amd64 ./amd64/nextpnr-ecp5 --quiet --placer heap --um-45k --package CABGA381 --speed 8 --json attosoc.json
./ld-linux-x32.so.2    --library-path x32   ./x32/nextpnr-ecp5   --quiet --placer heap --um-45k --package CABGA381 --speed 8 --json attosoc.json


Numbers (probably calculated wrong, see screenshots/):
  ecp5_evn:     ≈19% on E6400, ≈25% on i5-3450
  soc_ecp5_evn: ≈22% on E6400, ≈29% on i5-3450, ≈28% on i7-8565U, 23% on i9-9980XE with scheduler, and 29% without

screenshots/ and respective machines:
  0N  (screendump,                  image): Core 2 Duo E6400-based DDR2-666  HP rp5700 PoS system (https://twitter.com/nabijaczleweli/status/1204465652628672512)
  fbN (framebuffer 3840x2160 BGRA8, image): i5-3450-based          DDR3-1600 desktop                  (both on 5.6.7-1    kernels)
  lbN (framebuffer 640x480 BGRA8,   image): i7-8565U-based         DDR4-2400 Toshiba Tecra X40 laptop (        4.19.118-2 kernel)
  tN  (image):                              i9-9980XE, 5 GHz, hyperthreads off, latency-tuned™ HFT server     (4.9-based  kernel)


Everything based on HEAD clones of yosys/prjtrellis/nextpnr/icestorm around dawn CEST 25 May 2020: https://twitter.com/nabijaczleweli/status/1264771251165368323,
libraries (and the dynamic linkers) from up-to-date sid at that time.