Toolchain [FreeBSD WIKI]

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**Session Leader:** jhb

Notes: https://hackmd.io/BjHsH7MzTLuAcUF_P7JJxA#

====== Toolchain and Build ======

  * basic build requirements
    * compiler, etc. to bootstrap a new release
    * Guarantee: latest X release can build stable/X+1
    * Willing to guarantee: X-1 and X-2 userland will work on X-CURRENT kernel
  * cross-building from a non-%%FreeBSD%% host
    * patch to build without depending on tools in $PATH
    * Can we enumerate all build tools?
    * Once it works, add CI
    * Avoid host $PATH
    * Wrapper scripts for non-%%FreeBSD%% hosted builds
    * arichardson patch is ~ +1500 -69
      * https://github.com/arichardson/cheribsd/commits/crossbuild-aug2018
  * META_MODE
    * How do we finally turn it on? -- src.conf(5)
    * Can you use this without filemon.ko (cross-build)?
      * Would have to check in the dependencies, but wouldn't update them when making changes
    * LLVM sanitizers
      * more architectures (aarch64 and mips64 ASAN in review)
      * amd64 ASAN LLVM tests don't all pass
      * Need a matrix sowing what works
        * asan + threads issue?
      * kernel ubsan? (relatively straightforward)
      * Couple test suite with CI with sanitizers
      * which ones do we want?
        * ASAN
        * UBSAN
        * TSAN
        * profile?
        * xray?
          * msan?
      * Build infrastructure (make goo?) e.g. WITH_ASAN
      * Separate ASAN-built libraries as an option?
  * SSP in GENERIC-DEBUG? WITH_SSP=yes?
  * LLD
    * Current state
      * bootstrap + system linker on amd64, arm64
      * bootstrap linker on i386, armv7
    * Near future (12.0 if possible, else 13)
      * system linker on i386, armv7
      * armv7 flip on and watch for ports fallout
      * MIPS64? (requires -z notext)
  * Userland LTO
    * requires llvm-ar, possibly llvm-nm
    * requires rtld in 11+
    * option for base system? (WITH_LTO)
    * integration/deployment questions
      * -flto produces .o that contain llvm IR (not ELF)
      * llvm-ar emits llvm IR archives
      * new extension for LTO lib vs new libpath vs fat .a
  * Kernel LTO
    * barfed on Dtrace (ctfconvert is unhappy)
      * %%OpenBSD%% approach might address this
    * debugging and observability suffers
      * Need to be careful with optimizing out "unnecessary" memory zeroing etc.
      * Link some parts with -flto and not others?
      * -flto=thin vs -flto
  * C11 Atomics vs %%<%%machine/atomic.h>
    * marginally better asm
    * ordering with I/O (bus_space_barrier and bus_dmamap_sync)
    * issue with using C11 memory_order_consume to implement RCU in the Linux kernel (papers at [[http://www.rdrop.com/users/paulmck/RCU/]])
  * Enabling compiler support for mitigation features
    * per-arch considerations
    * CFI -- needs someone to actually try and test
    * safe stack -- ^^^
    * PIE binaries (WITH_PIE)
  * Compressed debug sections / .gdb_index
    * Enable on supported toolchains -- needs someone to do and test, probably shouldn't have WITH/WITHOUT, just enable by default
  * Switch llvm back to dwarf4 by default
    * emaste should do this tonight during the hacking lounge
  * clang support for more architectures?
    * RISC-V (can build test application with clang 8, not link)
    * MIPS? (do we care about mips32)
      * mips32 TLS segfaults
      * TLS model is always dynamic
    * powerpc
  * Using newer features (like ifuncs / optimized ifuncs)
    * Ok to start ditching GCC 4.2.x / ld.bfd 2.17.50
    * Requiring vs optionally using features in only one modern toolchain
    * Should we support both toolchains on platforms that support both? Generally yes.
    * Defer question about desupporting modern alternate toolchain.
  * Starting to use C++11 in userland on architectures?
  * Using external toolchain more often (e.g. in regular amd64 development)
    * Manual override to force existing system toolchain
    * Actual testing / documentation of llvm-xtoolchain port
  * LLVM vs elftoolchain
    * ar, nm - for LTO and large .a
      * bug fix needed for elftoolchain, possibly new functionality to be added
    * size, objcopy, readelf, strings - leave as elftc
  * External GCC (base/*)
  * Killing src/contrib/gcc
  * Killing src/contrib/binutils
  * Working with compiler-provided headers (std*.h, etc.)