stdenv: Nix-driven bootstrap of gcc
#### Summary
By default, when you type `make`, GCC will compile itself three
times. This PR inhibits that behavior by configuring GCC with
`--disable-bootstrap`, and reimplements the triple-rebuild using
Nix rather than `make`/`sh`.
#### Immediate Benefits
- Allow `gcc11` and `gcc12` on `aarch64` (without needing new
`bootstrapFiles`)
- Faster stdenv rebuilds: the third compilation of gcc
(i.e. stageCompare) is no longer a `drvInput` of the final stdenv.
This allows Nix to build stageCompare in parallel with the rest of
nixpkgs instead of in series.
- No more copying `libgcc_s` out of the bootstrap-files or other
derivations
- No more Frankenstein compiler: the final gcc and the libraries it
links against (mpfr, mpc, isl, glibc) are all built by the same
compiler (xgcc) instead of a mixture of the bootstrapFiles'
compiler and xgcc.
- No more [static lib{mpfr,mpc,gmp,isl}.a hack]
- Many other small `stdenv` hacks eliminated
- `gcc` and `clang` share the same codepath for more of `cc-wrapper`.
#### Future Benefits
- This should allow using a [foreign] `bootstrap-files` so long as
`hostPlatform.canExecute bootstrapFiles`.
- This should allow each of the libraries that ship with `gcc`
(lib{backtrace, atomic, cc1, decnumber, ffi, gomp, iberty,
offloadatomic, quadmath, sanitizer, ssp, stdc++-v3, vtv}) to be
built in separate (one-liner) derivations which `inherit src;`
from `gcc`, much like https://github.com/NixOS/nixpkgs/pull/132343
#### Incorporates
- https://github.com/NixOS/nixpkgs/pull/210004
- https://github.com/NixOS/nixpkgs/pull/36948 (unreverted)
- https://github.com/NixOS/nixpkgs/pull/210325
- https://github.com/NixOS/nixpkgs/pull/210118
- https://github.com/NixOS/nixpkgs/pull/210132
- https://github.com/NixOS/nixpkgs/pull/210109
- https://github.com/NixOS/nixpkgs/pull/213909
- https://github.com/NixOS/nixpkgs/pull/216136
- https://github.com/NixOS/nixpkgs/pull/216237
- https://github.com/NixOS/nixpkgs/pull/210019
- https://github.com/NixOS/nixpkgs/pull/216232
- https://github.com/NixOS/nixpkgs/pull/216016
- https://github.com/NixOS/nixpkgs/pull/217977
- https://github.com/NixOS/nixpkgs/pull/217995
#### Closes
- Closes #108305
- Closes #108111
- Closes #201254
- Closes #208412
#### Credits
This project was made possible by three important insights, none of
which were mine:
1. @ericson2314 was the first to advocate for this change, and
probably the first to appreciate its advantages. Nix-driven
(external) bootstrap is "cross by default".
2. @trofi has figured out a lot about how to get gcc to not mix up
the copy of `libstdc++` that it depends on with the copy that it
builds, by moving the `bootstrapFiles`' `libstdc++` into a
[versioned directory]. This allows a Nix-driven bootstrap of gcc
without the final gcc would still having references to the
`bootstrapFiles`.
3. Using the undocumented variable [`user-defined-trusted-dirs`]
when building glibc. When glibc `dlopen()`s `libgcc_s.so`, it
uses a completely different and totally special set of rules for
finding `libgcc_s.so`. This trick is the only way we can put
`libgcc_s.so` in its own separate outpath without creating
circular dependencies or dependencies on the bootstrapFiles. I
would never have guessed to use this (or that it existed!) if it
were not for a [comment in guix] which @Mic92 [mentioned].
My own role in this PR was basically: being available to go on a
coding binge at an opportune moment, so we wouldn't waste a
[crisis].
[aarch64-compare-ofborg]: https://github.com/NixOS/nixpkgs/pull/209870/checks?check_run_id=10662822938
[amd64-compare-ofborg]: https://github.com/NixOS/nixpkgs/pull/209870/checks?check_run_id=10662825857
[nonexistent sysroot]: https://github.com/NixOS/nixpkgs/pull/210004
[versioned directory]: https://github.com/NixOS/nixpkgs/pull/209054
[`user-defined-trusted-dirs`]: https://sourceware.org/legacy-ml/libc-help/2013-11/msg00026.html
[comment in guix]: 5e4ec82181/gnu/packages/gcc.scm (L253)
[mentioned]: https://github.com/NixOS/nixpkgs/pull/210112#issuecomment-1379608483
[crisis]: https://github.com/NixOS/nixpkgs/issues/108305
[foreign]: https://github.com/NixOS/nixpkgs/pull/170857#issuecomment-1170558348
[static lib{mpfr,mpc,gmp,isl}.a hack]: 2f1948af9c/pkgs/stdenv/linux/default.nix (L380)
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