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]: https://github.com/guix-mirror/guix/blob/5e4ec8218142eee8e6e148e787381a5ef891c5b1/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]: https://github.com/NixOS/nixpkgs/blob/2f1948af9c984ebb82dfd618e67dc949755823e2/pkgs/stdenv/linux/default.nix#L380

1 files changed, 4 insertions, 4 deletions

diff --git a/pkgs/development/compilers/gcc/11/default.nix b/pkgs/development/compilers/gcc/11/default.nix
index 6ad7ce27f25..b003a1c30fd 100644
--- a/pkgs/development/compilers/gcc/11/default.nix
+++ b/pkgs/development/compilers/gcc/11/default.nix
@@ -49,7 +49,7 @@ with builtins;
 
 let majorVersion = "11";
     version = "${majorVersion}.3.0";
-    disableBootstrap = !(with stdenv; targetPlatform == hostPlatform && hostPlatform == buildPlatform);
+    disableBootstrap = true;
 
     inherit (stdenv) buildPlatform hostPlatform targetPlatform;
 
@@ -250,9 +250,8 @@ lib.pipe (stdenv.mkDerivation ({
   targetConfig = if targetPlatform != hostPlatform then targetPlatform.config else null;
 
   buildFlags =
-    let target =
-          lib.optionalString (profiledCompiler) "profiled" +
-          lib.optionalString (targetPlatform == hostPlatform && hostPlatform == buildPlatform && !disableBootstrap) "bootstrap";
+    let target = lib.optionalString (profiledCompiler) "profiled"
+      + lib.optionalString (targetPlatform == hostPlatform && hostPlatform == buildPlatform && !disableBootstrap) "bootstrap";
     in lib.optional (target != "") target;
 
   inherit (callFile ../common/strip-attributes.nix { })
@@ -312,4 +311,5 @@ lib.pipe (stdenv.mkDerivation ({
 // optionalAttrs (enableMultilib) { dontMoveLib64 = true; }
 ))
 [
+  (callPackage ../common/libgcc.nix   { inherit langC langCC langJit; })
 ]