gcc: add common/checksum.nix

This commit adds `gcc/common/checksum.nix`, which contains code
common to both gcc11 and gcc12, implementing the `enableChecksum`
feature.

When gcc's built-in bootstrap (`--enable-bootstrap`) is used, gcc
compiles itself three times and compares a hash of the unlinked `.o`
files from the second and third compilation.  The
`enableChecksum=true` parameter performs the same comparison as part
of the `postInstall` phase.

Notably, `enableChecksum=true` can be used with `enableBootstrap=false`.

Co-authored-by: Sandro <sandro.jaeckel@gmail.com>

pkgs/development/compilers/gcc/11/default.nix

@@ -28,6 +28,8 @@
 , buildPackages
 , libxcrypt
 , disableGdbPlugin ? !enablePlugin
+, nukeReferences
+, callPackage
 }:
 
 # Make sure we get GNU sed.
@@ -312,4 +314,5 @@ lib.pipe (stdenv.mkDerivation ({
 ))
 [
   (callPackage ../common/libgcc.nix   { inherit langC langCC langJit; })
+  (callPackage ../common/checksum.nix { inherit langC langCC; })
 ]

pkgs/development/compilers/gcc/12/default.nix

@@ -29,6 +29,8 @@
 , buildPackages
 , libxcrypt
 , disableGdbPlugin ? !enablePlugin
+, nukeReferences
+, callPackage
 }:
 
 # Make sure we get GNU sed.
@@ -177,6 +179,7 @@ let majorVersion = "12";
         mpfr
         name
         noSysDirs
+        nukeReferences
         patchelf
         perl
         profiledCompiler
@@ -347,5 +350,6 @@ lib.pipe (stdenv.mkDerivation ({
 ))
 [
   (callPackage ../common/libgcc.nix   { inherit langC langCC langJit; })
+  (callPackage ../common/checksum.nix { inherit langC langCC; })
 ]
 

pkgs/development/compilers/gcc/common/checksum.nix

@@ -0,0 +1,39 @@
+{ lib
+, stdenv
+, nukeReferences
+, langC
+, langCC
+}:
+
+let
+  enableChecksum = (with stdenv; buildPlatform == hostPlatform && hostPlatform == targetPlatform) && langC && langCC;
+in
+(pkg: pkg.overrideAttrs (previousAttrs: lib.optionalAttrs enableChecksum {
+  outputs = previousAttrs.outputs ++ lib.optionals enableChecksum [ "checksum" ];
+  # This is a separate phase because gcc assembles its phase scripts
+  # in bash instead of nix (we should fix that).
+  preFixupPhases = (previousAttrs.preFixupPhases or []) ++ [ "postInstallSaveChecksumPhase" ];
+  #
+  # gcc uses an auxiliary utility `genchecksum` to md5-hash (most of) its
+  # `.o` and `.a` files prior to linking (in case the linker is
+  # nondeterministic).  Since we want to compare across gccs built from two
+  # separate derivations, we wrap `genchecksum` with a `nuke-references`
+  # call.  We also stash copies of the inputs to `genchecksum` in
+  # `$checksum/inputs/` -- this is extremely helpful for debugging since
+  # it's hard to get Nix to not delete the $NIX_BUILD_TOP of a successful
+  # build.
+  #
+  postInstallSaveChecksumPhase = ''
+    mv gcc/build/genchecksum gcc/build/.genchecksum-wrapped
+    cat > gcc/build/genchecksum <<\EOF
+    #!${runtimeShell}
+    ${nukeReferences}/bin/nuke-refs $@
+    for INPUT in "$@"; do install -Dt $INPUT $checksum/inputs/; done
+    exec build/.genchecksum-wrapped $@
+    EOF
+    chmod +x gcc/build/genchecksum
+    rm gcc/*-checksum.*
+    make -C gcc cc1-checksum.o cc1plus-checksum.o
+    install -Dt $checksum/checksums/ gcc/cc*-checksum.o
+  '';
+}))

pkgs/stdenv/linux/default.nix

@@ -331,8 +331,6 @@ in
         gmp      = prev.gmp.override { cxx = false; };
         gcc-unwrapped =
           (prev.gcc-unwrapped.override (commonGccOverrides // {
-            enablePlugins = false;
-
             # The most logical name for this package would be something like
             # "gcc-stage1".  Unfortunately "stage" is already reserved for the
             # layers of stdenv, so using "stage" in the name of this package