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build-support/cc-wrapper: pass in non-existent --sysroot= to untangle from libc

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I would like to add an extra `gcc` build step during linux bootstrap
(https://github.com/NixOS/nixpkgs/issues/208412). This makes it early
bootstrap compiler linked and targeted against `bootstrapTools` `glibc`
including it's headers.

Without this change `gcc`'s spec files always prefer `bootstrapTools` `glibc`
for header search path (passed in as --with-native-system-header-dir=). We'can't
override it with:

- `-I` option as it gets stacked before gcc-specific headers, we need to keep
  glibc headers after gcc as gcc cleans namespace up for C standard by using
  #include_next and by undefining system macros.
- `-idirafter` option as it gets appended after existing `glibc`-includes

This `--sysroot=/nix/store/does/not/exist` hack allows us to remove existing
`glibc` headers and add new ones with `-idirafter`.

We use `cc-cflags-before` instead of `libc-cflags` to allow user to define
their own `--sysroot=` (like `firefox` does).

To keep it working prerequisite cross-symlink in gcc.libs is required:
https://github.com/NixOS/nixpkgs/pull/209153
This commit is contained in:
Sergei Trofimovich 2023-01-10 07:43:51 +00:00
parent 3f305fab72
commit 8c80bd08b7
1 changed files with 7 additions and 0 deletions
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7
pkgs/build-support/cc-wrapper/default.nix
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@ -331,11 +331,18 @@ stdenv.mkDerivation {
# compile, because it uses "#include_next <limits.h>" to find the # compile, because it uses "#include_next <limits.h>" to find the
# limits.h file in ../includes-fixed. To remedy the problem, # limits.h file in ../includes-fixed. To remedy the problem,
# another -idirafter is necessary to add that directory again. # another -idirafter is necessary to add that directory again.
#
# We use --sysroot=/nix/store/does/not/exist to drop embedded default
# path to glibc headers gcc was built against. Without it -idirafter
# only appends to the list and outdated glibc headers end up being
# used. 'cc-cflags-before' is used to allow user's --sysroot= option
# to override our default.
+ optionalString (libc != null) ('' + optionalString (libc != null) (''
touch "$out/nix-support/libc-cflags" touch "$out/nix-support/libc-cflags"
touch "$out/nix-support/libc-ldflags" touch "$out/nix-support/libc-ldflags"
echo "-B${libc_lib}${libc.libdir or "/lib/"}" >> $out/nix-support/libc-crt1-cflags echo "-B${libc_lib}${libc.libdir or "/lib/"}" >> $out/nix-support/libc-crt1-cflags
'' + optionalString (!(cc.langD or false)) '' '' + optionalString (!(cc.langD or false)) ''
echo "--sysroot=/nix/store/does/not/exist" >> $out/nix-support/cc-cflags-before
echo "-idirafter ${libc_dev}${libc.incdir or "/include"}" >> $out/nix-support/libc-cflags echo "-idirafter ${libc_dev}${libc.incdir or "/include"}" >> $out/nix-support/libc-cflags
'' + optionalString (isGNU && (!(cc.langD or false))) '' '' + optionalString (isGNU && (!(cc.langD or false))) ''
for dir in "${cc}"/lib/gcc/*/*/include-fixed; do for dir in "${cc}"/lib/gcc/*/*/include-fixed; do