Skip to content
New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Jump to bottom

gccCrossStageStatic: enable dynamic libraries, rename it #238154

Merged
6 commits merged into from Jul 12, 2023
Merged

gccCrossStageStatic: enable dynamic libraries, rename it #238154

6 commits merged into from Jul 12, 2023

Commits on Jul 1, 2023

  1. lib.systems: introduce hasSharedLibraries 

    This commit adds `hasSharedLibraries` to `lib.systems`.

We need `plat.hasSharedLibraries` in order to know whether or not to
expect `gcc` (and many other tools) to emit shared libraries (like
`libgcc_s.so`).  Many of the GNU build scripts are smart enough that
if you configure them with `--enable-shared` on a platform (such as
`arm-none-eabi`) that doesn't support dynamic linking, they will
simply skip the shared libraries instead of aborting the
`configurePhase`.  Unfortunately the missing shared libraries in the
final build product cause very hard-to-troubleshoot problems later
on.

The alternative to introducing `hasSharedLibraries` would be to set
`isStatic` in these situations.  However doing so causes
`make-derivation.nix` to insert `-static` between the `pname` and
`hostPlatform` suffix, which is undesirable.

If at some point in the future we eliminate the `-static` suffix,
then `hasSharedLibraries` can be made equal to `!isStatic`.
    Adam Joseph committed Jul 1, 2023
    Configuration menu
    Copy the full SHA
    6980e6b View commit details
    Browse the repository at this point in the history

  2. gcc: use hasSharedLibraries instead of isStatic

    Adam Joseph committed Jul 1, 2023
    Configuration menu
    Copy the full SHA
    e41f217 View commit details
    Browse the repository at this point in the history

  3. gccCrossStageStatic: enable dynamic libraries, rename to gccWithoutTa… 

    …rgetLibc

This commit allows `gccCrossStageStatic` to build dynamically-linked
libraries.  Since is no longer restricted to building static
libraries its name is no longer appropriate, and this commit also
renames it to the more-accurate `gccWithoutTargetLibc`.

By default, you can't build a gcc that knows how to create dynamic
libraries unless you have already built the targetPlatform libc.

Because of this, our gcc cross-compiler is built in two stages:

  1. Build a cross-compiler (gccCrossStageStatic) that can build
     only static libraries.

  2. Use gccCrossStageStatic to compile the targetPlatform libc.

  3. Use the targetPlatform libc to build a fully-capable cross
     compiler.

You might notice that this pattern looks very similar to what we do
with `xgcc` in the stdenv bootstrap.  Indeed it is!  I would like to
work towards getting the existing stdenv bootstrap to handle cross
compilers as well.  However we don't want to cripple `stdenv.xgcc`
by taking away its ability to build dynamic libraries.

It turns out that the only thing gcc needs the targetPlatform libc
for is to emit a DT_NEEDED for `-lc` into `libgcc.so`.  That's it!
And since we don't use `gccCrossStageStatic` to build anything other
than libc, it's safe to omit the `DT_NEEDED` because that `libgcc`
will never be loaded by anything other than `libc`.  So `libc` will
already be in the process's address space.

Other people have noticed this; crosstool-ng has been using this
approach for a very long time:

  https://github.com/crosstool-ng/crosstool-ng/blob/36ad0b17a732aaffe4701d5d8d410d6e3e3abba9/scripts/build/cc/gcc.sh#L638-L640
    Adam Joseph committed Jul 1, 2023
    Configuration menu
    Copy the full SHA
    2affd45 View commit details
    Browse the repository at this point in the history

  4. gcc: s_crossStageStatic_withoutTargetLibc_ 

    This commit renames the `crossStageStatic` argument to the `gcc`
expression to `withoutTargetLibc`.  See previous commit for details.
    Adam Joseph committed Jul 1, 2023
    Configuration menu
    Copy the full SHA
    443dfc4 View commit details
    Browse the repository at this point in the history

  5. gcc: withoutTargetLibc: build libgcc_s.so 

    We want a `libgcc_s.so` to be built by the first stage
cross-compiler (withoutTargetLibc), since that is the compiler which
will compile the target libc.

This commit accomplishes that, by making three changes:

1. Replacing the `targetPlatform.libc == "msvcrt" &&` conditional
   with `enableShared`, so that the code which cross-build
   `libgcc_s.so` is used for all cross compilers capable of emitting
   shared libraries.

2. Removing the `targetPlatform == hostPlatform` guard from the code
   which produces the `libgcc` output.

3. Looking for build products in in "lib/${targetPlatform.config}/"
   rather than "lib/", so we will find them when cross compiling.
    Adam Joseph committed Jul 1, 2023
    Configuration menu
    Copy the full SHA
    63305d0 View commit details
    Browse the repository at this point in the history

  6. gcc: kludge to prevent mass-rebuild 

    This commit is reverted in #240596 (which must go to staging).
    Adam Joseph committed Jul 1, 2023
    Configuration menu
    Copy the full SHA
    96a2f1b View commit details
    Browse the repository at this point in the history