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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
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Adam Joseph
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Jul 1, 2023
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