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edit readme to document cross-compilation

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Umberto Valleriani 2023-08-24 09:03:26 +02:00
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README.md
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@ -272,6 +272,23 @@ This can be done by setting the `LTO_LINKING_FLAGS` to be something like
`"-g -Wl,-plugin-opt=save-temps"` which will be appended to the flags at link time.
This will at least preserve the bitcode files, even if `get-bc` will not be able to retrieve them for you.
## Cross-compilation notes
When cross-compiling a project (i.e. you pass the `--target=` or `-target` flag to the compiler),
you'll need to set the `GLLVM_OBJCOPY` variable to either
* `llvm-objcopy` to use LLVM's objcopy, which naturally supports all targets that clang does.
* `YOUR-TARGET-TRIPLE-objcopy` to use GNU's objcopy, since `objcopy` only supports the native architecture.
Example:
```sh
# test program
echo 'int main() { return 0; }' > a.c
clang --target=aarch64-linux-gnu a.c # works
gclang --target=aarch64-linux-gnu a.c # breaks
GLLVM_OBJCOPY=llvm-objcopy gclang --target=aarch64-linux-gnu a.c # works
GLLVM_OBJCOPY=aarch64-linux-gnu-objcopy gclang --target=aarch64-linux-gnu a.c # works if you have GNU's arm64 toolchain
```
## Developer tools
Debugging usually boils down to looking in the logs, maybe adding a print statement or two.