bitcode-overlay

Bitcode overlay for Nixpkgs

This Nixpkgs overlay can automatically build many Linux packages with LLVM bitcode included. Use it like this:

# Build the "lz4" program with embedded bitcode.
cd bcdb/nix/bitcode-overlay
nix-build -A pkgsBitcode.lz4

# Find the executables or shared libraries you want.
ls result*/bin/lz4

# The embedded bitcode is stored in the ".llvmbc" section.
objdump -h result*/bin/lz4 | grep llvmbc

# Use bc-imitate to extract and combine the bitcode,
# and add some metadata about dynamic linking.
bc-imitate extract result*/bin/lz4 > lz4.bc

Limitations

Many packages don't work out-of-the-box with Clang, or with its -fembed-bitcode option. This overlay provides fixes for a few packages, but many others will fail to build.

Building bitcode without the overlay

If you want to avoid Nix, you can also build bitcode manually by using the -fembed-bitcode option for Clang. For example:

git clone https://github.com/lz4/lz4
cd lz4

# The exact options you need will vary from package to package.
make lz4 CC=clang CFLAGS=-fembed-bitcode

# The embedded bitcode is stored in the ".llvmbc" section.
objdump -h lz4 | grep llvmbc

# Extract the compiled bitcode.
bc-imitate extract lz4 > lz4.bc

How the overlay works

See design.md for an explanation.