Background:
LLVM has some tools that run at build time. In native builds, these are
built as a part of the usual build, but in cross builds they need to
come from buildPackages.
In many scenarios this is a small problem because LLVM from
buildPackages is already available as a build; but if cross building a
version of LLVM which is not available (e.g. a new git commit of LLVM)
this results in two builds of LLVM and clang, one native and one for the
cross.
Full builds of LLVM are expensive; and unnecessary in this scenario. We
don't need a native LLVM, only a native copy of the tools which run at
build time. This is only tablegen and related tooling, which are cheap
to build.
Implementation-wise, we introduce a derivation llvmPackages.tblgen,
which specifies the tablegen targets which need to be built and has a
custom installPhase to copy them to the output.
A previous attempt in https://github.com/NixOS/nixpkgs/pull/359967
dropped the use of LLVM_TABLEGEN_EXE and friends on the grounds that
llvm can already cross build these things, but that is false since it's
necessary in that case to wire in a cross compiler. This PR avoids that
problem by allowing access to buildPackages.tblgen.
Signed-off-by: Peter Waller <p@pwaller.net>
Using ninja makes it consistent with the other llvm package builds, and
libclang is not necessary as a dependency.
Signed-off-by: Peter Waller <p@pwaller.net>
Also exposes owner, repo, and other attributes available on the original
monorepoSrc. These attributes were previously hidden behind a
runCommand that consumes monorepoSrc and hides this information away.
```
$ nix eval --raw .#llvmPackages_18.llvm.src.rev
llvmorg-18.1.8
$ nix eval --raw .#llvmPackages_git.llvm.src.rev
0e8555d4dbfdfeddc01dc2ecf9a9b6e804f7b645
```
Signed-off-by: Peter Waller <p@pwaller.net>
This change implements a leftover task from #307211, namely passing
monorepoSrc to the different llvmPackages_13 package expressions. Before
this change, all packages llvmPackages_13 would be built from a
subdirectory of the full LLVM monorepo tree. After this change only the
relevant directories are made available at build time. This
- reduces the size of the source that needs to be made available to the
builder.
- prevents LLVM from sidestepping our instructions and including extra
sources from other directories it shouldn't.
Since LLVM 12 and 13 don't have the `cmake` directory at the top level,
the runCommand expressions filtering the source need to be adjusted, but
this causes no rebuild for any other LLVM version (ofborg should confirm
this).
The only problem encountered was in lld:
- We need to make the patch to the inclusion of libunwind headers
unconditional now. lld needs this on non-darwin as well. In the
full monorepo, LLVM_MAIN_SRC_DIR would be set correctly, so the
patch wasn't necessary.
- The substitute mechanism for LLVM 12 and 13 can't be unified yet since
LLVM 12 still uses a non monorepo build, so we come up with a
different LLVM_MAIN_SRC_DIR.
Change was tested by building the following expression on x86_64-linux.
with import ./. {};
builtins.removeAttrs llvmPackages_13 [ "lldb" "lldbPlugins" ]'
lld was also tested on aarch64-darwin.
cmake flags have a 'last flag wins' logic, so by appending to the end of
the flags it is possible to override any cmake flag.
It also ignores (and warns) if a flag is unused, so passing flags across
all packages should be safe if you want to target one package.
In combination with #320261, this PR allows consistently overriding all
packages within LLVM with additional cmake arguments. Consistency here
means for example 'if you override LLVM, then all dependencies on it are
also see the overridden LLVM in their input'. Consistency is hard to
achieve with the other obvious way of implementing this as a user: if
you use overrideAttrs then you have to write a big mess of override code
in order to override all dependents, and this can be very difficult in a
cross-compilation scenario using crossSystem and useLLVM, for example.
With this PR it is possible to write an overlay which overlays
`llvmPackages` with `llvmPackage.override { devExtraCmakeFlags = [ ... ]; }`,
and then the toolchain used with useLLVM in effect should respect
these flags.
This is useful in development for experimenting with the effect of
various flags, hence the chosen name `devCmakeFlags`.
This won't work out of the box without #341855 applied, which fixes
override passthrough.
See-Also: #320261, #341855
Signed-off-by: Peter Waller <p@pwaller.net>
