[all-commits] [llvm/llvm-project] dce786: clang-tblgen build: avoid duplicate inclusion of l...

[Nicolai Hähnle via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: dce78646f07ff67f7d4887525e908dbf47d9e86b
      https://github.com/llvm/llvm-project/commit/dce78646f07ff67f7d4887525e908dbf47d9e86b
  Author: Nicolai Hähnle <nicolai.haehnle at amd.com>
  Date:   2022-10-28 (Fri, 28 Oct 2022)

  Changed paths:
    M clang/lib/Support/CMakeLists.txt
    M clang/utils/TableGen/CMakeLists.txt

  Log Message:
  -----------
  clang-tblgen build: avoid duplicate inclusion of libLLVMSupport

TableGen executables are supposed to never be linked against libLLVM-*.so,
even when LLVM_LINK_LLVM_DYLIB=ON, presumably for cross-compilation.

It turns out that clang-tblgen *did* link against libLLVM-*.so,
indirectly so via the clangSupport.

This lead to a regression in what should have been unrelated work of
cleaning up ManagedStatics in LLVMSupport. A running clang-tblgen
process ended up with two copies of a cl::opt static global:

- one from libLLVMSupport linked statically into clang-tblgen as a
  direct dependency
- one from libLLVMSupport linked into libLLVM-*.so, which clang-tblgen
  linked against due to the clangSupport dependency

For a bit more context, see the discussion at
https://discourse.llvm.org/t/flang-aarch64-dylib-buildbot-need-help-understanding-a-regression-in-clang-tblgen/64871/

None of the potential solutions I could find are perfect. Presumably one
possible solution would be to remove "Support" from the explicit
dependencies of clang-tblgen. However, relying on the transitive
inclusion via clangSupport seems risky, and in any case this wouldn't
address the issue of clang-tblgen surprisingly linking against libLLVM-*.so.

This change instead creates a second version of the clangSupport library
that is explicitly linked statically, to be used by clang-tblgen.

v2:
- define an alias so that clang-tblgen can always link against
  "clangSupport_tablegen"
- use add_llvm_library(... BUILDTREE_ONLY ...)

v3:
- use the object library version of clangSupport if available

Differential Revision: https://reviews.llvm.org/D134637