[all-commits] [llvm/llvm-project] b2f7b5: [CodeGen] Support bitcode input containing multipl...

[Fangrui Song via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: b2f7b5dbaefe4f2e3f8f279735ea3509a796693f
      https://github.com/llvm/llvm-project/commit/b2f7b5dbaefe4f2e3f8f279735ea3509a796693f
  Author: Fangrui Song <i at maskray.me>
  Date:   2023-07-21 (Fri, 21 Jul 2023)

  Changed paths:
    M clang/lib/CodeGen/CodeGenAction.cpp
    A clang/test/CodeGen/split-lto-unit-input.cpp

  Log Message:
  -----------
  [CodeGen] Support bitcode input containing multiple modules

When using -fsplit-lto-unit (explicitly specified or due to using
-fsanitize=cfi/-fwhole-program-vtables), the emitted LLVM IR contains a module
flag metadata `"EnableSplitLTOUnit"`. If a module contains both type metadata
and `"EnableSplitLTOUnit"`, `ThinLTOBitcodeWriter.cpp` will write two modules
into the bitcode file. Compiling the bitcode (not ThinLTO backend compilation)
will lead to an error due to `parseIR` requiring a single module.

```
% clang -flto=thin a.cc -c -o a.bc
% clang -c a.bc
% clang -fsplit-lto-unit -flto=thin a.cc -c -o a.bc
% clang -c a.bc
error: Expected a single module
1 error generated.
```

There are multiple ways to have just one module in a bitcode file
output: `-Xclang -fno-lto-unit`, not using features like `-fsanitize=cfi`,
using `-fsanitize=cfi` with `-fno-split-lto-unit`. I think whether a
bitcode input file contains 2 modules (internal implementation strategy)
should not be a criterion to require an additional driver option when
the user seek for a non-LTO compile action.

Let's place the extra module (if present) into CodeGenOptions::LinkBitcodeFiles
(originally for -cc1 -mlink-bitcode-file). Linker::linkModules will link the two
modules together. This patch makes the following commands work:

```
clang -S -emit-llvm a.bc
clang -S a.bc
clang -c a.bc
```

Reviewed By: ormris

Differential Revision: https://reviews.llvm.org/D154923