[PATCH] D116995: [gold] Ignore bitcode from sections inside object files

[Fangrui Song via Phabricator via llvm-commits]

MaskRay accepted this revision.
MaskRay added a comment.
This revision is now accepted and ready to land.

In D116995#3481661 <https://reviews.llvm.org/D116995#3481661>, @jyknight wrote:

> -fembed-bitcode was originally added by Apple to enable them to recompile
> 3rd-party-developers' software with a newer compiler, or for new CPU
> targets, without the source code. It uses the normal non-LTO optimization
> pipeline, and emits bitcode corresponding to the binary object also
> emitted.
>
> For LTO bitcode generation, we use a different pass manager configuration.
>
> That said, various other people seem to have had various other ideas about
> how things should work over time. So, I couldn't say conclusively that
> nobody will complain if we have gold not try to parse these sections...but
> it seems potentially reasonable to me.

I agree. I think for Mach-O the section is for LTO use. For ELF there are two usage:

- lld/LLVMgold.so: the section should be ignored. This patch makes LLVMgold.so behavior match lld, therefore LGTM.
- llgo (https://reviews.llvm.org/D4371): it places the bitcode for LTO into a special section `.llvmbc`.



================
Comment at: llvm/docs/ReleaseNotes.rst:170
 
+* The LLVM gold plugin will now ignore bitcode from the .llvmbc section of ELF
+  files when doing LTO.  https://github.com/llvm/llvm-project/issues/47216
----------------



================
Comment at: llvm/tools/gold/gold-plugin.cpp:540
+  // Only use bitcode files for LTO.  InputFile::create() will load bitcode
+  // from special sections within a binary object, this bitcode is typically
+  // generated by -fembed-bitcode and is not meant for LTO use.
----------------
special sections => .llvmbc

Be specific.


================
Comment at: llvm/tools/gold/gold-plugin.cpp:541
+  // from special sections within a binary object, this bitcode is typically
+  // generated by -fembed-bitcode and is not meant for LTO use.
+  if (identify_magic(BufferRef.getBuffer()) != file_magic::bitcode) {
----------------
"not meant for LTO use " when using LLVMgold.so.

For llgo(?) it was for LTO use: https://reviews.llvm.org/D4371


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