[ThinLTO] Reduce the number of renaming due to promotions (#183793)

Currently for thin-lto, the imported static global values (functions,
variables, etc) will be promoted/renamed from e.g., foo() to
foo.llvm.(). Such a renaming caused difficulties in live patching
since function name is changed ([1]).

It is possible that some global value names have to be promoted to avoid
name collision and linker failure. But in practice, majority of name
promotions can be avoided.

In [2], the suggestion is that thin-lto pre-link decides whether
a particular global value needs name promotion or not. If yes, later on
in thinBackend() the name will be promoted.

I compiled a particular linux kernel version (latest bpf-next tree)
and found 1216 global values with suffix .llvm.. With this patch,
the number of promoted functions is 2, 98% reduction from the
original kernel build.

If some native objects are not participating with LTO, name promotions
have to be done to avoid potential linker issues. So the current
implementation cannot be on by default. But in certain cases, e.g., linux kernel
build, people can enable lld flag --lto-whole-program-visibility to reduce the
number of functions like foo.llvm.().

For ThinLTOCodeGenerator.cpp which is used by llvm-lto tool and a
few other rare cases, reducing the number of renaming due to promotion,
is not implemented as lld flag '-lto-whole-program-visibility' is not
supported in ThinLTOCodeGenerator.cpp for now. In summary, this pull
request only supports llvm-lto2 style workflow.

The feature is off by default. To enable the future, lld flag
'-lto-whole-program-visibility'  and llvm flag
'-always-rename-promoted-locals=false' are needed.

The link [3] has more context for the pull request discussions.

[1] https://lpc.events/event/19/contributions/2212
[2] https://discourse.llvm.org/t/rfc-avoid-functions-like-foo-llvm-for-kernel-live-patch/89400
[3] https://github.com/llvm/llvm-project/pull/178587

1 files changed, 2 insertions, 0 deletions

diff --git a/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp b/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp
index cc0491b..f031611 100644
--- a/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp
+++ b/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp
@@ -1337,6 +1337,8 @@ static uint64_t getEncodedGVSummaryFlags(GlobalValueSummary::GVFlags Flags,
   unsigned ImportType = Flags.ImportType | ImportAsDecl;
   RawFlags |= (ImportType << 10); // 1 bit
 
+  RawFlags |= (Flags.NoRenameOnPromotion << 11); // 1 bit
+
   return RawFlags;
 }