Make explicit -fno-semantic-interposition (in -fpic mode) infer dso_local

-fno-semantic-interposition is currently the CC1 default. (The opposite
disables some interprocedural optimizations.) However, it does not infer
dso_local: on most targets accesses to ExternalLinkage functions/variables
defined in the current module still need PLT/GOT.

This patch makes explicit -fno-semantic-interposition infer dso_local,
so that PLT/GOT can be eliminated if targets implement local aliases
for AsmPrinter::getSymbolPreferLocal (currently only x86).

Currently we check whether the module flag "SemanticInterposition" is 0.
If yes, infer dso_local. In the future, we can infer dso_local unless
"SemanticInterposition" is 1: frontends other than clang will also
benefit from the optimization if they don't bother setting the flag.
(There will be risks if they do want ELF interposition: they need to set
"SemanticInterposition" to 1.)

1 files changed, 7 insertions, 0 deletions

diff --git a/llvm/lib/IR/Module.cpp b/llvm/lib/IR/Module.cpp
index 9ac1edb..1416cdc 100644
--- a/llvm/lib/IR/Module.cpp
+++ b/llvm/lib/IR/Module.cpp
@@ -600,6 +600,13 @@ void Module::setSemanticInterposition(bool SI) {
   addModuleFlag(ModFlagBehavior::Error, "SemanticInterposition", SI);
 }
 
+bool Module::noSemanticInterposition() const {
+  // Conservatively require an explicit zero value for now.
+  Metadata *MF = getModuleFlag("SemanticInterposition");
+  auto *Val = cast_or_null<ConstantAsMetadata>(MF);
+  return Val && cast<ConstantInt>(Val->getValue())->getZExtValue() == 0;
+}
+
 void Module::setOwnedMemoryBuffer(std::unique_ptr<MemoryBuffer> MB) {
   OwnedMemoryBuffer = std::move(MB);
 }