Implement -fsemantic-interposition

First attempt at implementing -fsemantic-interposition.

Rely on GlobalValue::isInterposable that already captures most of the expected
behavior.

Rely on a ModuleFlag to state whether we should respect SemanticInterposition or
not. The default remains no.

So this should be a no-op if -fsemantic-interposition isn't used, and if it is,
isInterposable being already used in most optimisation, they should honor it
properly.

Note that it only impacts architecture compiled with -fPIC and no pie.

Differential Revision: https://reviews.llvm.org/D72829

1 files changed, 14 insertions, 0 deletions

diff --git a/llvm/lib/IR/Module.cpp b/llvm/lib/IR/Module.cpp
index 3b35176..c2083f5 100644
--- a/llvm/lib/IR/Module.cpp
+++ b/llvm/lib/IR/Module.cpp
@@ -555,6 +555,20 @@ Metadata *Module::getProfileSummary(bool IsCS) {
                : getModuleFlag("ProfileSummary"));
 }
 
+bool Module::getSemanticInterposition() const {
+  Metadata *MF = getModuleFlag("SemanticInterposition");
+
+  auto *Val = cast_or_null<ConstantAsMetadata>(MF);
+  if (!Val)
+    return false;
+
+  return cast<ConstantInt>(Val->getValue())->getZExtValue();
+}
+
+void Module::setSemanticInterposition(bool SI) {
+  addModuleFlag(ModFlagBehavior::Error, "SemanticInterposition", SI);
+}
+
 void Module::setOwnedMemoryBuffer(std::unique_ptr<MemoryBuffer> MB) {
   OwnedMemoryBuffer = std::move(MB);
 }