[llvm][IR] Add per-global code model attribute (#72077)

This adds a per-global code model attribute, which can override the
target's code model to access global variables.

Suggested-by: Arthur Eubanks <aeubanks@google.com>
Link: https://discourse.llvm.org/t/how-to-best-implement-code-model-overriding-for-certain-values/71816
Link: https://discourse.llvm.org/t/rfc-add-per-global-code-model-attribute/74944

2 files changed, 32 insertions, 0 deletions

diff --git a/llvm/lib/IR/AsmWriter.cpp b/llvm/lib/IR/AsmWriter.cpp
index fabc79a..c67c32a 100644
--- a/llvm/lib/IR/AsmWriter.cpp
+++ b/llvm/lib/IR/AsmWriter.cpp
@@ -3672,6 +3672,27 @@ void AssemblyWriter::printGlobal(const GlobalVariable *GV) {
     printEscapedString(GV->getPartition(), Out);
     Out << '"';
   }
+  if (auto CM = GV->getCodeModel()) {
+    Out << ", code_model \"";
+    switch (*CM) {
+    case CodeModel::Tiny:
+      Out << "tiny";
+      break;
+    case CodeModel::Small:
+      Out << "small";
+      break;
+    case CodeModel::Kernel:
+      Out << "kernel";
+      break;
+    case CodeModel::Medium:
+      Out << "medium";
+      break;
+    case CodeModel::Large:
+      Out << "large";
+      break;
+    }
+    Out << '"';
+  }
 
   using SanitizerMetadata = llvm::GlobalValue::SanitizerMetadata;
   if (GV->hasSanitizerMetadata()) {
diff --git a/llvm/lib/IR/Globals.cpp b/llvm/lib/IR/Globals.cpp
index 7bd4503..51bdbeb 100644
--- a/llvm/lib/IR/Globals.cpp
+++ b/llvm/lib/IR/Globals.cpp
@@ -482,6 +482,8 @@ void GlobalVariable::copyAttributesFrom(const GlobalVariable *Src) {
   GlobalObject::copyAttributesFrom(Src);
   setExternallyInitialized(Src->isExternallyInitialized());
   setAttributes(Src->getAttributes());
+  if (auto CM = Src->getCodeModel())
+    setCodeModel(*CM);
 }
 
 void GlobalVariable::dropAllReferences() {
@@ -489,6 +491,15 @@ void GlobalVariable::dropAllReferences() {
   clearMetadata();
 }
 
+void GlobalVariable::setCodeModel(CodeModel::Model CM) {
+  unsigned CodeModelData = static_cast<unsigned>(CM) + 1;
+  unsigned OldData = getGlobalValueSubClassData();
+  unsigned NewData = (OldData & ~(CodeModelMask << CodeModelShift)) |
+                     (CodeModelData << CodeModelShift);
+  setGlobalValueSubClassData(NewData);
+  assert(getCodeModel() == CM && "Code model representation error!");
+}
+
 //===----------------------------------------------------------------------===//
 // GlobalAlias Implementation
 //===----------------------------------------------------------------------===//