[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

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

@@ -1144,6 +1144,23 @@ static bool getDecodedDSOLocal(unsigned Val) {
   }
 }
 
+static std::optional<CodeModel::Model> getDecodedCodeModel(unsigned Val) {
+  switch (Val) {
+  case 1:
+    return CodeModel::Tiny;
+  case 2:
+    return CodeModel::Small;
+  case 3:
+    return CodeModel::Kernel;
+  case 4:
+    return CodeModel::Medium;
+  case 5:
+    return CodeModel::Large;
+  }
+
+  return {};
+}
+
 static GlobalVariable::ThreadLocalMode getDecodedThreadLocalMode(unsigned Val) {
   switch (Val) {
     case 0: return GlobalVariable::NotThreadLocal;
@@ -3805,6 +3822,7 @@ Error BitcodeReader::parseGlobalVarRecord(ArrayRef<uint64_t> Record) {
   // dllstorageclass, comdat, attributes, preemption specifier,
   // partition strtab offset, partition strtab size] (name in VST)
   // v2: [strtab_offset, strtab_size, v1]
+  // v3: [v2, code_model]
   StringRef Name;
   std::tie(Name, Record) = readNameFromStrtab(Record);
 
@@ -3913,6 +3931,13 @@ Error BitcodeReader::parseGlobalVarRecord(ArrayRef<uint64_t> Record) {
     NewGV->setSanitizerMetadata(Meta);
   }
 
+  if (Record.size() > 17 && Record[17]) {
+    if (auto CM = getDecodedCodeModel(Record[17]))
+      NewGV->setCodeModel(*CM);
+    else
+      return error("Invalid global variable code model");
+  }
+
   return Error::success();
 }