[llvm] [NFCI][Globals]For GlobalObjects, add updateSectionPrefix and change setSectionPrefix to handle empty strings (PR #158460)

[Snehasish Kumar via llvm-commits]

================
@@ -0,0 +1,76 @@
+//===- GlobalObjectTest.cpp - Global object unit tests --------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/IR/GlobalObject.h"
+#include "llvm-c/Core.h"
+#include "llvm/AsmParser/Parser.h"
+#include "llvm/IR/Module.h"
+#include "llvm/Support/SourceMgr.h"
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+using namespace llvm;
+namespace {
+using testing::Eq;
+using testing::Optional;
+using testing::StrEq;
+
+static std::unique_ptr<Module> parseIR(LLVMContext &C, const char *IR) {
+  SMDiagnostic Err;
+  std::unique_ptr<Module> Mod = parseAssemblyString(IR, Err, C);
+  if (!Mod)
+    Err.print("GlobalObjectTests", errs());
+  return Mod;
+}
+
+static LLVMContext C;
+static std::unique_ptr<Module> M;
+
+class GlobalObjectTest : public testing::Test {
+public:
+  static void SetUpTestSuite() {
+    M = parseIR(C, R"(
+ at foo = global i32 3, !section_prefix !0
+ at bar = global i32 0
+
+!0 = !{!"section_prefix", !"hot"}
+)");
+  }
+};
+
+TEST_F(GlobalObjectTest, SectionPrefix) {
+  GlobalVariable *Foo = M->getGlobalVariable("foo");
+
+  // Initial section prefix is hot.
+  ASSERT_NE(Foo, nullptr);
+  ASSERT_THAT(Foo->getSectionPrefix(), Optional(StrEq("hot")));
+
+  // No actual update.
+  EXPECT_FALSE(Foo->updateSectionPrefix("hot"));
+
+  // Update prefix from hot to unlikely.
+  Foo->setSectionPrefix("unlikely");
+  EXPECT_THAT(Foo->getSectionPrefix(), Optional(StrEq("unlikely")));
+
+  // Update prefix to empty is the same as clear.
+  Foo->setSectionPrefix("");
+  EXPECT_THAT(Foo->getSectionPrefix(), Eq(std::nullopt));
+
+  GlobalVariable *Bar = M->getGlobalVariable("bar");
+
+  // Initial section prefix is empty.
+  ASSERT_NE(Bar, nullptr);
+  ASSERT_THAT(Bar->getSectionPrefix(), Eq(std::nullopt));
+
+  // No actual update.
+  EXPECT_FALSE(Bar->updateSectionPrefix(""));
----------------
snehasish wrote:

`updateSectionPrefix` has no impact since there is not section prefix metadata. If there was it would be dropped. Perhaps this check is more meaningful after L74? Similar to the check on L60.

https://github.com/llvm/llvm-project/pull/158460