[lld] [lld][ELF] Extend profile guided function ordering to ELF binaries (PR #117514)

[Ellis Hoag via llvm-commits]

================
@@ -0,0 +1,67 @@
+//===- BPSectionOrderer.cpp--------------------------------------*- C++ -*-===//
+//
+// 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 "BPSectionOrderer.h"
+#include "Config.h"
+#include "InputFiles.h"
+#include "InputSection.h"
+#include "lld/Common/BPSectionOrdererBase.h"
+#include "lld/Common/CommonLinkerContext.h"
+#include "llvm/ADT/DenseMap.h"
+#include "llvm/Support/BalancedPartitioning.h"
+#include "llvm/Support/TimeProfiler.h"
+
+#include "SymbolTable.h"
+#include "Symbols.h"
+
+using namespace llvm;
+using namespace lld::elf;
+
+llvm::DenseMap<const lld::elf::InputSectionBase *, int>
+lld::elf::runBalancedPartitioning(Ctx &ctx, llvm::StringRef profilePath,
+                                  bool forFunctionCompression,
+                                  bool forDataCompression,
+                                  bool compressionSortStartupFunctions,
+                                  bool verbose) {
+  size_t highestAvailablePriority = std::numeric_limits<int>::max();
+  // Collect all InputSectionBase objects from symbols and wrap them as
+  // BPSectionBase instances for balanced partitioning which follow the way
+  // '--symbol-ordering-file' does.
+  SmallVector<lld::BPSectionBase *> sections;
+
+  for (Symbol *sym : ctx.symtab->getSymbols())
+    if (auto *d = dyn_cast<Defined>(sym))
+      if (auto *sec = dyn_cast_or_null<InputSectionBase>(d->section))
+        if (sym->getSize() > 0)
+          sections.push_back(new BPSectionELF(sec, new BPSymbolELF(sym)));
+
+  for (ELFFileBase *file : ctx.objectFiles)
+    for (Symbol *sym : file->getLocalSymbols())
+      if (auto *d = dyn_cast<Defined>(sym))
+        if (auto *sec = dyn_cast_or_null<InputSectionBase>(d->section))
+          if (sym->getSize() > 0)
+            sections.push_back(new BPSectionELF(sec, new BPSymbolELF(sym)));
+
+  auto reorderedSections =
+      lld::BPSectionOrdererBase::reorderSectionsByBalancedPartitioning(
+          highestAvailablePriority, profilePath, forFunctionCompression,
+          forDataCompression, compressionSortStartupFunctions, verbose,
+          sections);
+
+  DenseMap<const InputSectionBase *, int> result;
+  for (const auto &[BPSectionBase, priority] : reorderedSections) {
+    if (const BPSectionELF *elfSection =
+            dyn_cast<BPSectionELF>(BPSectionBase)) {
+      result[elfSection->getSymbol()->getInputSection()] =
+          static_cast<int>(priority);
----------------
ellishg wrote:

If you use `try_emplace()` then it won't override sections that you have already set earlier. I think this is what we want.
```suggestion
      result.try_emplace(elfSection->getSymbol()->getInputSection(), static_cast<int>(priority));
```

https://github.com/llvm/llvm-project/pull/117514