[MC] Use StringRefs from pseudo_probe_desc section if it's mapped

Add `IsMMapped` flag to `buildGUID2FuncDescMap` controlling whether to
allocate a string in `FuncNameAllocator` or use StringRef directly.
Keep it false by default, only set it for BOLT use case because BOLT
keeps file sections in memory while processing them. llvm-profgen
constructs GUID2FuncDescMap and then releases the binary.

For medium sized binary with 0.8 GiB .pseudo_probe_desc section, this
saves 0.7 GiB peak RSS in perf2bolt.

Test Plan: no-op for llvm-profgen, NFC for perf2bolt

Reviewers: maksfb, dcci, wlei-llvm, rafaelauler, ayermolo

Reviewed By: wlei-llvm

Pull Request: #112996

Author: aaupov

bolt/lib/Rewrite/PseudoProbeRewriter.cpp

@@ -127,8 +127,8 @@ void PseudoProbeRewriter::parsePseudoProbe(bool ProfiledOnly) {
 
   StringRef Contents = PseudoProbeDescSection->getContents();
   if (!ProbeDecoder.buildGUID2FuncDescMap(
-          reinterpret_cast<const uint8_t *>(Contents.data()),
-          Contents.size())) {
+          reinterpret_cast<const uint8_t *>(Contents.data()), Contents.size(),
+          /*IsMMapped*/ true)) {
     errs() << "BOLT-WARNING: fail in building GUID2FuncDescMap\n";
     return;
   }

llvm/include/llvm/MC/MCPseudoProbe.h

@@ -431,7 +431,10 @@ class MCPseudoProbeDecoder {
   using Uint64Map = DenseMap<uint64_t, uint64_t>;
 
   // Decode pseudo_probe_desc section to build GUID to PseudoProbeFuncDesc map.
-  bool buildGUID2FuncDescMap(const uint8_t *Start, std::size_t Size);
+  // If pseudo_probe_desc section is mapped to memory and \p IsMMapped is true,
+  // uses StringRefs pointing to the section.
+  bool buildGUID2FuncDescMap(const uint8_t *Start, std::size_t Size,
+                             bool IsMMapped = false);
 
   // Decode pseudo_probe section to count the number of probes and inlined
   // function records for each function record.

llvm/lib/MC/MCPseudoProbe.cpp

@@ -375,7 +375,8 @@ ErrorOr<StringRef> MCPseudoProbeDecoder::readString(uint32_t Size) {
 }
 
 bool MCPseudoProbeDecoder::buildGUID2FuncDescMap(const uint8_t *Start,
-                                                 std::size_t Size) {
+                                                 std::size_t Size,
+                                                 bool IsMMapped) {
   // The pseudo_probe_desc section has a format like:
   // .section .pseudo_probe_desc,"",@progbits
   // .quad -5182264717993193164   // GUID
@@ -422,7 +423,8 @@ bool MCPseudoProbeDecoder::buildGUID2FuncDescMap(const uint8_t *Start,
     StringRef Name = cantFail(errorOrToExpected(readString(NameSize)));
 
     // Initialize PseudoProbeFuncDesc and populate it into GUID2FuncDescMap
-    GUID2FuncDescMap.emplace_back(GUID, Hash, Name.copy(FuncNameAllocator));
+    GUID2FuncDescMap.emplace_back(
+        GUID, Hash, IsMMapped ? Name : Name.copy(FuncNameAllocator));
   }
   assert(Data == End && "Have unprocessed data in pseudo_probe_desc section");
   assert(GUID2FuncDescMap.size() == FuncDescCount &&