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

[aaupov]

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

[llvmbot]

@llvm/pr-subscribers-bolt

@llvm/pr-subscribers-mc

Author: Amir Ayupov (aaupov)

Changes

Add IsMMapped flag to buildGUID2FuncDescMap controlling whether to
allocate a string in FuncNameAllocator or use StringRef directly.

This saves ~0.7 GiB peak RSS in perf2bolt processing a medium sized
binary with ~0.8 GiB .pseudo_probe_desc section.

This is because BOLT keeps file sections in memory while processing them
whereas llvm-profgen constructs GUID2FuncDescMap and then releases the
binary.

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

---

Full diff: https://github.com/llvm/llvm-project/pull/112996.diff

3 Files Affected:

• (modified) bolt/lib/Rewrite/PseudoProbeRewriter.cpp (+1-1)
• (modified) llvm/include/llvm/MC/MCPseudoProbe.h (+4-1)
• (modified) llvm/lib/MC/MCPseudoProbe.cpp (+4-2)

diff --git a/bolt/lib/Rewrite/PseudoProbeRewriter.cpp b/bolt/lib/Rewrite/PseudoProbeRewriter.cpp
index 8647df4b0edf82..4fecfe8c3c09b1 100644
--- a/bolt/lib/Rewrite/PseudoProbeRewriter.cpp
+++ b/bolt/lib/Rewrite/PseudoProbeRewriter.cpp
@@ -128,7 +128,7 @@ void PseudoProbeRewriter::parsePseudoProbe(bool ProfiledOnly) {
   StringRef Contents = PseudoProbeDescSection->getContents();
   if (!ProbeDecoder.buildGUID2FuncDescMap(
           reinterpret_cast<const uint8_t *>(Contents.data()),
-          Contents.size())) {
+          Contents.size(), /*IsMMapped*/true)) {
     errs() << "BOLT-WARNING: fail in building GUID2FuncDescMap\n";
     return;
   }
diff --git a/llvm/include/llvm/MC/MCPseudoProbe.h b/llvm/include/llvm/MC/MCPseudoProbe.h
index 4bfae9eba1a0aa..fd1f0557895446 100644
--- a/llvm/include/llvm/MC/MCPseudoProbe.h
+++ b/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.
diff --git a/llvm/lib/MC/MCPseudoProbe.cpp b/llvm/lib/MC/MCPseudoProbe.cpp
index 90d7588407068a..2a3761b2cfe718 100644
--- a/llvm/lib/MC/MCPseudoProbe.cpp
+++ b/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 &&

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[aaupov]

ping @wlei-llvm

[wlei-llvm]

Sorry for the delay, LGTM.