[llvm] [ProfData] Improve efficiency of reader (PR #169730)

[via llvm-commits]

https://github.com/mxms0 created https://github.com/llvm/llvm-project/pull/169730

Pre-reserve space in the map before inserting. In release builds, 9.4% of all CPU time is spent in llvm::sampleprof::ProfileSymbolList::add. Of that 9.4%, roughly half is in llvm::DenseMapBase::grow.

>From 592a87b371844731bac6469e8efb6731f102f461 Mon Sep 17 00:00:00 2001
From: mxms <mxms at google.com>
Date: Wed, 26 Nov 2025 20:55:18 +0000
Subject: [PATCH] [ProfData] Improve efficiency of reader

Pre-reserve space in the map before inserting. In release builds, 9.4%
of all CPU time is spent in llvm::sampleprof::ProfileSymbolList::add. Of
that 9.4%, roughly half is in llvm::DenseMapBase::grow.
---
 llvm/include/llvm/ProfileData/SampleProf.h |  1 +
 llvm/lib/ProfileData/SampleProf.cpp        | 25 +++++++++++++++++-----
 2 files changed, 21 insertions(+), 5 deletions(-)

diff --git a/llvm/include/llvm/ProfileData/SampleProf.h b/llvm/include/llvm/ProfileData/SampleProf.h
index 3dd34aba2d716..6bce5c718e598 100644
--- a/llvm/include/llvm/ProfileData/SampleProf.h
+++ b/llvm/include/llvm/ProfileData/SampleProf.h
@@ -1658,6 +1658,7 @@ class ProfileSymbolList {
   }
 
   unsigned size() { return Syms.size(); }
+  void reserve(size_t Size) { Syms.reserve(Size); }
 
   void setToCompress(bool TC) { ToCompress = TC; }
   bool toCompress() { return ToCompress; }
diff --git a/llvm/lib/ProfileData/SampleProf.cpp b/llvm/lib/ProfileData/SampleProf.cpp
index ac7513ef2cb49..5f0af79ef33df 100644
--- a/llvm/lib/ProfileData/SampleProf.cpp
+++ b/llvm/lib/ProfileData/SampleProf.cpp
@@ -399,15 +399,30 @@ LLVM_DUMP_METHOD void FunctionSamples::dump() const { print(dbgs(), 0); }
 std::error_code ProfileSymbolList::read(const uint8_t *Data,
                                         uint64_t ListSize) {
   const char *ListStart = reinterpret_cast<const char *>(Data);
-  uint64_t Size = 0;
+  uint64_t Offset = 0;
   uint64_t StrNum = 0;
-  while (Size < ListSize && StrNum < ProfileSymbolListCutOff) {
-    StringRef Str(ListStart + Size);
+  uint64_t ExpectedCount = 0;
+
+  Scan forward to see how many elements we expect.
+  while (Offset < ListSize) {
+    if (ListStart[Offset] == '\0') ExpectedCount++;
+    Offset++;
+  }
+
+  reserve(ExpectedCount);
+
+  Offset = 0;
+
+  while (Offset < ListSize && StrNum < ProfileSymbolListCutOff) {
+    StringRef Str(ListStart + Offset);
     add(Str);
-    Size += Str.size() + 1;
+    Offset += Str.size() + 1;
     StrNum++;
   }
-  if (Size != ListSize && StrNum != ProfileSymbolListCutOff)
+
+  assert(ExpectedCount == StrNum);
+
+  if (Offset != ListSize && StrNum != ProfileSymbolListCutOff)
     return sampleprof_error::malformed;
   return sampleprof_error::success;
 }