[PATCH] D138555: Cache memory buffer's name length

[serge via Phabricator via llvm-commits]

serge-sans-paille created this revision.
serge-sans-paille added reviewers: zturner, labath, nikic, rriddle.
Herald added a subscriber: hiraditya.
Herald added a project: All.
serge-sans-paille requested review of this revision.
Herald added a project: LLVM.
Herald added a subscriber: llvm-commits.

This avoids repeated calls to strlen while we already know its value.

When preprocessing sqlite3.c, this gives a surprising 2% speedup.

Full benchmark available here: https://llvm-compile-time-tracker.com/compare.php?from=5279e6a7d677cdf4488883b77aacab911318100c&to=389601b0dbdf23cf25167ddfc49b3af5742ebd9a&stat=instructions:u


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D138555

Files:
  llvm/lib/Support/MemoryBuffer.cpp


Index: llvm/lib/Support/MemoryBuffer.cpp
===================================================================
--- llvm/lib/Support/MemoryBuffer.cpp
+++ llvm/lib/Support/MemoryBuffer.cpp
@@ -58,12 +58,10 @@
 // MemoryBufferMem implementation.
 //===----------------------------------------------------------------------===//
 
-/// CopyStringRef - Copies contents of a StringRef into a block of memory and
-/// null-terminates it.
+/// CopyStringRef - Copies contents of a StringRef into a block of memory.
 static void CopyStringRef(char *Memory, StringRef Data) {
   if (!Data.empty())
     memcpy(Memory, Data.data(), Data.size());
-  Memory[Data.size()] = 0; // Null terminate string.
 }
 
 namespace {
@@ -77,8 +75,10 @@
   SmallString<256> NameBuf;
   StringRef NameRef = Alloc.Name.toStringRef(NameBuf);
 
-  char *Mem = static_cast<char *>(operator new(N + NameRef.size() + 1));
-  CopyStringRef(Mem + N, NameRef);
+  char *Mem =
+      static_cast<char *>(operator new(N + sizeof(size_t) + NameRef.size()));
+  *reinterpret_cast<size_t *>(Mem + N) = NameRef.size();
+  CopyStringRef(Mem + N + sizeof(size_t), NameRef);
   return Mem;
 }
 
@@ -98,7 +98,8 @@
 
   StringRef getBufferIdentifier() const override {
     // The name is stored after the class itself.
-    return StringRef(reinterpret_cast<const char *>(this + 1));
+    return StringRef(reinterpret_cast<const char *>(this + 1) + sizeof(size_t),
+                     *reinterpret_cast<const size_t *>(this + 1));
   }
 
   MemoryBuffer::BufferKind getBufferKind() const override {
@@ -221,7 +222,8 @@
 
   StringRef getBufferIdentifier() const override {
     // The name is stored after the class itself.
-    return StringRef(reinterpret_cast<const char *>(this + 1));
+    return StringRef(reinterpret_cast<const char *>(this + 1) + sizeof(size_t),
+                     *reinterpret_cast<const size_t *>(this + 1));
   }
 
   MemoryBuffer::BufferKind getBufferKind() const override {
@@ -301,7 +303,8 @@
   // that MemoryBuffer and data are aligned so PointerIntPair works with them.
   SmallString<256> NameBuf;
   StringRef NameRef = BufferName.toStringRef(NameBuf);
-  size_t StringLen = sizeof(MemBuffer) + NameRef.size() + 1;
+
+  size_t StringLen = sizeof(MemBuffer) + sizeof(size_t) + NameRef.size();
   size_t RealLen = StringLen + Size + 1 + BufAlign.value();
   if (RealLen <= Size) // Check for rollover.
     return nullptr;
@@ -310,13 +313,15 @@
     return nullptr;
 
   // The name is stored after the class itself.
-  CopyStringRef(Mem + sizeof(MemBuffer), NameRef);
+  *reinterpret_cast<size_t *>(Mem + sizeof(MemBuffer)) =
+      NameRef.size(); // Null terminate buffer.
+  CopyStringRef(Mem + sizeof(MemBuffer) + sizeof(size_t), NameRef);
 
   // The buffer begins after the name and must be aligned.
   char *Buf = (char *)alignAddr(Mem + StringLen, BufAlign);
-  Buf[Size] = 0; // Null terminate buffer.
 
   auto *Ret = new (Mem) MemBuffer(StringRef(Buf, Size), true);
+  Buf[Size] = 0; // Null terminate buffer.
   return std::unique_ptr<WritableMemoryBuffer>(Ret);
 }
 


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