Cache memory buffer's name length

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=d14c2d408dccd8c6defa7d151e9a96be3cac8cc3&to=04f0641c1cbdcd0bdbd11cd910ca6091420bf52e&stat=instructions:u

Recommit 1824432174b3166b40bce59477beb5821170748e, with restored '\0' at the end of buffer name.

Differential Revision: https://reviews.llvm.org/D138555

diff --git a/llvm/lib/Support/MemoryBuffer.cpp b/llvm/lib/Support/MemoryBuffer.cpp
index 6bb046e..77c509c 100644 (file)
--- a/llvm/lib/Support/MemoryBuffer.cpp
+++ b/llvm/lib/Support/MemoryBuffer.cpp
@@ -77,8 +77,10 @@ void *operator new(size_t N, const NamedBufferAlloc &Alloc) {
   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() + 1));
+  *reinterpret_cast<size_t *>(Mem + N) = NameRef.size();
+  CopyStringRef(Mem + N + sizeof(size_t), NameRef);
   return Mem;
 }
 
@@ -98,7 +100,8 @@ public:
 
   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 +224,8 @@ public:
 
   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 +305,8 @@ WritableMemoryBuffer::getNewUninitMemBuffer(size_t Size,
   // 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() + 1;
   size_t RealLen = StringLen + Size + 1 + BufAlign.value();
   if (RealLen <= Size) // Check for rollover.
     return nullptr;
@@ -310,7 +315,8 @@ WritableMemoryBuffer::getNewUninitMemBuffer(size_t Size,
     return nullptr;
 
   // The name is stored after the class itself.
-  CopyStringRef(Mem + sizeof(MemBuffer), NameRef);
+  *reinterpret_cast<size_t *>(Mem + sizeof(MemBuffer)) = NameRef.size();
+  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);