[llvm][mustache] Avoid excessive hash lookups in EscapeStringStream (llvm#160166)

The naive char-by-char lookup performed OK, but we can skip ahead to the
next match, avoiding all the extra hash lookups in the key map. Likely
there is a faster method than this, but its already a 42% win in the
BM_Mustache_StringRendering/Escaped benchmark, and an order of magnitude
improvement for BM_Mustache_LargeOutputString.

| Benchmark | Before (ns) | After (ns) | Speedup |
| :--- | ---: | ---: | ---: |
| `StringRendering/Escaped` | 29,440,922 | 16,583,603 | ~44% |
| `LargeOutputString` | 15,139,251 | 929,891 | ~94% |
| `HugeArrayIteration` | 102,148,245 | 95,943,960 | ~6% |
| `PartialsRendering` | 308,330,014 | 303,556,563 | ~1.6% |

Unreported benchmarks, like those for parsing, had no significant
change.

Author: ilovepi

llvm/lib/Support/Mustache.cpp

@@ -397,26 +397,40 @@ class EscapeStringStream : public raw_ostream {
 public:
   explicit EscapeStringStream(llvm::raw_ostream &WrappedStream,
                               EscapeMap &Escape)
-      : Escape(Escape), WrappedStream(WrappedStream) {
+      : Escape(Escape), EscapeChars(Escape.keys().begin(), Escape.keys().end()),
+        WrappedStream(WrappedStream) {
     SetUnbuffered();
   }
 
 protected:
   void write_impl(const char *Ptr, size_t Size) override {
-    llvm::StringRef Data(Ptr, Size);
-    for (char C : Data) {
-      auto It = Escape.find(C);
-      if (It != Escape.end())
-        WrappedStream << It->getSecond();
-      else
-        WrappedStream << C;
+    StringRef Data(Ptr, Size);
+    size_t Start = 0;
+    while (Start < Size) {
+      // Find the next character that needs to be escaped.
+      size_t Next = Data.find_first_of(EscapeChars.str(), Start);
+
+      // If no escapable characters are found, write the rest of the string.
+      if (Next == StringRef::npos) {
+        WrappedStream << Data.substr(Start);
+        return;
+      }
+
+      // Write the chunk of text before the escapable character.
+      if (Next > Start)
+        WrappedStream << Data.substr(Start, Next - Start);
+
+      // Look up and write the escaped version of the character.
+      WrappedStream << Escape[Data[Next]];
+      Start = Next + 1;
     }
   }
 
   uint64_t current_pos() const override { return WrappedStream.tell(); }
 
 private:
   EscapeMap &Escape;
+  SmallString<8> EscapeChars;
   llvm::raw_ostream &WrappedStream;
 };