[demangler] Buffer peeking needs buffer

[urnathan]

Summary

The output buffer has a 'back' member, which returns NUL when you try it with an empty buffer. But there are no use cases that need that additional functionality. This makes the 'back' member behave more like STL containers' back members. (It still returns a value, not a reference.)

[urnathan]

rebase & ping?

[dblaikie]

Sure, if that works.

libcxxabi/src/demangle/Utility.h

void insert(size_t Pos, const char *S, size_t N) {
     std::memcpy(Buffer + Pos, S, N);
     CurrentPosition += N;
   }
 
   size_t getCurrentPosition() const { return CurrentPosition; }
   void setCurrentPosition(size_t NewPos) { CurrentPosition = NewPos; }
 
   char back() const {
-    return CurrentPosition ? Buffer[CurrentPosition - 1] : '\0';
+    assert(CurrentPosition);
+    return Buffer[CurrentPosition - 1];
   }
 
   bool empty() const { return CurrentPosition == 0; }
 
   char *getBuffer() { return Buffer; }
   char *getBufferEnd() { return Buffer + CurrentPosition - 1; }
   size_t getBufferCapacity() const { return BufferCapacity; }
 };

llvm/include/llvm/Demangle/Utility.h

void insert(size_t Pos, const char *S, size_t N) {
     std::memcpy(Buffer + Pos, S, N);
     CurrentPosition += N;
   }
 
   size_t getCurrentPosition() const { return CurrentPosition; }
   void setCurrentPosition(size_t NewPos) { CurrentPosition = NewPos; }
 
   char back() const {
-    return CurrentPosition ? Buffer[CurrentPosition - 1] : '\0';
+    assert(CurrentPosition);
+    return Buffer[CurrentPosition - 1];
   }
 
   bool empty() const { return CurrentPosition == 0; }
 
   char *getBuffer() { return Buffer; }
   char *getBufferEnd() { return Buffer + CurrentPosition - 1; }
   size_t getBufferCapacity() const { return BufferCapacity; }
 };