[llvm][Support] Avoid intermediate heap allocations in `StringSaver`

[jansvoboda11]

Summary

The Twine::str() function currently always allocates heap memory via std::string. However, some instances of Twine don't need an intermediate buffer at all, and the rest can attempt to print into a stack buffer first.

This is intentionally not making use of Twine::isSingleStringLiteral() from D157010 to skip saving the string in the bump-pointer allocator, since the StringSaver documentation suggests that MUST happen for every given string.

llvm/include/llvm/Support/StringSaver.h

 public:
   StringSaver(BumpPtrAllocator &Alloc) : Alloc(Alloc) {}
 
   BumpPtrAllocator &getAllocator() const { return Alloc; }
 
   // All returned strings are null-terminated: *save(S).end() == 0.
   StringRef save(const char *S) { return save(StringRef(S)); }
   StringRef save(StringRef S);
-  StringRef save(const Twine &S) { return save(StringRef(S.str())); }
+  StringRef save(const Twine &S);
   StringRef save(const std::string &S) { return save(StringRef(S)); }
 };
 
 /// Saves strings in the provided stable storage and returns a StringRef with a
 /// stable character pointer. Saving the same string yields the same StringRef.
 ///
 /// Compared to StringSaver, it does more work but avoids saving the same string
 /// multiple times.
 ///
 /// Compared to StringPool, it performs fewer allocations but doesn't support
 /// refcounting/deletion.
 class UniqueStringSaver final {
   StringSaver Strings;
   llvm::DenseSet<llvm::StringRef> Unique;
 
 public:
   UniqueStringSaver(BumpPtrAllocator &Alloc) : Strings(Alloc) {}
 
   // All returned strings are null-terminated: *save(S).end() == 0.
   StringRef save(const char *S) { return save(StringRef(S)); }
   StringRef save(StringRef S);
-  StringRef save(const Twine &S) { return save(StringRef(S.str())); }
+  StringRef save(const Twine &S);
   StringRef save(const std::string &S) { return save(StringRef(S)); }
 };
 
 } // namespace llvm
 #endif

llvm/lib/Support/StringSaver.cpp

 //===-- StringSaver.cpp ---------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #include "llvm/Support/StringSaver.h"
 
+#include "llvm/ADT/SmallString.h"
+
 using namespace llvm;
 
 StringRef StringSaver::save(StringRef S) {
   char *P = Alloc.Allocate<char>(S.size() + 1);
   if (!S.empty())
     memcpy(P, S.data(), S.size());
   P[S.size()] = '\0';
   return StringRef(P, S.size());
 }
 
+StringRef StringSaver::save(const Twine &S) {
+  SmallString<128> Storage;
+  return save(S.toStringRef(Storage));
+}
+
 StringRef UniqueStringSaver::save(StringRef S) {
   auto R = Unique.insert(S);
   if (R.second)                 // cache miss, need to actually save the string
     *R.first = Strings.save(S); // safe replacement with equal value
   return *R.first;
 }
+
+StringRef UniqueStringSaver::save(const Twine &S) {
+  SmallString<128> Storage;
+  return save(S.toStringRef(Storage));
+}