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

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.

Reviewed By: benlangmuir

Differential Revision: https://reviews.llvm.org/D157015

llvm/include/llvm/Support/StringSaver.h

@@ -29,7 +29,7 @@ public:
   // 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)); }
 };
 
@@ -51,7 +51,7 @@ public:
   // 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)); }
 };
 

llvm/lib/Support/StringSaver.cpp

@@ -8,6 +8,8 @@
 
 #include "llvm/Support/StringSaver.h"
 
+#include "llvm/ADT/SmallString.h"
+
 using namespace llvm;
 
 StringRef StringSaver::save(StringRef S) {
@@ -18,9 +20,19 @@ StringRef StringSaver::save(StringRef S) {
   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));
+}