use StringMap in llvm-dwp when merging debug_str.dwo section

[dblaikie]

Looks good. Could you include some comparison numbers, if you have them, in the commit message? (What target did you test (clang is an easy one), time and memory usage would be good to know)

[dblaikie]

Oh, wait. This will increase memory usage by copying the strings into the string map which is is unfortunate... Any way to avoid that?

[zhuna8616]

correct StringRef in make_pair

[zhuna8616]

My bad... I forgot to convert const char * to StringRef.

StringRef alone doesn't seem to do any copy. StringMap copies the key into the map. The old implementation uses const char * and has none of such problem. I can think of no other way to it if we persist using StringMap.

[MaskRay]

StringMap clones the strings. DenseMap<CachedHashStringRef, uint32_t> may be more efficient.

[dblaikie]

In D154341#4471422, @zhuna8616 wrote:

My bad... I forgot to convert const char * to StringRef.

StringRef alone doesn't seem to do any copy. StringMap copies the key into the map. The old implementation uses const char * and has none of such problem. I can think of no other way to it if we persist using StringMap.

Yeah - maybe lld has something that can be generalized and avoids this issue. I think it also has multithreading in string merging too.

llvm/include/llvm/DWP/DWPStringPool.h

static bool isEqual(const char *LHS, const char *RHS) {
       if (RHS == getTombstoneKey())
         return LHS == getTombstoneKey();
       return strcmp(LHS, RHS) == 0;
     }
   };
 
   MCStreamer &Out;
   MCSection *Sec;
-  DenseMap<const char *, uint32_t, CStrDenseMapInfo> Pool;
+  StringMap<uint32_t> Pool;
   uint32_t Offset = 0;
 
 public:
   DWPStringPool(MCStreamer &Out, MCSection *Sec) : Out(Out), Sec(Sec) {}
 
   uint32_t getOffset(const char *Str, unsigned Length) {
     assert(strlen(Str) + 1 == Length && "Ensure length hint is correct");
 
-    auto Pair = Pool.insert(std::make_pair(Str, Offset));
+    auto Pair = Pool.insert(std::make_pair(StringRef(Str, Length), Offset));
     if (Pair.second) {
       Out.switchSection(Sec);
       Out.emitBytes(StringRef(Str, Length));
       Offset += Length;
     }
 
     return Pair.first->second;
   }
 };
 } // namespace llvm
 
 #endif // LLVM_DWP_DWPSTRINGPOOL_H