[PATCH] D114394: Compile-time computation of string attribute hashes

[serge via Phabricator via cfe-commits]

serge-sans-paille added a comment.

> Frontends not written in C++ will always be going through the AttributeKey::get() API, which will be slower than the initial state (I think -- we still have to calculate the hash dynamically, but now we also need to intern an AttributeKey by performing an actual hash table lookup.)

I ran the following benchmark

  #include "llvm-c/Core.h"
  
  int main() {
    LLVMContextRef ctx = LLVMContextCreate();
    LLVMAttributeRef Key = LLVMCreateStringAttribute(ctx, "mykey", 5, "myvalue", 7);
    unsigned size;
    for(int i = 0; i <10000; ++i)
    for(int j = 0; j <10000; ++j)
      LLVMGetStringAttributeValue(Key, &size);
    return size;
  }

linking dynamically with libLLVMCore.so , the compiler doesn't optimize away the loop content.

Before the patch: 0.39s , ~ 2.5G instructions
With the patch: 0.30s, ~2.3G instructions

Why is that so? A side effect of the patch is that srings are memoized into a StringPool, so less allocation etc as soon as we lookup the attribute more than once.


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  https://reviews.llvm.org/D114394/new/

https://reviews.llvm.org/D114394