[llvm] 7bd4f1a - [memprof] Use BLAKE for FrameId (#109501)

[via llvm-commits]

Author: Kazu Hirata
Date: 2024-09-24T15:32:26-07:00
New Revision: 7bd4f1a0ed64aaf13e6acdafaaf50c4e7bf1d4dc

URL: https://github.com/llvm/llvm-project/commit/7bd4f1a0ed64aaf13e6acdafaaf50c4e7bf1d4dc
DIFF: https://github.com/llvm/llvm-project/commit/7bd4f1a0ed64aaf13e6acdafaaf50c4e7bf1d4dc.diff

LOG: [memprof] Use BLAKE for FrameId (#109501)

This patch uses a stronger hash function for FrameId.

Without this patch, I've observed hash collisions in fairly common
scenarios.  Specifically, for a given GUID, I see three pairs of hash
collisions in the two-dimensional range 1 <= LineOffset <= 70 and
1 <= Column <= 50, which may be a bit too frequent.  With the new
function, I don't see collisions at all even for a large profile.

Impact on serialization/deserialization should be as follows:

- Up to indexed memprof format Version 2, inclusive: The FrameIds
  computed with the new hash function will show up as part of the
  profile file.  However, the deserializer only treats FrameIds as
  keys (but not hash values) to retrieve Frames from the on-disk hash
  table, so a change of the hash function shouldn't matter.

- For indexed memprof format Version 3, FrameIds do not show up at all
  in the resulting profile file.  FrameIds are only used to break ties
  when we sort Frames in writeMemProfFrameArray.

Added: 
    

Modified: 
    llvm/include/llvm/ProfileData/MemProf.h

Removed: 
    


################################################################################
diff  --git a/llvm/include/llvm/ProfileData/MemProf.h b/llvm/include/llvm/ProfileData/MemProf.h
index 892865a0a26fea..f8121d35732518 100644
--- a/llvm/include/llvm/ProfileData/MemProf.h
+++ b/llvm/include/llvm/ProfileData/MemProf.h
@@ -7,8 +7,10 @@
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/IR/GlobalValue.h"
 #include "llvm/ProfileData/MemProfData.inc"
+#include "llvm/Support/BLAKE3.h"
 #include "llvm/Support/Endian.h"
 #include "llvm/Support/EndianStream.h"
+#include "llvm/Support/HashBuilder.h"
 #include "llvm/Support/raw_ostream.h"
 
 #include <bitset>
@@ -308,24 +310,19 @@ struct Frame {
        << "        Inline: " << IsInlineFrame << "\n";
   }
 
-  // Return a hash value based on the contents of the frame. Here we don't use
-  // hashing from llvm ADT since we are going to persist the hash id, the hash
-  // combine algorithm in ADT uses a new randomized seed each time.
+  // Return a hash value based on the contents of the frame. Here we use a
+  // cryptographic hash function to minimize the chance of hash collisions.  We
+  // do persist FrameIds as part of memprof formats up to Version 2, inclusive.
+  // However, the deserializer never calls this function; it uses FrameIds
+  // merely as keys to look up Frames proper.
   inline FrameId hash() const {
-    auto HashCombine = [](auto Value, size_t Seed) {
-      std::hash<decltype(Value)> Hasher;
-      // The constant used below is the 64 bit representation of the fractional
-      // part of the golden ratio. Used here for the randomness in their bit
-      // pattern.
-      return Hasher(Value) + 0x9e3779b97f4a7c15 + (Seed << 6) + (Seed >> 2);
-    };
-
-    size_t Result = 0;
-    Result ^= HashCombine(Function, Result);
-    Result ^= HashCombine(LineOffset, Result);
-    Result ^= HashCombine(Column, Result);
-    Result ^= HashCombine(IsInlineFrame, Result);
-    return static_cast<FrameId>(Result);
+    llvm::HashBuilder<llvm::TruncatedBLAKE3<8>, llvm::endianness::little>
+        HashBuilder;
+    HashBuilder.add(Function, LineOffset, Column, IsInlineFrame);
+    llvm::BLAKE3Result<8> Hash = HashBuilder.final();
+    FrameId Id;
+    std::memcpy(&Id, Hash.data(), sizeof(Hash));
+    return Id;
   }
 };