[llvm] r263869 - Add a comment on partial hashing of Metadata

[Mehdi Amini via llvm-commits]

Author: mehdi_amini
Date: Fri Mar 18 20:06:24 2016
New Revision: 263869

URL: http://llvm.org/viewvc/llvm-project?rev=263869&view=rev
Log:
Add a comment on partial hashing of Metadata

Following r263866, on D. Blaikie suggestion.

From: Mehdi Amini <mehdi.amini at apple.com>

Modified:
    llvm/trunk/lib/IR/LLVMContextImpl.h

Modified: llvm/trunk/lib/IR/LLVMContextImpl.h
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/IR/LLVMContextImpl.h?rev=263869&r1=263868&r2=263869&view=diff
==============================================================================
--- llvm/trunk/lib/IR/LLVMContextImpl.h (original)
+++ llvm/trunk/lib/IR/LLVMContextImpl.h Fri Mar 18 20:06:24 2016
@@ -365,6 +365,10 @@ template <> struct MDNodeKeyImpl<DIDeriv
            ExtraData == RHS->getRawExtraData();
   }
   unsigned getHashValue() const {
+    // Intentionally computes the hash on a subset of the operands for
+    // performance reason. The subset has to be significant enough to avoid
+    // collision "most of the time". There is no correctness issue in case of
+    // collision because of the full check above.
     return hash_combine(Tag, Name, File, Line, Scope, BaseType, Flags);
   }
 };
@@ -421,6 +425,10 @@ template <> struct MDNodeKeyImpl<DICompo
            Identifier == RHS->getRawIdentifier();
   }
   unsigned getHashValue() const {
+    // Intentionally computes the hash on a subset of the operands for
+    // performance reason. The subset has to be significant enough to avoid
+    // collision "most of the time". There is no correctness issue in case of
+    // collision because of the full check above.
     return hash_combine(Name, File, Line, BaseType, Scope, Elements,
                         TemplateParams);
   }
@@ -517,6 +525,10 @@ template <> struct MDNodeKeyImpl<DISubpr
            Variables == RHS->getRawVariables();
   }
   unsigned getHashValue() const {
+    // Intentionally computes the hash on a subset of the operands for
+    // performance reason. The subset has to be significant enough to avoid
+    // collision "most of the time". There is no correctness issue in case of
+    // collision because of the full check above.
     return hash_combine(Name, Scope, File, Type, Line);
   }
 };