[clang] [Serialization] Use specialized decl hash function for GlobalDeclID (PR #95730)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Chuanqi Xu (ChuanqiXu9)

<details>
<summary>Changes</summary>

See the comment:
https://github.com/llvm/llvm-project/pull/92083#issuecomment-2168121729

After the patch, https://github.com/llvm/llvm-project/pull/92083, the lower 32 bits of DeclID can be the same commonly. This may produce many collisions. It will be best to change the default hash implementation for uint64_t. But sent this one as a quick workaround.

---
Full diff: https://github.com/llvm/llvm-project/pull/95730.diff


1 Files Affected:

- (modified) clang/include/clang/AST/DeclID.h (+5-1) 


``````````diff
diff --git a/clang/include/clang/AST/DeclID.h b/clang/include/clang/AST/DeclID.h
index 32d2ed41a374a..4ad7afb463b18 100644
--- a/clang/include/clang/AST/DeclID.h
+++ b/clang/include/clang/AST/DeclID.h
@@ -230,7 +230,11 @@ template <> struct DenseMapInfo<clang::GlobalDeclID> {
   }
 
   static unsigned getHashValue(const GlobalDeclID &Key) {
-    return DenseMapInfo<DeclID>::getHashValue(Key.get());
+    // Our default hash algorithm for 64 bits integer may not be very good.
+    // In GlobalDeclID's case, it is pretty common that the lower 32 bits can
+    // be same.
+    return DenseMapInfo<uint32_t>::getHashValue(Key.getModuleFileIndex()) ^
+           DenseMapInfo<uint32_t>::getHashValue(Key.getLocalDeclIndex());
   }
 
   static bool isEqual(const GlobalDeclID &L, const GlobalDeclID &R) {

``````````

</details>


https://github.com/llvm/llvm-project/pull/95730