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

[Chuanqi Xu via cfe-commits]

https://github.com/ChuanqiXu9 created https://github.com/llvm/llvm-project/pull/95730

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.

>From 50923aa33f09b2530cfe492a53f70296f9ce9107 Mon Sep 17 00:00:00 2001
From: Chuanqi Xu <yedeng.yd at linux.alibaba.com>
Date: Mon, 17 Jun 2024 11:32:35 +0800
Subject: [PATCH] [Serialization] Use specialized decl hash function for
 GlobalDeclID

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.
---
 clang/include/clang/AST/DeclID.h | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

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) {