[PATCH] D55971: [ELF] .gnu.hash bloom filter: use Shift2 = 6 instead of 2

[Fangrui Song via Phabricator via llvm-commits]

MaskRay updated this revision to Diff 179217.
MaskRay retitled this revision from "[ELF] .gnu.hash bloom filter: use Shift2 = 26 instead of 2" to "[ELF] .gnu.hash bloom filter: use Shift2 = 6 instead of 2".
MaskRay edited the summary of this revision.
MaskRay added a comment.

Update description


Repository:
  rLLD LLVM Linker

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D55971/new/

https://reviews.llvm.org/D55971

Files:
  ELF/SyntheticSections.cpp
  ELF/SyntheticSections.h
  test/ELF/gnu-hash-table-rwsegment.s
  test/ELF/gnu-hash-table.s


Index: test/ELF/gnu-hash-table.s
===================================================================
--- test/ELF/gnu-hash-table.s
+++ test/ELF/gnu-hash-table.s
@@ -62,7 +62,7 @@
 # EMPTY-NEXT:   Num Buckets: 1
 # EMPTY-NEXT:   First Hashed Symbol Index: 2
 # EMPTY-NEXT:   Num Mask Words: 1
-# EMPTY-NEXT:   Shift Count: 6
+# EMPTY-NEXT:   Shift Count: 26
 # EMPTY-NEXT:   Bloom Filter: [0x0]
 # EMPTY-NEXT:   Buckets: [0]
 # EMPTY-NEXT:   Values: []
@@ -121,8 +121,8 @@
 # I386-NEXT:   Num Buckets: 1
 # I386-NEXT:   First Hashed Symbol Index: 4
 # I386-NEXT:   Num Mask Words: 1
-# I386-NEXT:   Shift Count: 6
-# I386-NEXT:   Bloom Filter: [0x4004204]
+# I386-NEXT:   Shift Count: 26
+# I386-NEXT:   Bloom Filter: [0x4000204]
 # I386-NEXT:   Buckets: [4]
 # I386-NEXT:   Values: [0xB8860BA, 0xB887389]
 # I386-NEXT: }
@@ -181,8 +181,8 @@
 # X86_64-NEXT:   Num Buckets: 1
 # X86_64-NEXT:   First Hashed Symbol Index: 4
 # X86_64-NEXT:   Num Mask Words: 1
-# X86_64-NEXT:   Shift Count: 6
-# X86_64-NEXT:   Bloom Filter: [0x400000000004204]
+# X86_64-NEXT:   Shift Count: 26
+# X86_64-NEXT:   Bloom Filter: [0x400000000000204]
 # X86_64-NEXT:   Buckets: [4]
 # X86_64-NEXT:   Values: [0xB8860BA, 0xB887389]
 # X86_64-NEXT: }
@@ -241,8 +241,8 @@
 # PPC64-NEXT:   Num Buckets: 1
 # PPC64-NEXT:   First Hashed Symbol Index: 4
 # PPC64-NEXT:   Num Mask Words: 1
-# PPC64-NEXT:   Shift Count: 6
-# PPC64-NEXT:   Bloom Filter: [0x400000000004204]
+# PPC64-NEXT:   Shift Count: 26
+# PPC64-NEXT:   Bloom Filter: [0x400000000000204]
 # PPC64-NEXT:   Buckets: [4]
 # PPC64-NEXT:   Values: [0xB8860BA, 0xB887389]
 # PPC64-NEXT: }
Index: test/ELF/gnu-hash-table-rwsegment.s
===================================================================
--- test/ELF/gnu-hash-table-rwsegment.s
+++ test/ELF/gnu-hash-table-rwsegment.s
@@ -8,8 +8,8 @@
 # CHECK-NEXT:   Num Buckets: 1
 # CHECK-NEXT:   First Hashed Symbol Index: 1
 # CHECK-NEXT:   Num Mask Words: 1
-# CHECK-NEXT:   Shift Count: 6
-# CHECK-NEXT:   Bloom Filter: [0x400000000004204]
+# CHECK-NEXT:   Shift Count: 26
+# CHECK-NEXT:   Bloom Filter: [0x400000000000204]
 # CHECK-NEXT:   Buckets: [1]
 # CHECK-NEXT:   Values: [0xB8860BA, 0xB887389]
 # CHECK-NEXT: }
Index: ELF/SyntheticSections.h
===================================================================
--- ELF/SyntheticSections.h
+++ ELF/SyntheticSections.h
@@ -619,6 +619,7 @@
   void addSymbols(std::vector<SymbolTableEntry> &Symbols);
 
 private:
+  // See the comment in writeBloomFilter.
   enum { Shift2 = 6 };
 
   void writeBloomFilter(uint8_t *Buf);
Index: ELF/SyntheticSections.cpp
===================================================================
--- ELF/SyntheticSections.cpp
+++ ELF/SyntheticSections.cpp
@@ -2178,6 +2178,8 @@
   for (const Entry &Sym : Symbols) {
     size_t I = (Sym.Hash / C) & (MaskWords - 1);
     uint64_t Val = readUint(Buf + I * Config->Wordsize);
+    // We choose Shift2 = 6 to give us 6 bits (nearly) independent from the low
+    // 6 bits, and thus less collision.
     Val |= uint64_t(1) << (Sym.Hash % C);
     Val |= uint64_t(1) << ((Sym.Hash >> Shift2) % C);
     writeUint(Buf + I * Config->Wordsize, Val);


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