[ELF] .gnu.hash bloom filter: use Shift2 = 26 instead of 6

Summary:
For the 2-bit bloom filter, we currently pick the bits Hash%64 and Hash>>6%64 (Shift2=6), but bits [6:...] are also used to select a word, causing a loss of precision.

In this patch, we choose Shift2=26, with is suggested by Ambrose Feinstein.

Note, Shift2 is computed as maskbitslog2 in bfd/elflink.c and gold/dynobj.cc
It is varying with the number of dynamic symbols but we don't
necessarily copy its rule.

Reviewers: ruiu, espindola

Reviewed By: ruiu

Subscribers: emaste, arichardson, llvm-commits

Differential Revision: https://reviews.llvm.org/D55971

4 files changed, 13 insertions, 10 deletions

diff --git a/lld/ELF/SyntheticSections.cpp b/lld/ELF/SyntheticSections.cpp
index 293d8452c89..dafd2d02def 100644
--- a/lld/ELF/SyntheticSections.cpp
+++ b/lld/ELF/SyntheticSections.cpp
@@ -2176,6 +2176,8 @@ void GnuHashTableSection::writeTo(uint8_t *Buf) {
 void GnuHashTableSection::writeBloomFilter(uint8_t *Buf) {
   unsigned C = Config->Is64 ? 64 : 32;
   for (const Entry &Sym : Symbols) {
+    // When C = 64, we choose a word with bits [6:...] and set 1 to two bits in
+    // the word using bits [0:5] and [26:31].
     size_t I = (Sym.Hash / C) & (MaskWords - 1);
     uint64_t Val = readUint(Buf + I * Config->Wordsize);
     Val |= uint64_t(1) << (Sym.Hash % C);
diff --git a/lld/ELF/SyntheticSections.h b/lld/ELF/SyntheticSections.h
index 63d7897e12a..6fc40d355d5 100644
--- a/lld/ELF/SyntheticSections.h
+++ b/lld/ELF/SyntheticSections.h
@@ -619,7 +619,8 @@ public:
   void addSymbols(std::vector<SymbolTableEntry> &Symbols);
 
 private:
-  enum { Shift2 = 6 };
+  // See the comment in writeBloomFilter.
+  enum { Shift2 = 26 };
 
   void writeBloomFilter(uint8_t *Buf);
   void writeHashTable(uint8_t *Buf);
diff --git a/lld/test/ELF/gnu-hash-table-rwsegment.s b/lld/test/ELF/gnu-hash-table-rwsegment.s
index b1a50fbde3a..ab1b252bc5d 100644
--- a/lld/test/ELF/gnu-hash-table-rwsegment.s
+++ b/lld/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: }
diff --git a/lld/test/ELF/gnu-hash-table.s b/lld/test/ELF/gnu-hash-table.s
index 642b445ec5d..9dcd4a081e3 100644
--- a/lld/test/ELF/gnu-hash-table.s
+++ b/lld/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: }