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[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

llvm-svn: 349966
This commit is contained in:
Fangrui Song 2018-12-21 21:59:34 +00:00
parent b264d69de7
commit fe36417f6e
4 changed files with 13 additions and 10 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

2
lld/ELF/SyntheticSections.cpp
View File

@ -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);

3
lld/ELF/SyntheticSections.h
View File

@ -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);

4
lld/test/ELF/gnu-hash-table-rwsegment.s
View File

@ -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: }

14
lld/test/ELF/gnu-hash-table.s
View File

@ -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: }