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Make .gnu.hash section smaller.
Our on-disk hash table was unnecessarily large. The cost of collision is
not high in the .gnu.hash table because each symbol in the .gnu.hash
table has a hash value with it. So, for each collided symbol, the
dynamic linker just compares an integer, which is pretty cheap.
This patch increases the load factor by about 8. Here's a comparison.
$ readelf --histogram libclangSema.so.6.0.0svn-new-lld
Histogram for `.gnu.hash' bucket list length (total of 582 buckets):
Length Number % of total Coverage
0 11 ( 1.9%)
1 35 ( 6.0%) 1.5%
2 93 ( 16.0%) 9.5%
3 108 ( 18.6%) 23.4%
4 121 ( 20.8%) 44.1%
5 86 ( 14.8%) 62.6%
6 63 ( 10.8%) 78.8%
7 38 ( 6.5%) 90.2%
8 18 ( 3.1%) 96.4%
9 6 ( 1.0%) 98.7%
10 3 ( 0.5%) 100.0%
$ readelf --histogram libclangSema.so.6.0.0svn-old-lld
Histogram for `.gnu.hash' bucket list length (total of 4093 buckets):
Length Number % of total Coverage
0 1498 ( 36.6%)
1 1545 ( 37.7%) 37.7%
2 712 ( 17.4%) 72.5%
3 251 ( 6.1%) 90.9%
4 66 ( 1.6%) 97.3%
5 16 ( 0.4%) 99.3%
6 5 ( 0.1%) 100.0%
$ readelf --histogram libclangSema.so.6.0.0svn-bfd
Histogram for `.gnu.hash' bucket list length (total of 1004 buckets):
Length Number % of total Coverage
0 92 ( 9.2%)
1 227 ( 22.6%) 9.8%
2 266 ( 26.5%) 32.6%
3 222 ( 22.1%) 61.2%
4 115 ( 11.5%) 81.0%
5 55 ( 5.5%) 92.8%
6 21 ( 2.1%) 98.2%
7 6 ( 0.6%) 100.0%
$ readelf --histogram libclangSema.so.6.0.0svn-gold
Histogram for `.gnu.hash' bucket list length (total of 2053 buckets):
Length Number % of total Coverage
0 671 ( 32.7%)
1 709 ( 34.5%) 30.4%
2 470 ( 22.9%) 70.7%
3 141 ( 6.9%) 88.9%
4 54 ( 2.6%) 98.2%
5 5 ( 0.2%) 99.2%
6 3 ( 0.1%) 100.0%
Differential Revision: https://reviews.llvm.org/D40683
llvm-svn: 319503
This commit is contained in:
Rui Ueyama
parent
2c1e68237f
commit
1cf7f9cc80
@ -1775,22 +1775,6 @@ static uint32_t hashGnu(StringRef Name) {
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return H;
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}
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// Returns a number of hash buckets to accomodate given number of elements.
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// We want to choose a moderate number that is not too small (which
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// causes too many hash collisions) and not too large (which wastes
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// disk space.)
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//
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// We return a prime number because it (is believed to) achieve good
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// hash distribution.
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static size_t getBucketSize(size_t NumSymbols) {
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// List of largest prime numbers that are not greater than 2^n + 1.
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for (size_t N : {131071, 65521, 32749, 16381, 8191, 4093, 2039, 1021, 509,
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251, 127, 61, 31, 13, 7, 3, 1})
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if (N <= NumSymbols)
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return N;
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return 0;
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}
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// Add symbols to this symbol hash table. Note that this function
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// destructively sort a given vector -- which is needed because
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// GNU-style hash table places some sorting requirements.
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@ -1813,7 +1797,12 @@ void GnuHashTableSection::addSymbols(std::vector<SymbolTableEntry> &V) {
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Symbols.push_back({B, Ent.StrTabOffset, hashGnu(B->getName())});
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}
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NBuckets = getBucketSize(Symbols.size());
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// We chose load factor 4 for the on-disk hash table. For each hash
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// collision, the dynamic linker will compare a uint32_t hash value.
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// Since the integer comparison is quite fast, we believe we can make
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// the load factor even larger. 4 is just a conservative choice.
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NBuckets = std::max<size_t>(Symbols.size() / 4, 1);
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std::stable_sort(Symbols.begin(), Symbols.end(),
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[&](const Entry &L, const Entry &R) {
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return L.Hash % NBuckets < R.Hash % NBuckets;
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@ -34,6 +34,15 @@
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# CHECK-NEXT: Section: .text
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# CHECK-NEXT: }
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# CHECK-NEXT: Symbol {
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# CHECK-NEXT: Name: baz
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# CHECK-NEXT: Value:
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# CHECK-NEXT: Size:
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# CHECK-NEXT: Binding: Global
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# CHECK-NEXT: Type:
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# CHECK-NEXT: Other:
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# CHECK-NEXT: Section: Undefined
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# CHECK-NEXT: }
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# CHECK-NEXT: Symbol {
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# CHECK-NEXT: Name: foo
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# CHECK-NEXT: Value:
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# CHECK-NEXT: Size:
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@ -51,15 +60,6 @@
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# CHECK-NEXT: Other:
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# CHECK-NEXT: Section: Undefined
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# CHECK-NEXT: }
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# CHECK-NEXT: Symbol {
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# CHECK-NEXT: Name: baz
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# CHECK-NEXT: Value:
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# CHECK-NEXT: Size:
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# CHECK-NEXT: Binding: Global
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# CHECK-NEXT: Type:
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# CHECK-NEXT: Other:
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# CHECK-NEXT: Section: Undefined
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# CHECK-NEXT: }
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# CHECK-NEXT: ]
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# CHECK-NOT: NEEDED
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@ -90,6 +90,15 @@
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# CHECK2-NEXT: Section: .text
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# CHECK2-NEXT: }
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# CHECK2-NEXT: Symbol {
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# CHECK2-NEXT: Name: baz
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# CHECK2-NEXT: Value:
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# CHECK2-NEXT: Size:
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# CHECK2-NEXT: Binding: Global
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# CHECK2-NEXT: Type:
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# CHECK2-NEXT: Other:
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# CHECK2-NEXT: Section: Undefined
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# CHECK2-NEXT: }
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# CHECK2-NEXT: Symbol {
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# CHECK2-NEXT: Name: qux
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# CHECK2-NEXT: Value:
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# CHECK2-NEXT: Size:
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@ -107,15 +116,6 @@
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# CHECK2-NEXT: Other:
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# CHECK2-NEXT: Section: .text
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# CHECK2-NEXT: }
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# CHECK2-NEXT: Symbol {
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# CHECK2-NEXT: Name: baz
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# CHECK2-NEXT: Value:
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# CHECK2-NEXT: Size:
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# CHECK2-NEXT: Binding: Global
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# CHECK2-NEXT: Type:
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# CHECK2-NEXT: Other:
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# CHECK2-NEXT: Section: Undefined
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# CHECK2-NEXT: }
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# CHECK2-NEXT: ]
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# CHECK2-NOT: NEEDED
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