[IR] Switch AttributeList to use an array for O(1) access

Summary:
Before this change, AttributeLists stored a pair of index and
AttributeSet. This is memory efficient if most arguments do not have
attributes. However, it requires doing a search over the pairs to test
an argument or function attribute. Profiling shows that this loop was
0.76% of the time in 'opt -O2' of sqlite3.c, because LLVM constantly
tests values for nullability.

This was worth about 2.5% of mid-level optimization cycles on the
sqlite3 amalgamation. Here are the full perf results:
https://reviews.llvm.org/P7995

Here are just the before and after cycle counts:
```
$ perf stat -r 5 ./opt_before -O2 sqlite3.bc -o /dev/null
    13,274,181,184      cycles                    #    3.047 GHz                      ( +-  0.28% )
$ perf stat -r 5 ./opt_after -O2 sqlite3.bc -o /dev/null
    12,906,927,263      cycles                    #    3.043 GHz                      ( +-  0.51% )
```

This patch *does not* change the indices used to query attributes, as
requested by reviewers. Tracking whether an index is usable for array
indexing is a huge pain that affects many of the internal APIs, so it
would be good to come back later and do a cleanup to remove this
internal adjustment.

Reviewers: pete, chandlerc

Subscribers: javed.absar, llvm-commits

Differential Revision: https://reviews.llvm.org/D32819

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@303654 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 11 insertions, 6 deletions

diff --git a/lib/Bitcode/Writer/BitcodeWriter.cpp b/lib/Bitcode/Writer/BitcodeWriter.cpp
index 1f8b50342c2..e29b225adea 100644
--- a/lib/Bitcode/Writer/BitcodeWriter.cpp
+++ b/lib/Bitcode/Writer/BitcodeWriter.cpp
@@ -660,10 +660,12 @@ void ModuleBitcodeWriter::writeAttributeTable() {
 
   SmallVector<uint64_t, 64> Record;
   for (unsigned i = 0, e = Attrs.size(); i != e; ++i) {
-    const AttributeList &A = Attrs[i];
-    for (unsigned i = 0, e = A.getNumSlots(); i != e; ++i)
-      Record.push_back(
-          VE.getAttributeGroupID({A.getSlotIndex(i), A.getSlotAttributes(i)}));
+    AttributeList AL = Attrs[i];
+    for (unsigned i = AL.index_begin(), e = AL.index_end(); i != e; ++i) {
+      AttributeSet AS = AL.getAttributes(i);
+      if (AS.hasAttributes())
+        Record.push_back(VE.getAttributeGroupID({i, AS}));
+    }
 
     Stream.EmitRecord(bitc::PARAMATTR_CODE_ENTRY, Record);
     Record.clear();
diff --git a/lib/Bitcode/Writer/ValueEnumerator.cpp b/lib/Bitcode/Writer/ValueEnumerator.cpp
index fd76400331d..bb626baabd1 100644
--- a/lib/Bitcode/Writer/ValueEnumerator.cpp
+++ b/lib/Bitcode/Writer/ValueEnumerator.cpp
@@ -902,8 +902,11 @@ void ValueEnumerator::EnumerateAttributes(AttributeList PAL) {
   }
 
   // Do lookups for all attribute groups.
-  for (unsigned i = 0, e = PAL.getNumSlots(); i != e; ++i) {
-    IndexAndAttrSet Pair = {PAL.getSlotIndex(i), PAL.getSlotAttributes(i)};
+  for (unsigned i = PAL.index_begin(), e = PAL.index_end(); i != e; ++i) {
+    AttributeSet AS = PAL.getAttributes(i);
+    if (!AS.hasAttributes())
+      continue;
+    IndexAndAttrSet Pair = {i, AS};
     unsigned &Entry = AttributeGroupMap[Pair];
     if (Entry == 0) {
       AttributeGroups.push_back(Pair);