[IR] Use map for string attributes (NFC)

Attributes are currently stored as a simple list. Enum attributes
additionally use a bitset to allow quickly determining whether an
attribute is set. String attributes on the other hand require a
full scan of the list. As functions tend to have a lot of string
attributes (at least when clang is used), this is a noticeable
performance issue.

This patch adds an additional name => attribute map to the
AttributeSetNode, which allows querying string attributes quickly.
This results in a 3% reduction in instructions retired on CTMark.
Changes to memory usage seem to be in the noise (attribute sets are
uniqued, and we don't tend to have more than a few dozen or hundred
unique attribute sets, so adding an extra map does not have a
noticeable cost.)

Differential Revision: https://reviews.llvm.org/D78859

llvm/lib/IR/AttributeImpl.h

@@ -16,6 +16,7 @@
 #define LLVM_LIB_IR_ATTRIBUTEIMPL_H
 
 #include "llvm/ADT/ArrayRef.h"
+#include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/FoldingSet.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/IR/Attributes.h"
@@ -181,6 +182,8 @@ class AttributeSetNode final
   /// Bitset with a bit for each available attribute Attribute::AttrKind.
   uint8_t AvailableAttrs[12] = {};
 
+  DenseMap<StringRef, Attribute> StringAttrs;
+
   AttributeSetNode(ArrayRef<Attribute> Attrs);
 
 public:

llvm/lib/IR/Attributes.cpp

@@ -770,7 +770,9 @@ AttributeSetNode::AttributeSetNode(ArrayRef<Attribute> Attrs)
                 "Too many attributes");
 
   for (const auto &I : *this) {
-    if (!I.isStringAttribute()) {
+    if (I.isStringAttribute()) {
+      StringAttrs.insert({ I.getKindAsString(), I });
+    } else {
       Attribute::AttrKind Kind = I.getKindAsEnum();
       AvailableAttrs[Kind / 8] |= 1ULL << (Kind % 8);
     }
@@ -857,10 +859,7 @@ AttributeSetNode *AttributeSetNode::get(LLVMContext &C, const AttrBuilder &B) {
 }
 
 bool AttributeSetNode::hasAttribute(StringRef Kind) const {
-  for (const auto &I : *this)
-    if (I.hasAttribute(Kind))
-      return true;
-  return false;
+  return StringAttrs.count(Kind);
 }
 
 Attribute AttributeSetNode::getAttribute(Attribute::AttrKind Kind) const {
@@ -873,10 +872,7 @@ Attribute AttributeSetNode::getAttribute(Attribute::AttrKind Kind) const {
 }
 
 Attribute AttributeSetNode::getAttribute(StringRef Kind) const {
-  for (const auto &I : *this)
-    if (I.hasAttribute(Kind))
-      return I;
-  return {};
+  return StringAttrs.lookup(Kind);
 }
 
 MaybeAlign AttributeSetNode::getAlignment() const {