[llvm] [IR][Attribute] Add support for intersecting AttributeLists; NFC (PR #109719)

[Nikita Popov via llvm-commits]

================
@@ -903,6 +956,134 @@ AttributeSet AttributeSet::removeAttributes(LLVMContext &C,
   return get(C, B);
 }
 
+std::optional<AttributeSet>
+AttributeSet::intersectWith(LLVMContext &C, AttributeSet Other) const {
+  if (*this == Other)
+    return *this;
+
+  AttrBuilder Intersected(C);
+  // Iterate over both attr sets at once.
+  auto ItBegin0 = begin();
+  auto ItEnd0 = end();
+  auto ItBegin1 = Other.begin();
+  auto ItEnd1 = Other.end();
+
+  while (ItBegin0 != ItEnd0 || ItBegin1 != ItEnd1) {
+    // Loop through all attributes in both this and Other in sorted order. If
+    // the attribute is only present in one of the sets, it will be set in
+    // Attr0. If it is present in both sets both Attr0 and Attr1 will be set.
+    Attribute Attr0, Attr1;
+    if (ItBegin1 == ItEnd1)
+      Attr0 = *ItBegin0++;
+    else if (ItBegin0 == ItEnd0)
+      Attr1 = *ItBegin1++;
+    else {
+      int Cmp = ItBegin0->cmpKind(*ItBegin1);
+      if (Cmp == 0) {
+        Attr0 = *ItBegin0++;
+        Attr1 = *ItBegin1++;
+      } else if (Cmp < 0)
+        Attr0 = *ItBegin0++;
+      else
+        Attr0 = *ItBegin1++;
+    }
+    assert(Attr0.isValid() ||
+           Attr1.isValid() && "Iteration should never yield no valid attrs");
+
+    // If we don't have both attributes, then fail if the attribute is
+    // must-preserve or drop it otherwise.
+    if (!Attr0.isValid() || !Attr1.isValid()) {
----------------
nikic wrote:

```suggestion
    assert(Attr0.isValid() && "Iteration should never yield no valid attrs");

    // If we don't have both attributes, then fail if the attribute is
    // must-preserve or drop it otherwise.
    if (!Attr1.isValid()) {
```

https://github.com/llvm/llvm-project/pull/109719