[LLD] Add CLASS syntax to SECTIONS (#95323)

This allows the input section matching algorithm to be separated from
output section descriptions. This allows a group of sections to be
assigned to multiple output sections, providing an explicit version of
--enable-non-contiguous-regions's spilling that doesn't require altering
global linker script matching behavior with a flag. It also makes the
linker script language more expressive even if spilling is not intended,
since input section matching can be done in a different order than
sections are placed in an output section.

The implementation reuses the backend mechanism provided by
--enable-non-contiguous-regions, so it has roughly similar semantics and
limitations. In particular, sections cannot be spilled into or out of
INSERT, OVERWRITE_SECTIONS, or /DISCARD/. The former two aren't
intrinsic, so it may be possible to relax those restrictions later.

lld/ELF/ScriptParser.cpp

@@ -87,6 +87,8 @@ private:
   OutputDesc *readOverlaySectionDescription();
   OutputDesc *readOutputSectionDescription(StringRef outSec);
   SmallVector<SectionCommand *, 0> readOverlay();
+  SectionClassDesc *readSectionClassDescription();
+  StringRef readSectionClassName();
   SmallVector<StringRef, 0> readOutputSectionPhdrs();
   std::pair<uint64_t, uint64_t> readInputSectionFlags();
   InputSectionDescription *readInputSectionDescription(StringRef tok);
@@ -605,6 +607,33 @@ SmallVector<SectionCommand *, 0> ScriptParser::readOverlay() {
   return v;
 }
 
+SectionClassDesc *ScriptParser::readSectionClassDescription() {
+  StringRef name = readSectionClassName();
+  SectionClassDesc *desc = make<SectionClassDesc>(name);
+  if (!script->sectionClasses.insert({CachedHashStringRef(name), desc}).second)
+    setError("section class '" + name + "' already defined");
+  expect("{");
+  while (auto tok = till("}")) {
+    if (tok == "(" || tok == ")") {
+      setError("expected filename pattern");
+    } else if (peek() == "(") {
+      InputSectionDescription *isd = readInputSectionDescription(tok);
+      if (!isd->classRef.empty())
+        setError("section class '" + name + "' references class '" +
+                 isd->classRef + "'");
+      desc->sc.commands.push_back(isd);
+    }
+  }
+  return desc;
+}
+
+StringRef ScriptParser::readSectionClassName() {
+  expect("(");
+  StringRef name = unquote(next());
+  expect(")");
+  return name;
+}
+
 void ScriptParser::readOverwriteSections() {
   expect("{");
   while (auto tok = till("}"))
@@ -619,7 +648,12 @@ void ScriptParser::readSections() {
       for (SectionCommand *cmd : readOverlay())
         v.push_back(cmd);
       continue;
-    } else if (tok == "INCLUDE") {
+    }
+    if (tok == "CLASS") {
+      v.push_back(readSectionClassDescription());
+      continue;
+    }
+    if (tok == "INCLUDE") {
       readInclude();
       continue;
     }
@@ -822,8 +856,14 @@ ScriptParser::readInputSectionDescription(StringRef tok) {
     expect("(");
     if (consume("INPUT_SECTION_FLAGS"))
       std::tie(withFlags, withoutFlags) = readInputSectionFlags();
-    InputSectionDescription *cmd =
-        readInputSectionRules(next(), withFlags, withoutFlags);
+
+    tok = next();
+    InputSectionDescription *cmd;
+    if (tok == "CLASS")
+      cmd = make<InputSectionDescription>(StringRef{}, withFlags, withoutFlags,
+                                          readSectionClassName());
+    else
+      cmd = readInputSectionRules(tok, withFlags, withoutFlags);
     expect(")");
     script->keptSections.push_back(cmd);
     return cmd;
@@ -832,6 +872,9 @@ ScriptParser::readInputSectionDescription(StringRef tok) {
     std::tie(withFlags, withoutFlags) = readInputSectionFlags();
     tok = next();
   }
+  if (tok == "CLASS")
+    return make<InputSectionDescription>(StringRef{}, withFlags, withoutFlags,
+                                         readSectionClassName());
   return readInputSectionRules(tok, withFlags, withoutFlags);
 }
 
@@ -951,8 +994,12 @@ OutputDesc *ScriptParser::readOverlaySectionDescription() {
       std::tie(withFlags, withoutFlags) = readInputSectionFlags();
       tok = till("");
     }
-    osd->osec.commands.push_back(
-        readInputSectionRules(tok, withFlags, withoutFlags));
+    if (tok == "CLASS")
+      osd->osec.commands.push_back(make<InputSectionDescription>(
+          StringRef{}, withFlags, withoutFlags, readSectionClassName()));
+    else
+      osd->osec.commands.push_back(
+          readInputSectionRules(tok, withFlags, withoutFlags));
   }
   osd->osec.phdrs = readOutputSectionPhdrs();
   return osd;