[ELF] Add REVERSE input section description keyword

The `REVERSE` keyword is described here:

https://sourceware.org/bugzilla/show_bug.cgi?id=27565

It complements `SORT` by allowing the order of input sections to be reversed.

This is particularly useful for order-dependent sections such as .init_array,
where `REVERSE` can be used to either detect static initialization order fiasco
issues or as a mechanism to maintain .ctors element order while transitioning to
the modern .init_array. Such a transition is described here:

https://discourse.llvm.org/t/is-it-possible-to-manually-specify-init-array-order/68649

Differential Revision: https://reviews.llvm.org/D145381

3 files changed, 11 insertions, 1 deletions

diff --git a/lld/ELF/Config.h b/lld/ELF/Config.h
index 61faea4e9bce..249fe3c30ece 100644
--- a/lld/ELF/Config.h
+++ b/lld/ELF/Config.h
@@ -73,7 +73,14 @@ enum class UnresolvedPolicy { ReportError, Warn, Ignore };
 enum class OrphanHandlingPolicy { Place, Warn, Error };
 
 // For --sort-section and linkerscript sorting rules.
-enum class SortSectionPolicy { Default, None, Alignment, Name, Priority };
+enum class SortSectionPolicy {
+  Default,
+  None,
+  Alignment,
+  Name,
+  Priority,
+  Reverse,
+};
 
 // For --target2
 enum class Target2Policy { Abs, Rel, GotRel };
diff --git a/lld/ELF/LinkerScript.cpp b/lld/ELF/LinkerScript.cpp
index 668a79b5edd7..9207751cb7d2 100644
--- a/lld/ELF/LinkerScript.cpp
+++ b/lld/ELF/LinkerScript.cpp
@@ -457,6 +457,8 @@ static void sortSections(MutableArrayRef<InputSectionBase *> vec,
     return llvm::stable_sort(vec, nameComparator);
   case SortSectionPolicy::Priority:
     return llvm::stable_sort(vec, priorityComparator);
+  case SortSectionPolicy::Reverse:
+    return std::reverse(vec.begin(), vec.end());
   }
 }
 
diff --git a/lld/ELF/ScriptParser.cpp b/lld/ELF/ScriptParser.cpp
index e83779bcebff..db7f0268cef6 100644
--- a/lld/ELF/ScriptParser.cpp
+++ b/lld/ELF/ScriptParser.cpp
@@ -660,6 +660,7 @@ StringMatcher ScriptParser::readFilePatterns() {
 
 SortSectionPolicy ScriptParser::peekSortKind() {
   return StringSwitch<SortSectionPolicy>(peek())
+      .Case("REVERSE", SortSectionPolicy::Reverse)
       .Cases("SORT", "SORT_BY_NAME", SortSectionPolicy::Name)
       .Case("SORT_BY_ALIGNMENT", SortSectionPolicy::Alignment)
       .Case("SORT_BY_INIT_PRIORITY", SortSectionPolicy::Priority)