[PATCH] D145381: [ELF] Add REVERSE input section description keyword

[Justin Cady via Phabricator via llvm-commits]

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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


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D145381

Files:
  lld/ELF/Config.h
  lld/ELF/LinkerScript.cpp
  lld/ELF/ScriptParser.cpp
  lld/test/ELF/linkerscript/sort-init.s


Index: lld/test/ELF/linkerscript/sort-init.s
===================================================================
--- lld/test/ELF/linkerscript/sort-init.s
+++ lld/test/ELF/linkerscript/sort-init.s
@@ -9,6 +9,14 @@
 # CHECK:      Hex dump of section '.init_array':
 # CHECK-NEXT: 0x00000001 00010203 04050607
 
+## Test REVERSE can be used to reverse the order of .init_array and .ctors
+
+# RUN: ld.lld -T %t/reverse.lds %t.o -o %t2.out
+# RUN: llvm-readelf -x .init_array %t2.out | FileCheck %s --check-prefix=CHECK2
+
+# CHECK2:      Hex dump of section '.init_array':
+# CHECK2-NEXT: 0x00000001 04030201 00050706
+
 #--- asm
 .globl _start
 _start:
@@ -41,3 +49,11 @@
     *(.init_array .ctors)
   }
 }
+
+#--- reverse.lds
+SECTIONS {
+  .init_array : {
+    *(REVERSE(SORT_BY_INIT_PRIORITY(.init_array.* .ctors.*)) SORT_BY_INIT_PRIORITY(foo*))
+    *(REVERSE(.init_array .ctors))
+  }
+}
Index: lld/ELF/ScriptParser.cpp
===================================================================
--- lld/ELF/ScriptParser.cpp
+++ lld/ELF/ScriptParser.cpp
@@ -660,6 +660,7 @@
 
 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)
Index: lld/ELF/LinkerScript.cpp
===================================================================
--- lld/ELF/LinkerScript.cpp
+++ lld/ELF/LinkerScript.cpp
@@ -457,6 +457,8 @@
     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());
   }
 }
 
Index: lld/ELF/Config.h
===================================================================
--- lld/ELF/Config.h
+++ lld/ELF/Config.h
@@ -73,7 +73,14 @@
 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 };


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