[ELF] Add REVERSE input section description keyword

[justincady]

Summary

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

[MaskRay]

lld/ELF/Config.h

82

[MaskRay]

Normally I'd like a confirmation from binutils, but the semantics seem pretty clear, so ld.lld adopting the feature first may be fine :)
At any rate I left a comment on https://sourceware.org/bugzilla/show_bug.cgi?id=27565

[justincady]

Add trailing comma to SortSectionPolicy enum as suggested

[justincady]

Fixed the comma in latest revision.

lld/ELF/Config.h

 
 // For --unresolved-symbols.
 enum class UnresolvedPolicy { ReportError, Warn, Ignore };
 
 // For --orphan-handling.
 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 };
 
 // For tracking ARM Float Argument PCS
 enum class ARMVFPArgKind { Default, Base, VFP, ToolChain };
 
 // For -z noseparate-code, -z separate-code and -z separate-loadable-segments.

lld/ELF/LinkerScript.cpp

static void sortSections(MutableArrayRef<InputSectionBase *> vec,
   case SortSectionPolicy::None:
     return;
   case SortSectionPolicy::Alignment:
     return llvm::stable_sort(vec, alignmentComparator);
   case SortSectionPolicy::Name:
     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());
   }
 }
 
 // Sort sections as instructed by SORT-family commands and --sort-section
 // option. Because SORT-family commands can be nested at most two depth
 // (e.g. SORT_BY_NAME(SORT_BY_ALIGNMENT(.text.*))) and because the command
 // line option is respected even if a SORT command is given, the exact
 // behavior we have here is a bit complicated. Here are the rules.

lld/ELF/ScriptParser.cpp

StringMatcher ScriptParser::readFilePatterns() {
 
   while (!errorCount() && !consume(")"))
     Matcher.addPattern(SingleStringMatcher(next()));
   return Matcher;
 }
 
 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)
       .Case("SORT_NONE", SortSectionPolicy::None)
       .Default(SortSectionPolicy::Default);
 }
 
 SortSectionPolicy ScriptParser::readSortKind() {

lld/test/ELF/linkerscript/sort-init.s

 # REQUIRES: x86
 ## Test SORT_BY_INIT_PRIORITY can be used to convert .ctors into .init_array
 
 # RUN: split-file %s %t
 # RUN: llvm-mc -filetype=obj -triple=x86_64 %t/asm -o %t.o
 # RUN: ld.lld -T %t/lds %t.o -o %t.out
 # RUN: llvm-readelf -x .init_array %t.out | FileCheck %s
 
 # 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:
   nop
 
 .section foo, "aw", @init_array
   .byte 5
 

 
 #--- lds
 SECTIONS {
   .init_array : {
     *(SORT_BY_INIT_PRIORITY(.init_array.* .ctors.*) SORT_BY_INIT_PRIORITY(foo*))
     *(.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))
+  }
+}