[ELF] - Allow multiple comdats when producing relocatable output.

[grimar]

Summary

Previously LLD would fail for case when there are multiple comdats and -r.

That happened because it merged all ".group" (SHT_GROUP) sections into single
output, producing broken result. Such sections may have similar name, alignment and flags
and other properties. I think all we need to do for fix is to produce separate output
section for each such input one.

[grimar]

I think this is not correct. I'll revisit it tomorrow.

[ruiu]

How can this happen? I believe we dedup SHT_GROUP sections even if the -r option is given, so we want to emit only one SHT_GROUP for each unique name, no?

[grimar]

In D33643#772824, @ruiu wrote:

How can this happen? I believe we dedup SHT_GROUP sections even if the -r option is given, so we want to emit only one SHT_GROUP for each unique name, no?

Dedup does not help here, because groups are different. We have group A and section named ".group" + group .B with section ".group" too.
Afted deduplication we still have 2 sections named ".group". And idea is that we should emit different output sections for each, because
after deduplication we definetely know them are different groups.

[ruiu]

LGTM with this change.

ELF/OutputSections.cpp

363–365 ↗

(On Diff #100669)

Then this explanation doesn't capture why we are doing this.

Sections with the SHT_GROUP attribute reach here only when the -r option is given. Such sections define "section groups", and InputFiles.cpp has dedup'ed section groups by their signatures. For the -r, we want to pass through all SHT_GROUP sections without merging them because merging them creates broken section contents.

lld/trunk/ELF/OutputSections.cpp

void elf::reportDiscarded(InputSectionBase *IS) {
   if (!Config->PrintGcSections)
     return;
   message("removing unused section from '" + IS->Name + "' in file '" +
           IS->File->getName());
 }
 
 void OutputSectionFactory::addInputSec(InputSectionBase *IS,
                                        StringRef OutsecName) {
+  // Sections with the SHT_GROUP attribute reach here only when the - r option
+  // is given. Such sections define "section groups", and InputFiles.cpp has
+  // dedup'ed section groups by their signatures. For the -r, we want to pass
+  // through all SHT_GROUP sections without merging them because merging them
+  // creates broken section contents.
+  if (IS->Type == SHT_GROUP) {
+    OutputSection *Out = nullptr;
+    addInputSec(IS, OutsecName, Out);
+    return;
+  }
+
   SectionKey Key = createKey(IS, OutsecName);
   OutputSection *&Sec = Map[Key];
   return addInputSec(IS, OutsecName, Sec);
 }
 
 void OutputSectionFactory::addInputSec(InputSectionBase *IS,
                                        StringRef OutsecName,
                                        OutputSection *&Sec) {

lld/trunk/test/ELF/Inputs/relocatable-comdat-multiple.s

+.section .text.c,"axG",@progbits,bbb,comdat
+.section .text.d,"axG",@progbits,bbb,comdat

lld/trunk/test/ELF/relocatable-comdat-multiple.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
+# RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %S/Inputs/relocatable-comdat-multiple.s -o %t2.o
+# RUN: ld.lld -r %t.o %t2.o -o %t
+# RUN: llvm-readobj -elf-section-groups %t | FileCheck %s
+
+# CHECK:      Groups {
+# CHECK-NEXT:   Group {
+# CHECK-NEXT:     Name: .group
+# CHECK-NEXT:     Index: 2
+# CHECK-NEXT:     Type: COMDAT
+# CHECK-NEXT:     Signature: aaa
+# CHECK-NEXT:     Section(s) in group [
+# CHECK-NEXT:       .text.a
+# CHECK-NEXT:       .text.b
+# CHECK-NEXT:     ]
+# CHECK-NEXT:   }
+# CHECK-NEXT:   Group {
+# CHECK-NEXT:     Name: .group
+# CHECK-NEXT:     Index: 5
+# CHECK-NEXT:     Type: COMDAT
+# CHECK-NEXT:     Signature: bbb
+# CHECK-NEXT:     Section(s) in group [
+# CHECK-NEXT:       .text.c
+# CHECK-NEXT:       .text.d
+# CHECK-NEXT:     ]
+# CHECK-NEXT:   }
+# CHECK-NEXT: }
+
+.section .text.a,"axG",@progbits,aaa,comdat
+.section .text.b,"axG",@progbits,aaa,comdat