[ELF] -r: allow SHT_X86_64_UNWIND to be merged into SHT_PROGBITS

[MaskRay]

Summary

• For .cfi_*, GCC/GNU as emits SHT_PROGBITS type .eh_frame sections.
• Since rL252300, clang emits SHT_X86_64_UNWIND type .eh_frame sections (originated from Solaris, documented in the x86-64 psABI).
• Some assembly use .section .eh_frame,"a",@unwind to generate SHT_X86_64_UNWIND .eh_frame sections.

In a non-relocatable link, input .eh_frame are combined and there is
only one SyntheticSection .eh_frame in the output section, so the
"section type mismatch" diagnostic does not fire.

In a relocatable link, there is no SyntheticSection .eh_frame. .eh_frame of
mixed types can trigger the diagnostic. This patch fixes it by adding another
special case 0x70000001 (= SHT_X86_64_UNWIND) to canMergeToProgbits().

ld.lld -r gcc.o clang.o => error: section type mismatch for .eh_frame

There was a discussion "RFC: Usefulness of SHT_X86_64_UNWIND" on the x86-64-abi
mailing list. Folks are not wild about making the psABI value 0x70000001 into
gABI, but a few think defining 0x70000001 for .eh_frame may be a good idea for a
new architecture.

[psmith]

I think this should be ok for .ARM.exidx sections as they have a SHF_LINK_ORDER dependency which prevents them from being merged in relocatable links.

[grimar]

There is little cost losing the "section type mismatch" diagnostic for other architectures, so we don't bother checking e_machine

Is there any reason why you don't want to check the e_machine to make the code more readable, straightforward and
don't lose the "section type mismatch" diagnostic?

[MaskRay]

In D85785#2212794, @grimar wrote:

There is little cost losing the "section type mismatch" diagnostic for other architectures, so we don't bother checking e_machine

Is there any reason why you don't want to check the e_machine to make the code more readable, straightforward and
don't lose the "section type mismatch" diagnostic?

Because I don't think the "section type mismatch" diagnostic is useful for any SHT_*_UNWIND.

However, given the previous comment about SHT_ARM_EXIDX, I think checking config->e_machine may be fine.

[MaskRay]

Check emachine

[grimar]

LGTM

[MaskRay]

@hans This is a good candidate for release/11.x

[hans]

In D85785#2237433, @MaskRay wrote:

@hans This is a good candidate for release/11.x

Okay, 04d70cd0f07dab371abf586627ce9ac09e04362c.

lld/ELF/OutputSections.cpp

: BaseCommand(OutputSectionKind),
                   /*Info*/ 0, /*Link*/ 0) {}
 
 // We allow sections of types listed below to merged into a
 // single progbits section. This is typically done by linker
 // scripts. Merging nobits and progbits will force disk space
 // to be allocated for nobits sections. Other ones don't require
 // any special treatment on top of progbits, so there doesn't
 // seem to be a harm in merging them.
+//
+// NOTE: clang since rL252300 emits SHT_X86_64_UNWIND .eh_frame sections. Allow
+// them to be merged into SHT_PROGBITS .eh_frame (GNU as .cfi_*).
 static bool canMergeToProgbits(unsigned type) {
   return type == SHT_NOBITS || type == SHT_PROGBITS || type == SHT_INIT_ARRAY ||
          type == SHT_PREINIT_ARRAY || type == SHT_FINI_ARRAY ||
-         type == SHT_NOTE;
+         type == SHT_NOTE ||
+         (type == SHT_X86_64_UNWIND && config->emachine == EM_X86_64);
 }
 
 // Record that isec will be placed in the OutputSection. isec does not become
 // permanent until finalizeInputSections() is called. The function should not be
 // used after finalizeInputSections() is called. If you need to add an
 // InputSection post finalizeInputSections(), then you must do the following:
 //
 // 1. Find or create an InputSectionDescription to hold InputSection.

lld/test/ELF/eh-frame-type.test

 ## Show that LLD can handle .eh_frame sections of different types.
 
 # RUN: yaml2obj %s -o %t1.o -D TYPE=SHT_PROGBITS
 # RUN: ld.lld %t1.o -o %t1
 # RUN: llvm-readobj -S %t1 | FileCheck %s
 
 # RUN: yaml2obj %s -o %t2.o -D TYPE=SHT_X86_64_UNWIND
 # RUN: ld.lld %t2.o -o %t2
 # RUN: llvm-readobj -S %t2 | FileCheck %s
 
 # RUN: ld.lld %t1.o %t2.o -o %tboth
 # RUN: llvm-readobj -S %tboth | FileCheck %s
 
+# RUN: ld.lld -r %t1.o %t2.o -o %tboth.ro
+# RUN: llvm-readobj -S %tboth.ro | FileCheck %s
+
 # CHECK:      Name: .eh_frame
 # CHECK-NEXT: Type: SHT_PROGBITS
 
 --- !ELF
 FileHeader:
   Class:   ELFCLASS64
   Data:    ELFDATA2LSB
   Type:    ET_REL
   Machine: EM_X86_64
 Sections:
   - Name:  .eh_frame
     Type:  [[TYPE]]
     Flags: [ SHF_ALLOC ]