[ELF] - Set the sh_entsize for mergable sections

[grimar]

Summary

Previously sh_entsize field was not set for MergeOutputSection.
Patch fixes that.

That should resolve the https://llvm.org/bugs/show_bug.cgi?id=26975

[grimar]

ELF/OutputSections.cpp

1223

I think there is no rule what EntSize to take. Gold uses the first size he saw here:

.section	.rodata.1,"aMS",@progbits,2
.asciz	"a"

.section        .rodata.1,"aMS",@progbits,1
.asciz	"b"
...
            
  EntSize == 2:
  [ 2] .rodata.1         PROGBITS         0000000000000000  00000040  0000000000000004  0000000000000002

.section	.rodata.1,"aMS",@progbits,1
.asciz	"a"

.section        .rodata.1,"aMS",@progbits,2
.asciz	"b"

...

  EntSize == 1:               
  [ 2] .rodata.1         PROGBITS         0000000000000000  00000040  0000000000000004  0000000000000001

So I think there is no need in any conditions and we can just assign here.

[emaste]

Perhaps sections with different sh_entsize should not be mergable?
But I think this is fine either way.

[• rafael]

LGTM.

Thanks!

[ruiu]

lld/trunk/ELF/OutputSections.cpp

1223 ↗

(On Diff #51014)

Setting a header value from addSection would be a bit confusing because it runs multiple times. I'd do that in finalize().

[grimar]

lld/trunk/ELF/OutputSections.cpp

1223 ↗

(On Diff #51014)

Am I missing something, but we dont have access to sh_entsize from finalize. I mean we dont store a list of added sections to take it there.

[ruiu]

Ah, right. I thought that we have a list of input sections, but we actually
don't have that.

ELF/OutputSections.cpp

 void MergeOutputSection<ELFT>::addSection(InputSectionBase<ELFT> *C) {
   auto *S = cast<MergeInputSection<ELFT>>(C);
   S->OutSec = this;
   this->updateAlign(S->Align);
 
   ArrayRef<uint8_t> D = S->getSectionData();
   StringRef Data((const char *)D.data(), D.size());
   uintX_t EntSize = S->getSectionHdr()->sh_entsize;
+  this->Header.sh_entsize = EntSize;
 
   // If this is of type string, the contents are null-terminated strings.
   if (this->Header.sh_flags & SHF_STRINGS) {
     uintX_t Offset = 0;
     while (!Data.empty()) {
       size_t End = findNull(Data, EntSize);
       if (End == StringRef::npos)
         fatal("string is not null terminated");

test/ELF/merge-string.s

 // CHECK-NEXT:   SHF_STRINGS
 // CHECK-NEXT: ]
 // CHECK-NEXT: Address:         0x1C8
 // CHECK-NEXT: Offset:  0x1C8
 // CHECK-NEXT: Size:    4
 // CHECK-NEXT: Link: 0
 // CHECK-NEXT: Info: 0
 // CHECK-NEXT: AddressAlignment: 1
-// CHECK-NEXT: EntrySize: 0
+// CHECK-NEXT: EntrySize: 1
 // CHECK-NEXT: SectionData (
 // CHECK-NEXT:   0000: 61626300                             |abc.|
 // CHECK-NEXT: )
 
 // NOTAIL:      Name:    .rodata
 // NOTAIL-NEXT: Type:    SHT_PROGBITS
 // NOTAIL-NEXT: Flags [
 // NOTAIL-NEXT:   SHF_ALLOC
 // NOTAIL-NEXT:   SHF_MERGE
 // NOTAIL-NEXT:   SHF_STRINGS
 // NOTAIL-NEXT: ]
 // NOTAIL-NEXT: Address:         0x1C8
 // NOTAIL-NEXT: Offset:  0x1C8
 // NOTAIL-NEXT: Size:    7
 // NOTAIL-NEXT: Link: 0
 // NOTAIL-NEXT: Info: 0
 // NOTAIL-NEXT: AddressAlignment: 1
-// NOTAIL-NEXT: EntrySize: 0
+// NOTAIL-NEXT: EntrySize: 1
 // NOTAIL-NEXT: SectionData (
 // NOTAIL-NEXT:   0000: 61626300 626300                     |abc.bc.|
 // NOTAIL-NEXT: )
 
 // CHECK:      Name: .rodata
 // CHECK-NEXT: Type: SHT_PROGBITS
 // CHECK-NEXT: Flags [
 // CHECK-NEXT:   SHF_ALLOC
 // CHECK-NEXT:   SHF_MERGE
 // CHECK-NEXT:   SHF_STRINGS
 // CHECK-NEXT: ]
 // CHECK-NEXT: Address: 0x1CC
 // CHECK-NEXT: Offset: 0x1CC
 // CHECK-NEXT: Size: 4
 // CHECK-NEXT: Link: 0
 // CHECK-NEXT: Info: 0
 // CHECK-NEXT: AddressAlignment: 2
-// CHECK-NEXT: EntrySize: 0
+// CHECK-NEXT: EntrySize: 2
 // CHECK-NEXT: SectionData (
 // CHECK-NEXT:   0000: 14000000                             |....|
 // CHECK-NEXT: )
 
 
 // CHECK:      Name:    bar
 // CHECK-NEXT: Value:   0x1C9
 
 // CHECK:      Name:    foo
 // CHECK-NEXT: Value:   0x1C8
 
 // CHECK:      Name: zed
 // CHECK-NEXT: Value: 0x1CC
 // CHECK-NEXT: Size: 0

test/ELF/merge.s

 // CHECK-NEXT:      SHF_MERGE
 // CHECK-NEXT:    ]
 // CHECK-NEXT:    Address: 0x10120
 // CHECK-NEXT:    Offset: 0x120
 // CHECK-NEXT:    Size: 8
 // CHECK-NEXT:    Link: 0
 // CHECK-NEXT:    Info: 0
 // CHECK-NEXT:    AddressAlignment: 4
-// CHECK-NEXT:    EntrySize: 0
+// CHECK-NEXT:    EntrySize: 4
 // CHECK-NEXT:    SectionData (
 // CHECK-NEXT:      0000: 10000000 42000000
 // CHECK-NEXT:    )
 
 
 // Address of the constant 0x10 = 0x10120 = 65824
 // Address of the constant 0x42 = 0x10124 = 65828