[ELF] Name MergeSyntheticSection using an input section instead of the output section

[MaskRay]

Summary

In a link map, the input section name gives more information. See the updated
merge-entsize.s for an example. The output file is unchanged.

Compiler generated input sections with the SHF_MERGE flag have names such as
.rodata.str1.1 and .rodata.cstN, and are not affected by -fdata-sections.

[peter.smith]

I agree with the change. This should help distinguish the cases where there are more than one MergeSection within the same OutputSection.

lld/ELF/OutputSections.cpp

for (InputSectionBase *s : isd->sectionBases) {
         // section.
         //
         // SHF_STRINGS section with different alignments should not be merged.
         return sec->flags == ms->flags && sec->entsize == ms->entsize &&
                (sec->addralign == ms->addralign || !(sec->flags & SHF_STRINGS));
       });
       if (i == mergeSections.end()) {
         MergeSyntheticSection *syn =
-            createMergeSynthetic(name, ms->type, ms->flags, ms->addralign);
+            createMergeSynthetic(s->name, ms->type, ms->flags, ms->addralign);
         mergeSections.push_back(syn);
         i = std::prev(mergeSections.end());
         syn->entsize = ms->entsize;
         isd->sections.push_back(syn);
       }
       (*i)->addSection(ms);
     }
 

lld/test/ELF/merge-entsize.s

 // REQUIRES: x86
 // RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
-// RUN: ld.lld %t.o -o %t
+// RUN: ld.lld %t.o -o %t -M | FileCheck %s --check-prefix=MAP
 // RUN: llvm-readobj -S %t | FileCheck %s
 
+// MAP:      .rodata{{$}}
+// MAP-NEXT:         <internal>:(.rodata.1)
+// MAP-NEXT:         <internal>:(.rodata.2)
+
         .section  .rodata.1,"aM",@progbits,1
         .p2align 2
         .byte 0x42
 
 // sh_addralign = 4 while sh_entsize = 3.
 // sh_entsize is not necessarily a power of 2 and it can be unrelated to sh_addralign.
         .section  .rodata.2,"aM",@progbits,3
         .p2align 2

lld/test/ELF/merge-string-align.s

 // REQUIRES: x86
 // RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
-// RUN: ld.lld %t.o -o %t.so -shared
+// RUN: ld.lld %t.o -o %t.so -shared -M | FileCheck %s --check-prefix=MAP
 // RUN: llvm-readobj -S --section-data %t.so | FileCheck %s
 
+// MAP:      .rodata{{$}}
+// MAP-NEXT:         <internal>:(.rodata.foo)
+// MAP-NEXT: .rodata2
+// MAP-NEXT:         <internal>:(.rodata2)
+
         .section        .rodata.foo,"aMS",@progbits,1
         .align  16
         .asciz "foo"
 
         .section        .rodata.foo2,"aMS",@progbits,1
         .align  16
         .asciz "foo"