ELF2: Merge .{text,rodata,data,bss}.* sections.

[ruiu]

Summary

Previously, we used input section names as output section names. That resulted
that we created lots of sections for comdat or -f{function,data}-section sections.

This patch is reduces the number of sections by dropping suffix from
all section names which start with ".text.", ".rodata.", ".data." or ".bss.".
GNU linker does this using the internal linker script, but for LLD I just
chose to do that directly.

Interestingly, this makes the linker faster. Time to link Clang is this.

Before:

real    0m0.537s
user    0m0.433s
sys     0m0.104s

After:

real    0m0.390s
user    0m0.268s
sys     0m0.120s

It make sense because previously we created 57659 sections, and it's now
only 27.

[• rafael]

Thanks!

I always compile with -fno-unique-section-names, which is why I didn't notice this.

ELF/Writer.cpp

371

Pass in the name and you can drop the template.

test/elf2/section-name.s

28

foo.a?

[Eugene.Zelenko]

Committed in r250315.

ELF/Writer.cpp

for (DefinedCommon<ELFT> *C : Syms) {
     Off = RoundUpToAlignment(Off, Align);
     C->OffsetInBSS = Off;
     Off += Sym.st_size;
   }
 
   Out<ELFT>::Bss->setSize(Off);
 }
 
+template <class ELFT>
+static StringRef getOutputSection(InputSection<ELFT> *Sec) {
+  StringRef S = Sec->getSectionName();
+  if (S.startswith(".text."))
+    return ".text";
+  if (S.startswith(".rodata."))
+    return ".rodata";
+  if (S.startswith(".data."))
+    return ".data";
+  if (S.startswith(".bss."))
+    return ".bss";
+  return S;
+}
+
 // Create output section objects and add them to OutputSections.
 template <class ELFT> void Writer<ELFT>::createSections() {
   // .interp needs to be on the first page in the output file.
   if (needsInterpSection())
     OutputSections.push_back(Out<ELFT>::Interp);
 
   SmallDenseMap<SectionKey<ELFT::Is64Bits>, OutputSection<ELFT> *> Map;
 
if (!isOutputDynamic())
     Symtab.addIgnoredSym("__tls_get_addr");
 
   for (const std::unique_ptr<ObjectFile<ELFT>> &F : Symtab.getObjectFiles()) {
     for (InputSection<ELFT> *C : F->getSections()) {
       if (!C || C == &InputSection<ELFT>::Discarded)
         continue;
       const Elf_Shdr *H = C->getSectionHdr();
       uintX_t OutFlags = H->sh_flags & ~SHF_GROUP;
-      SectionKey<ELFT::Is64Bits> Key{C->getSectionName(), H->sh_type, OutFlags};
+      SectionKey<ELFT::Is64Bits> Key{getOutputSection(C), H->sh_type, OutFlags};
       OutputSection<ELFT> *&Sec = Map[Key];
       if (!Sec) {
         Sec = new (CAlloc.Allocate())
             OutputSection<ELFT>(Key.Name, Key.Type, Key.Flags);
         OutputSections.push_back(Sec);
       }
       Sec->addSection(C);
       scanRelocs(*C);

test/elf2/section-name.s

+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
+# RUN: ld.lld2 %t -o %tout
+# RUN: llvm-readobj -sections %tout | FileCheck %s
+# REQUIRES: x86
+
+.global _start
+.text
+_start:
+
+.section .text.a,"ax"
+.section .text.,"ax"
+.section .rodata.a,"a"
+.section .rodata,"a"
+.section .data.a,"aw"
+.section .data,"aw"
+.section .bss.a,"",@nobits
+.section .bss,"",@nobits
+.section .foo.a,"aw"
+.section .foo,"aw"
+
+// CHECK:     Name: .rodata
+// CHECK-NOT: Name: .rodata.a
+// CHECK:     Name: .text
+// CHECK-NOT: Name: .text.a
+// CHECK:     Name: .data
+// CHECK-NOT: Name: .data.a
+// CHECK:     Name: .foo
+// CHECK:     Name: .foo
+// CHECK:     Name: .bss
+// CHECK-NOT: Name: .bss.a