[ELF] - Do not crash when linkerscript applies fill to .bss.

[grimar]

Summary

I found that during visual inspection of code while wrote different patch.
Script in testcase probably have nothing common with real life, but
we segfault currently using it.

I think if output section is known NOBITS, there is no need to create
writers threads for doing nothing or proccess any filler logic that is useless here.
We can just early return, that is what this patch do.

[ruiu]

lld/trunk/ELF/LinkerScript.cpp

1083–1084

Why don't you write this at the very beginning of this function?

[grimar]

lld/trunk/ELF/LinkerScript.cpp

1083–1084

Done in r304777.

lld/trunk/ELF/LinkerScript.cpp

template <class ELFT> void OutputSectionCommand::writeTo(uint8_t *Buf) {
   // -compress-debug-sections option. Write it together with header.
   if (!Sec->CompressedData.empty()) {
     memcpy(Buf, Sec->ZDebugHeader.data(), Sec->ZDebugHeader.size());
     memcpy(Buf + Sec->ZDebugHeader.size(), Sec->CompressedData.data(),
            Sec->CompressedData.size());
     return;
   }
 
+  if (Sec->Type == SHT_NOBITS)
+    return;
+
   // Write leading padding.
   ArrayRef<InputSection *> Sections = Sec->Sections;
   uint32_t Filler = getFiller();
   if (Filler)
     fill(Buf, Sections.empty() ? Sec->Size : Sections[0]->OutSecOff, Filler);
 
   parallelForEachN(0, Sections.size(), [=](size_t I) {
     InputSection *IS = Sections[I];

lld/trunk/test/ELF/linkerscript/bss-fill.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
+# RUN: echo "SECTIONS { .bss : { . += 0x10000; *(.bss) } =0xFF };" > %t.script
+# RUN: ld.lld -o %t --script %t.script %t.o
+
+.section .bss,"",@nobits
+.short 0