[ELF] mergeSections: remove non-alive MergeInputSection

[MaskRay]

Summary

This makes it conform to what the comment says. Otherwise when getErrPlace() is called afterwards, cast<InputSection>(D) will cause incompatible cast as MergeInputSection is not a subclass of InputSection.

[MaskRay]

This supersedes https://reviews.llvm.org/D50419

[MaskRay]

Update description with arc diff --edit --verbatim (see if this updates the Phabricator summary)

[grimar]

Cool! Can you add a test?

[MaskRay]

I don't know how to add a test checking the elements of InputSections.

Alternatively, I can check whether getErrorPlace will cause cast<> assertion failure. But how can I make it error?

[MaskRay]

Add test

[MaskRay]

.rodata.str2.1 -> .rodata.str1.2

[MaskRay]

Simplify test

[MaskRay]

.

[grimar]

LGTM with a nit.

test/ELF/x86-64-reloc-error2.s

13 ↗

(On Diff #160859)

You do not need to reveal the implementation details in the test cases.
They might change and will make such comment outdated.

All you need is to say is something like:
"This mergeable section will be garbage collected. We had a crash issue in that case. Test it."

lld/trunk/ELF/SyntheticSections.cpp

void elf::mergeSections() {
   std::vector<MergeSyntheticSection *> MergeSections;
   for (InputSectionBase *&S : InputSections) {
     MergeInputSection *MS = dyn_cast<MergeInputSection>(S);
     if (!MS)
       continue;
 
     // We do not want to handle sections that are not alive, so just remove
     // them instead of trying to merge.
-    if (!MS->Live)
+    if (!MS->Live) {
+      S = nullptr;
       continue;
+    }
 
     StringRef OutsecName = getOutputSectionName(MS);
     uint32_t Alignment = std::max<uint32_t>(MS->Alignment, MS->Entsize);
 
     auto I = llvm::find_if(MergeSections, [=](MergeSyntheticSection *Sec) {
       // While we could create a single synthetic section for two different
       // values of Entsize, it is better to take Entsize into consideration.
       //

lld/trunk/test/ELF/x86-64-reloc-error2.s

 # REQUIRES: x86
 # RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
-# RUN: not ld.lld %t.o -o /dev/null 2>&1 | FileCheck %s
+# RUN: not ld.lld --entry=func --gc-sections %t.o -o /dev/null 2>&1 | FileCheck %s
 
 ## Check we are able to find a function symbol that encloses
 ## a given location when reporting error messages.
 # CHECK: {{.*}}.o:(function func): relocation R_X86_64_32S out of range: -281474974609408 is not in [-2147483648, 2147483647]
 
+# This mergeable section will be garbage collected. We had a crash issue in that case. Test it.
+.section .rodata.str1,"aMS",@progbits,1
+.asciz "a"
+
 .section .text.func, "ax", %progbits
 .globl func
 .type func,@function
-.size func, 0x10
 func:
- movq func - 0x1000000000000, %rdx
+  movq $func - 0x1000000000000, %rdx
+.size func, .-func