Fix --gc-sections crash.

We would crash when a non-alloca section pointed to a gced part of a
merge section.

That can happen when a C/c++ constant in put in a merge section and
debug info is present.

llvm-svn: 282845

lld/ELF/InputSection.cpp

@@ -623,7 +623,9 @@ typename ELFT::uint MergeInputSection<ELFT>::getOffset(uintX_t Offset) const {
   // If Offset is not at beginning of a section piece, it is not in the map.
   // In that case we need to search from the original section piece vector.
   const SectionPiece &Piece = *this->getSectionPiece(Offset);
-  assert(Piece.Live);
+  if (!Piece.Live)
+    return 0;
+
   uintX_t Addend = Offset - Piece.InputOff;
   return Piece.OutputOff + Addend;
 }

lld/test/ELF/gc-sections-non-alloc-to-merge.s

+# REQUIRES: x86
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
+# RUN: ld.lld %t.o -o %t --gc-sections
+# RUN: llvm-readobj -s --elf-output-style=GNU %t | FileCheck %s
+
+# CHECK:  .merge1     PROGBITS    {{[0-9a-z]*}} {{[0-9a-z]*}} 000004
+
+        .global _start
+_start:
+        .quad .Lfoo
+
+        .section        .merge1,"aM",@progbits,4
+        .p2align        2
+.Lfoo:
+        .long 1
+.Lbar:
+        .long 2
+
+        .section bar
+        .quad .Lbar