[ELF] - Do not segfault when using --gc-sections with linker script

Patch fixes PR32024.

Sections that were not marked as Live has null output section.
Previously we tried to access that field and segfaulted.

Differential revision: https://reviews.llvm.org/D30188

llvm-svn: 295727

lld/ELF/LinkerScript.cpp

@@ -503,6 +503,8 @@ template <class ELFT> void LinkerScript<ELFT>::process(BaseCommand &Base) {
         continue;
 
     auto *IB = static_cast<InputSectionBase<ELFT> *>(ID);
+    if (!IB->Live)
+      continue;
     switchTo(IB->OutSec);
     if (auto *I = dyn_cast<InputSection<ELFT>>(IB))
       output(I);

lld/test/ELF/linkerscript/sections-gc.s

@@ -0,0 +1,19 @@
+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
+# RUN: echo "SECTIONS { .text : { *(.text*) } }" > %t.script
+# RUN: ld.lld %t --gc-sections --script %t.script -o %t1
+# RUN: llvm-objdump -section-headers %t1 | FileCheck %s
+
+# CHECK:      Sections:
+# CHECK-NEXT:  Name      Size
+# CHECK:       .text     00000001
+
+.section .text.foo, "ax"
+.global _start
+_start:
+  nop
+
+.section .text.bar, "ax"
+.global bar
+bar:
+  nop