[LLD][ELF] Eliminate symbols of merged .ARM.exidx sections.

GNU tools generate mapping symbols "$d" for .ARM.exidx sections. The
symbols are added to the symbol table much earlier than the merging
takes place, and after that, they become dangling. Before the patch,
LLD output those symbols as SHN_ABS with the value of 0. The patch
removes such symbols from the symbol table.

Differential Revision: https://reviews.llvm.org/D78820

Author: igorkudrin

lld/ELF/Writer.cpp

@@ -681,6 +681,15 @@ static bool shouldKeepInSymtab(const Defined &sym) {
   if (config->copyRelocs && sym.used)
     return true;
 
+  // Exclude local symbols pointing to .ARM.exidx sections.
+  // They are probably mapping symbols "$d", which are optional for these
+  // sections. After merging the .ARM.exidx sections, some of these symbols
+  // may become dangling. The easiest way to avoid the issue is not to add
+  // them to the symbol table from the beginning.
+  if (config->emachine == EM_ARM && sym.section &&
+      sym.section->type == SHT_ARM_EXIDX)
+    return false;
+
   if (config->discard == DiscardPolicy::None)
     return true;
   if (config->discard == DiscardPolicy::All)

lld/test/ELF/arm-exidx-mapping-symbols.s

@@ -0,0 +1,26 @@
+// REQUIRES: arm
+// RUN: llvm-mc -filetype=obj -triple=armv7a-none-linux-gnueabi %s -o %t
+// RUN: ld.lld %t -o %t2
+// RUN: llvm-readelf -s %t2 | FileCheck %s
+// CHECK-NOT: $d.exidx.foo
+
+/// Test that symbols which point to input .ARM.exidx sections are eliminated.
+/// These symbols might be produced, for example, by GNU tools.
+
+    .syntax unified
+    .section .text.foo,"axG",%progbits,foo,comdat
+foo:
+    bx lr
+
+/// GNU as adds mapping symbols "$d" for .ARM.exidx sections it generates.
+/// llvm-mc does not do that, so reproduce that manually.
+    .section .ARM.exidx.text.foo,"ao?",%0x70000001,.text.foo
+$d.exidx.foo:
+    .reloc 0, R_ARM_NONE, __aeabi_unwind_cpp_pr0
+    .long .text.foo(PREL31)
+    .long 0x80b0b0b0
+
+    .section .text.h,"ax"
+    .global __aeabi_unwind_cpp_pr0
+__aeabi_unwind_cpp_pr0:
+    bx lr