[ELF] When placing orphans, handle case when last section is dead

[phosek]

Summary

r315292 introduced a change that's supposed consistently ignore
"dead" output sections when placing orphans. Unfortunately, that
change doesn't handle the special case when the orphan section is
second to last section and the last section is dead (e.g. because
it's being discarded) introducing a regression in some cases.

This change handles this case by using the same predicate when
checking the last section.

[ruiu]

LGTM

ELF/Writer.cpp

1014

nit: we usually use P for a pair, so maybe Pred is a better name.

ELF/Writer.cpp

findOrphanPos(std::vector<BaseCommand *>::iterator B,
   for (; I != E; ++I) {
     auto *CurSec = dyn_cast<OutputSection>(*I);
     if (!CurSec || !CurSec->Live)
       continue;
     if (getRankProximity(Sec, CurSec) != Proximity ||
         Sec->SortRank < CurSec->SortRank)
       break;
   }
-  auto J = std::find_if(llvm::make_reverse_iterator(I),
-                        llvm::make_reverse_iterator(B), [](BaseCommand *Cmd) {
-                          auto *OS = dyn_cast<OutputSection>(Cmd);
-                          return OS && OS->Live;
-                        });
+
+  auto P = [](BaseCommand *Cmd) {
+  };
+
+  auto J = std::find_if(llvm::make_reverse_iterator(I),
+                        llvm::make_reverse_iterator(B), P);
   I = J.base();
 
   // As a special case, if the orphan section is the last section, put
   // it at the very end, past any other commands.
   // This matches bfd's behavior and is convenient when the linker script fully
   // specifies the start of the file, but doesn't care about the end (the non
   // alloc sections for example).
-  auto NextSec = std::find_if(
-      I, E, [](BaseCommand *Cmd) { return isa<OutputSection>(Cmd); });
+  auto NextSec = std::find_if(I, E, P);
   if (NextSec == E)
     return E;
 
   while (I != E && shouldSkip(*I))
     ++I;
   return I;
 }
 

test/ELF/linkerscript/orphan-discard.s

+# REQUIRES: x86
+# RUN: llvm-mc -position-independent -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
+# RUN: echo "SECTIONS { \
+# RUN:  . = 0xffffffff80000000; \
+# RUN:  .text : ALIGN(4096) { *(.text) } \
+# RUN:  .data : ALIGN(4096) { *(.data) } \
+# RUN:  .bss : ALIGN(4096) { *(.bss); } \
+# RUN:  . = ALIGN(4096); \
+# RUN:  _end = .; \
+# RUN:  /DISCARD/ : { *(.comment) } \
+# RUN: }" > %t.script
+# RUN: ld.lld -o %t --script %t.script %t.o
+# RUN: llvm-readelf -s -symbols %t | FileCheck %s
+
+# CHECK: .bss NOBITS ffffffff80002000 002008 000002 00 WA 0 0 4096
+# CHECK: ffffffff80003000 0 NOTYPE GLOBAL DEFAULT 3 _end
+
+.section .text, "ax"
+  ret
+
+.section .data, "aw"
+  .quad 0
+
+.section .bss, "", @nobits
+  .short 0