[lld/mac] Test zerofill sections after __thread_bss

Real zerofill sections go after __thread_bss, since zerofill sections
must all be at the end of their segment and __thread_bss must be right
after __thread_data.

Works fine already, but wasn't tested as far as I can tell.

Also tweak comment about zerofill sections a bit.

No behavior change.

Differential Revision: https://reviews.llvm.org/D104609

lld/MachO/OutputSegment.cpp

@@ -130,8 +130,10 @@ static int sectionOrder(OutputSection *osec) {
         .Case(section_names::codeSignature, std::numeric_limits<int>::max())
         .Default(osec->inputOrder);
   }
-  // ZeroFill sections must always be the at the end of their segments,
-  // otherwise subsequent sections may get overwritten with zeroes at runtime.
+  // ZeroFill sections must always be the at the end of their segments:
+  // dyld checks if a segment's file size is smaller than its in-memory
+  // size to detect if a segment has zerofill sections, and if so it maps
+  // the missing tail as zerofill.
   if (sectionType(osec->flags) == S_ZEROFILL)
     return std::numeric_limits<int>::max();
   return osec->inputOrder;

lld/test/MachO/tlv-dylib.s

@@ -69,11 +69,29 @@
 # FLAGS-NEXT:   segname __DATA
 # FLAGS-NEXT:      addr
 # FLAGS-NEXT:      size 0x0000000000000008
-# FLAGS-NEXT:    offset
+# FLAGS-NEXT:    offset 0
 # FLAGS-NEXT:     align 2^3 (8)
 # FLAGS-NEXT:    reloff 0
 # FLAGS-NEXT:    nreloc 0
 # FLAGS-NEXT:      type S_THREAD_LOCAL_ZEROFILL
+# FLAGS:       sectname __bss
+# FLAGS-NEXT:   segname __DATA
+# FLAGS-NEXT:      addr
+# FLAGS-NEXT:      size 0x0000000000002000
+# FLAGS-NEXT:    offset 0
+# FLAGS-NEXT:     align 2^0 (1)
+# FLAGS-NEXT:    reloff 0
+# FLAGS-NEXT:    nreloc 0
+# FLAGS-NEXT:      type S_ZEROFILL
+# FLAGS:       sectname __common
+# FLAGS-NEXT:   segname __DATA
+# FLAGS-NEXT:      addr
+# FLAGS-NEXT:      size 0x0000000000004000
+# FLAGS-NEXT:    offset 0
+# FLAGS-NEXT:     align 2^14 (16384)
+# FLAGS-NEXT:    reloff 0
+# FLAGS-NEXT:    nreloc 0
+# FLAGS-NEXT:      type S_ZEROFILL
 
 #--- libtlv.s
 .section __DATA,__thread_vars,thread_local_variables
@@ -95,6 +113,12 @@ _main:
 
 ## Add some TLVs to test too, so that we can test the ordering
 ## of __thread_ptrs, __thread_data, and __thread_bss.
+## Also add a .bss and a .comm for good measure too. Since they
+## are both zerofill, they end up after __thread_bss.
+.comm _com, 0x4000
+.bss
+.zero 0x2000
+
 .section __DATA,__thread_data,thread_local_regular
 _tfoo$tlv$init:
   .quad 123