update tests and comments

Signed-off-by: Peter Rong <[email protected]>

Author: DataCorrupted

lld/MachO/BPSectionOrderer.cpp

@@ -61,6 +61,14 @@ getRelocHash(const Reloc &reloc,
   return getRelocHash(kind, sectionIdx.value_or(0), 0, reloc.addend);
 }
 
+// Get a hash of the unwind info (after relocation).
+// This hash is not 100% accurate, but it's good enough for compression.
+//
+// Unwind info will be eliminated if it is the same with its neighboors.
+// We want to order functions such that the ones with similar unwind info
+// can stay together.
+// See more here:
+// https://faultlore.com/blah/compact-unwinding/#page-tables
 static uint64_t getUnwindInfoEncodingHash(const InputSection *isec) {
   for (Symbol *sym : isec->symbols) {
     if (auto *d = dyn_cast_or_null<Defined>(sym)) {

lld/MachO/UnwindInfoSection.h

@@ -42,6 +42,7 @@ struct CompactUnwindEntry {
   Symbol *personality;
   InputSection *lsda;
 
+  // Relocate the entry to the given Symbol.
   void relocateOneCompactUnwindEntry(const Defined *d);
 };
 

lld/test/MachO/bp-section-orderer-stress.s

@@ -29,11 +29,15 @@ profiled_functions = function_names[: int(num_functions / 2)]
 function_contents = [
     f"""
 {name}:
+  .cfi_startproc
+  .cfi_personality 155, _personality_{i % 5}
+  .cfi_lsda 16, _exception{i % 3}
   add w0, w0, #{i % 4096}
   add w1, w1, #{i % 10}
   add w2, w0, #{i % 20}
   adrp x3, {name}@PAGE
   ret
+  .cfi_endproc
 """
     for i, name in enumerate(function_names)
 ]
@@ -78,6 +82,26 @@ with open(assembly_filepath, "w") as f:
 _main:
   ret
 
+_personality_0:
+  ret
+_personality_1:
+  ret
+_personality_2:
+  ret
+_personality_3:
+  ret
+_personality_4:
+  ret
+
+_exception0:
+  .quad 0x4200
+
+_exception1:
+  .quad 0x4210
+
+_exception2:
+  .quad 0x4220
+
 {"".join(function_contents)}
 
 .data