[sanitizer] Remove tag from StackDepotNode

[vitalybuka]

Summary

And share storage with size.

Depends on D111615.

[dvyukov]

Remove size from StackDepotNode

s/size/tag/?

[dvyukov]

But this does not reduce size of the node, it's still 16 bytes. So what's the point?

[vitalybuka]

3 of https://reviews.llvm.org/D111614#3058999
plus it a part of StackTrace::tag so maybe more consistent to keep with the rest of trace, e.g. sharing word with StackTrace::size

[vitalybuka]

In D111616#3057473, @dvyukov wrote:

Remove size from StackDepotNode

s/size/tag/?

done

[vitalybuka]

Also I am not convinced that we need the tag at all, but @eugenis has a different opinion D110880

[vitalybuka]

rebase

[vitalybuka]

rebase

compiler-rt/lib/sanitizer_common/sanitizer_stackdepot.cpp

 namespace __sanitizer {
 
 static PersistentAllocator<uptr> traceAllocator;
 
 struct StackDepotNode {
   using hash_type = u64;
   hash_type stack_hash;
   u32 link;
-  u32 tag;
 
   static const u32 kTabSizeLog = SANITIZER_ANDROID ? 16 : 20;
+  static const u32 kStackSizeBits = 16;
 
   typedef StackTrace args_type;
   bool eq(hash_type hash, const args_type &args) const {
     return hash == stack_hash;
   }
   static uptr allocated() { return traceAllocator.allocated(); }
   static hash_type hash(const args_type &args) {
     MurMur2Hash64Builder H(args.size * sizeof(uptr));
int StackDepotHandle::use_count() const {
   return atomic_load_relaxed(&useCounts[id_]);
 }
 
 void StackDepotHandle::inc_use_count_unsafe() {
   atomic_fetch_add(&useCounts[id_], 1, memory_order_relaxed);
 }
 
 void StackDepotNode::store(u32 id, const args_type &args, hash_type hash) {
-  tag = args.tag;
   stack_hash = hash;
   uptr *stack_trace = traceAllocator.alloc(args.size + 1);
-  *stack_trace = args.size;
+  CHECK_LT(args.size, 1 << kStackSizeBits);
+  *stack_trace = args.size + (args.tag << kStackSizeBits);
   internal_memcpy(stack_trace + 1, args.trace, args.size * sizeof(uptr));
   tracePtrs[id] = stack_trace;
 }
 
 StackDepotNode::args_type StackDepotNode::load(u32 id) const {
   const uptr *stack_trace = tracePtrs[id];
   if (!stack_trace)
     return {};
-  return args_type(stack_trace + 1, *stack_trace, tag);
+  uptr size = *stack_trace & ((1 << kStackSizeBits) - 1);
+  uptr tag = *stack_trace >> kStackSizeBits;
+  return args_type(stack_trace + 1, size, tag);
 }
 
 StackDepotStats StackDepotGetStats() { return theDepot.GetStats(); }
 
 u32 StackDepotPut(StackTrace stack) { return theDepot.Put(stack); }
 
 StackDepotHandle StackDepotPut_WithHandle(StackTrace stack) {
   return StackDepotNode::get_handle(theDepot.Put(stack));