[BOLT] Fix runOnEachFunctionWithUniqueAllocId (llvm#90039)

When runOnEachFunctionWithUniqueAllocId is invoked with
ForceSequential=true, then the current implementation runs the function
with AllocId==0, which is the Id for the shared, non-unique, default
AnnotationAllocator.

However, the documentation for runOnEachFunctionWithUniqueAllocId
states:
```
/// Perform the work on each BinaryFunction except those that are rejected
/// by SkipPredicate, and create a unique annotation allocator for each
/// task. This should be used whenever the work function creates annotations to
/// allow thread-safe annotation creation.
```

Therefore, even when ForceSequential==true, a unique AllocId should be
used, i.e. different from 0.

In the current upstream BOLT this is presumably not depended on, but it
is needed to reduce memory usage for analyses that use a lot of
memory/annotations. Examples are the pac-ret and stack-clash analyses
that currently have prototype implementations as described in
https://discourse.llvm.org/t/rfc-bolt-based-binary-analysis-tool-to-verify-correctness-of-security-hardening/78148
These analyses use the DataFlowAnalysis framework to sometimes store
quite a lot of information on each MCInst. They run in parallel on each
function. When the dataflow analysis is finished, the annotations on
each MCInst can be removed, hugely saving on memory consumption. The
only annotations that need to remain are those that indicate some
unexpected properties somewhere in the binary.

Fixing this bug enables implementing the deletion of the memory used by
those huge number of DataFlowAnalysis annotations (by invoking
BC.MIB->freeValuesAllocator(AllocatorId)), even when run with
--no-threads. Without this bug fixed, the invocation of
BC.MIB->freeValuesAllocator(AllocatorId) results in also the memory for
all other annotations to be deleted, as AllocatorId is 0.

---------

Co-authored-by: Maksim Panchenko <maks@meta.com>

Author: kbeyls

bolt/lib/Core/ParallelUtilities.cpp

@@ -188,8 +188,20 @@ void runOnEachFunctionWithUniqueAllocId(
     LLVM_DEBUG(T.stopTimer());
   };
 
+  unsigned AllocId = 1;
+  auto EnsureAllocatorExists = [&BC](unsigned AllocId) {
+    if (!BC.MIB->checkAllocatorExists(AllocId)) {
+      MCPlusBuilder::AllocatorIdTy Id =
+          BC.MIB->initializeNewAnnotationAllocator();
+      (void)Id;
+      assert(AllocId == Id && "unexpected allocator id created");
+    }
+  };
+
   if (opts::NoThreads || ForceSequential) {
-    runBlock(BC.getBinaryFunctions().begin(), BC.getBinaryFunctions().end(), 0);
+    EnsureAllocatorExists(AllocId);
+    runBlock(BC.getBinaryFunctions().begin(), BC.getBinaryFunctions().end(),
+             AllocId);
     return;
   }
   // This lock is used to postpone task execution
@@ -205,19 +217,13 @@ void runOnEachFunctionWithUniqueAllocId(
   ThreadPoolInterface &Pool = getThreadPool();
   auto BlockBegin = BC.getBinaryFunctions().begin();
   unsigned CurrentCost = 0;
-  unsigned AllocId = 1;
   for (auto It = BC.getBinaryFunctions().begin();
        It != BC.getBinaryFunctions().end(); ++It) {
     BinaryFunction &BF = It->second;
     CurrentCost += computeCostFor(BF, SkipPredicate, SchedPolicy);
 
     if (CurrentCost >= BlockCost) {
-      if (!BC.MIB->checkAllocatorExists(AllocId)) {
-        MCPlusBuilder::AllocatorIdTy Id =
-            BC.MIB->initializeNewAnnotationAllocator();
-        (void)Id;
-        assert(AllocId == Id && "unexpected allocator id created");
-      }
+      EnsureAllocatorExists(AllocId);
       Pool.async(runBlock, BlockBegin, std::next(It), AllocId);
       AllocId++;
       BlockBegin = std::next(It);